Delco Electronic Ignition

The Delco Distributor and Electronic Ignition System
by Tony Rhodes
January 2013
THIS IS A PRELIMINARY DRAFT OF THIS
ARTICLE. IT CONTAINS ALMOST ALL OF THE
SCANS OF THE ORIGINAL MANUFACTURER
DATA THAT I EXPECT TO USE, BUT ONLY
SOME OF THE TEXT. MUCH OF MY TEXT IS
INCOMPLETE AND PRELIMINARY. PLEASE
EXCUSE THE APPEARANCE!
When I got my 1980 TR7 Spider in 2001, it had
recently stopped running. It would turn over, but it
would not start. It was delivered to me as a “nonrunner” on an 18-wheel auto transport. I had to
get a tow truck to get it off the transporter and
bring it to my house. I fixed it easily by cleaning
the green corrosion and glop off the contacts
of the wire from the distributor to the electronic
ignition coil unit. At that time I had no idea what
this electronic ignition was, but it looked old, clunky
and unreliable. It was on my short list of items to
be replaced. It looked like a unique device and
it was then 21 years old. I am suspicious of old
electronics in harsh environments. I was not a big
fan of electronic ignition in general because when
it goes bad, you don’t just run poorly. You don’t run
at all. It is hard to diagnose and repair elec­tronic
ignition faults, especially at the side of the road.
When your electronic ignition goes bad, it is time for
a tow. Points ignition, on the other hand, typically
de­grades slowly with ample warning to allow you
to get to a convenient place before it conks out.
nearly IMPOSSIBLE to properly adjust the points
on the TR7. After installing the UK-spec distributor
and painstakingly adjusting the points, I swore to
myself that when it became time to re-adjust the
points, I would install an after-market electronic
ignition. My foray into after-market electronic
ignition for the points-type Delco distributor is a
story all by itself. Lets just say that the Newtronic
optical pickup for the TR7 is not well designed. I
removed that ignition almost immediately. In the
meantime, I had learned that the clunky-looking
original electronic ignition was in actuality a GM
High Energy Ignition (HEI) system. The HEI system
is loved by American hotrodders for its simplicity,
reliability and ease of use. The part I was most
concerned about failing, the ignition module, is
inexpensive and easily available at any auto parts
store. I decided to go back to the original distributor
and module/coil.
After re-fitting the Delco electronic ignition, it
ran well for several years, then it began to get
progressively harder to start. Finally it required
liberal use of starting fluid and lots of cranking to
get the car to fire. Once it was running, it would
have a terrible miss for some seconds then the
other cylinders would start to fire. Then it would run
nicely and smoothly with no problems until the next
time it had not been run in 24 hours. Ultimately, the
car would not start at all. When that happened, I
checked all the electrical connections and they all
seemed to be clean and tight. I then tool the coil
wire off the distributor and attached a spark tester
to the coil wire. When the engine was cranked, no
spark was present. No spark was present, even
with a tiny gap to jump. Darn! I concluded that the
coil must be bad because the module had been
replaced just a few months prior in attempts to
cure the hard starting. The new module seemed
to help the hard staring marginally, but it was still
ridiculously hard to start.
I also had a plan to replace the poorly func­tioning
dual downdraft Weber 32-36 DGEV carbs with
some simple and reliable SU carbs. The SU’s
on my TR4A work well and are easy to maintain.
Plus they don’t have that pesky rubber diaphragm
found on Zenith Stromberg carbure­tors. That’s the
diaphragm that always rips and makes you run lean.
The distributor on the car used vacuum retard, and
the SU carburetors do not have a port for vacuum
retard. So when I installed the SU carburetors, I
decided to also install the matching UK-spec Delco
points distributor. After installation, I found out why Assuming that the electrical connections and the
the Brits are keen on replacing the points with new module were not the culprits, I concluded that
some sort of after-mar­ket electronic ignition. It is the original 32 year old epoxy coil was no longer
functional. I started the hunt for the mythical TR7
HEI coil. For along time before this I had been
keeping my eyes open at places like Ebay for a
NOS TR7 coil, and I never saw one. I had read
somewhere that the same Delco-France module/
coil unit had been used by Peugeot. I then
searched for a coil listed as be­ing compatible with
the Peugeot 504, or 604 from the early 80’s. On
Ebay I found someone selling two coils for the
Peugeot 604. In the photos, it these looked to
be very similar to the TR7 coil, except that they
had a male “HEI style” high tension wire terminal
(it looks like the terminal of a sparkplug). I bought
one of them and found that it was a perfect fit in
the original mounting holes on the cast aluminum
frame. I also found a GM D525 coil that also looked
very promising. The epoxy coil itself looked a little
larger than the TR7 or Peugeot coils, but the frame
looked about right, so I ordered one of them. The
Delco D525 is easily and inexpensively available
at car parts places like NAPA, and the quality is
typically quite good. When I got the D525, I found
that it DID fit the frame, but it uses the alternate set
of holes that were already drilled in the frame. I just
had to tap them for 4mm x 0.7 metric threads. The
Delco electrical part of the coil was bigger than
the TR7 and Peu­geot coil, so I guessed it might
put out more spark energy. I decided to learn how
to measure the parameters of coils to determine
which ones are more “power­ful” than others. Refer
to Appendix NN for information on how to measure
the coil electrical parameters and what they mean
for spark power.
manually triggered the ignition module with a
points distributor. I got a good spark. This made
me conclude that the problem had to o with the
distributor. I checked the re­sistance of the reluctor
pickup in the distributor and I got about 800 ohms
which is apparently a good number, so there
appeared to be no break in a wire. There was no
continuity between the reluctor and ground, so it
was not shorted out. I did see some sort of AC
signal coming from the reluctor, but I had no data
to indicate what sort of voltage to expect, nor what
voltage is required to trigger the ignition module. I
suspected that my pickup was putting out a voltage
at the very edge of enough to trigger the module.
Maybe the magnet in the reluctor was weak.
I needed to get another distributor for comparison.
As luck would have it, a Delco elec­tronic distributor
had recently been on Ebay, and had not sold.
I contacted the seller and bought it. As soon as
it arrived I tried it on my electronic ignition. I did
not need to install the distributor. I could hold the
distributor and turn the rotor man­ually. As I did so,
I succeeded in getting spark from the coil! I could
trigger the coil down to one revolution every 4
seconds. I think cranking speed is faster than that.
Unfortunately, the Ebay distributor was in rough
shape. I could not drop the distributor in place of
my own unit. I would have to dismantle the new
distributor and transfer the pickup to my distributor.
Before I did that, I looked for a new or NOS reluctor
pickup. Thre are hundreds of vari­ations on a theme
available. I found two likely looking 4-cylinder
reluctor pickups and ordered them. They were less
than $20 each including shipment. Unfortunately,
neither looked like a good match, an I did not
anticipate being able to drop the pickup into my
When I left off, my car was not starting, and distributor directy. But maybe the magnetic sensor
appeared not to have any spark. Since the module coil in one of the new ones would be a match for
was quite new, I had concluded that the coil was the original in my car. And maybe the magnetic
not working properly, and was not able to deliver a pole piece would be interchangeable.
good spark. I searched for a replace­ment coil, and
settled on the D525 as a coil which ought to deliver
a good spark at cranking speeds. Unfortunately,
when I reinstalled the upgraded ignition there was
still no spark! As I was testing coils, I had found
a schematic for a transistor interface between
points coil and an HEI module. I put that interface
on the electronic ignition as it sat on the car and
Appendix AAA
Why HEI is a Good System
Where does the “High Energy” of a High Energy
Ignition come from? Ignition designers always knew
that they could get more spark energy if they could
have more current in the coil. They could achieve
this by having more current. Energy is proportional
to the square of the current. Unfortunately points
can handle only up to about 4 amps before the
lifetime of the points is seriously degraded due
to arcing at the moment the points open. Once
electronic switching was robust enough to survive
the induced voltages from switching a coil,
automotive designers did
what they always wanted
to do: Get more current
through the coil. But it is not
quite as simple as that. It
takes time to get the energy
into the coil. Charging
time
is
proportional
to the coil inductance
divided by the resistance.
Lower resistance allows
a greater current, but a
large inductance slows
down the ability to build up
that current. An inductor
has magnetic “inertia”
which only allows current to build slowly. At high
RPMs there is less time to charge the coil, so a
high inductance coil will not have enough time
to fully charge. So, designers needed to lower
the inductance to a degree, depending on the
application. For a high revving V8, they needed a
low inductance coil to get full energy at high RPMs.
Unfortunately, spark energy is directly proportional
to inductance, so to keep spark energy constant,
current would need to be increased a little as the
inductance drops. This issue was later addressed
by using multiple coils, eventually one per cylinder,
as found on many performance engines today. In
the 1970’s, they used one coil, with a low enough
inductance to allow adequate charging over the
full RPM range used by a particular application.
Our 4-cylinder engines can still use a fairly high
inductance coil without much of a tradeoff at high
RPM.
Another factor that the HEI designers addressed
was heat. If you charge a coil longer than
necessary, then you are turning all that current into
heat. You only need to start charging the coil at a
time dependent on the RPM. You want the coil to
have JUST reached full current at the moment the
next spark needs to be created. The HEI ignition
module has a current limining circuit so no more
than a safe current is provided to the coil. But
before that current limit is reached the module has
Figure 1. This shows the relative spark energy vs RPM of the HEI
coils which fit the TR7 electronic ignition frame. The Points curve
is for a standard 12 volt coil and 60 degrees of dwell, not the VERY
short 39 degree dwell specified for the TR7.
minimal resistance to current flow, and therefore
produces minimal heat. In current limiting mode,
there is a lot of heat being generated by the ignition
module. But the module measures the time it
takes to reach the current limit and starts to charge
the coil just early enough to barely hit the current
limit. Current is limited by the coil resistance and
inductance, not the ignition module most of the
time. Therefore, there s very little heat production
by the module, and as little heat production in
the coil as possible. Just a few watts in the coil,
compared to the 70 watts that 3 ohm coil might
produce with points!
So, the HEI designers were able to achieve high
spark energy while having a more efficient ignition
system and less waste heat. But how much better engines have double the number of spark events.
is it really?
So, the 7000 RPM point in Figure 1 would be 4666
RPM on a 6 cylinder and 3500 RPM on an eight.
The D525 coil’s output is dropping quickly at 7000
Figure 1 shows the relative spark energies vs. RPM RPM (on a 4-cylinder), and on a 6 or 8 cylinder
of the several coils listed in Chart 1 in Appendix car, the spark energy may not be adequate at
CCC. A 12 volt (3 ohm) coil with points is shown for high RPM. The graph suggests that the D525 coil
comparison. You can see that the low inductance may be intended for 4 cylinder engines, and the
coils tend to maintain a more constant spark Peugeot coil, with its very flat graph, may have
energy over the RPM range. This is because they been intended for a 6 cylinder engine. It appears
have time to fully saturate the magnetic field. High that both the D525 and Peugeot coils are superior
inductance coils have do not allow the coil current to the original TR7 coil for a 4-cylinder engine.
to climb as quickly, and at high RPM they do not I chose to use the Delco D525 on my upgraded
get as much current by the time the spark needs ignition system.
to fire. The data used to create these graphs was
based on a mathematical model of the coil and
sparkplug. The current was limited to 5.5A, and
Other TR7 Ignition Systems
dwell was adjusted to allow the coil to just have The TR7 initially used the Lucas Opus electronic
time to reach the current limit before the next spark ignition in the US, and it was always supplied
event. This is the same way the HEI ignition module with a points system in the UK. The Opus system
works. The 3 ohm coil does not use any external had an extremely high failure rate, and many or
current limiter, its inductance and resistance limit most of the original Opus systems have been
the current to a safe value for points. The dwell replaced. Triumph dealers were using the Lucas
was set to a fixed 60 degrees as used in te TR2-4a CEI (Constant Energy Ignition) systems as the
4-cylinder engines. This dwell value is considerably replacement for the Opus. The Lucas CEI uses
longer than the recommended 39 degrees for the a variable reluctor pickup similar to the US Delco
UK points-equipped TR7, but it allows much better distributor, and its ignition module was simply a
coil saturation. It is not clear to me why Triumph standard 4-pin HEI module in an enclosure. I am
recommended 39 degrees of dwell for the TR7. not sure what coil was used with the CEI system,
Some other points-equipped cars ( e.g. VW) use a but it was not a typical HEI “E-Core” coil. It was
dwell of about 52 degrees.
some sort of cylindrical coil. I strongly suspect
it was a standard 1.5 ohm coil with no ballast
The Delco D525 coil has a higher inductance than resistance. Figure 2 shows the relative spark
the TR7 or Peugeot coils, and
as a result has a much higher
spark energy. But it has trouble
maintaining that energy all the
way to red line. The original
TR7 coil has a comparatively
poor spark energy, but it
appears to be intended to be
an HEI version of the 3 ohm
coil, with improved high RPM
performance. The Peugeot coil
is about midway between the
D525 and the TR7 HEI coils.
Six cylinder engines have 50%
more spark events for the same
RPM compared to a 4 cylinder. And 8 cylinder
Figure 2. Relative spark energy of various coils. The points
curves are for a 12 volt, 3 ohm coil with 60 or 39 degrees of dwell.
energies of points with 39 and 60 degrees of dwell,
the stock TR7 HEI, and the Lucas CEI system. The
39 degree dwell energies are well below those of
the 60 degree settings for the 3 ohm coil. The CEI
system starts off fairly well, but the resistive losses
at higher RPM cause a steep drop in spark power
at redline. The CEI is only slightly better than points
with a 60 degree dwell, and it is isightly nferior to
the TR7 original HEI at red line. This difference is
largely due to the higher resistance of the primary
windings limiting the speed of charging of the coil
compared to the HEI type coils.
In summary, there are a number of ignition options
for the TR7. It appears that the very best is the
Delco HEI distributor with the Delco D525 coil or
the “Peugeot” coil. The Lucas CEI appears to
have a better spark energy overall than the original
HEI system supplied by Triumph, but not as good
as an improved HEI system.
Appendix BBB
ADVANCE CURVES
With the help of the TR7-TR8 forum, I was able to
obtain the data on the various Delco distributors fitted
to the TR7. The tables are included at the end of this
article. A graph of the centrifugal advance curves is
shown in Figure 1 here. I was surprised by the wide
variety of curves that were used. I would have thought
that for a specific state of tune (mostly determined by
the cam and compression), the advance curves would
have been very similar. The “early” UK points type
distributor was used with a high compression engine
economy. It might make a few MPG difference.
Advance Measurements
Unfortunately I do not have a “distributor machine”
which spins a distributor and can measure the advance
across the RPM range. All I can do is photograph the
rotation of the advance mechanism with no advance and
maximum advance. By superimposing the two images,
I can measure the degrees of rotation of the advance
mechanism.
I measured the amount of centrifugal advance
for the UK distributor. This unit does not use
any limit bushings to control the advance.
The window alone controls the maximum.
The measurements showed that it had 9.75
degrees of distributor advance (19.5 degrees
at the crank).
When I stripped a US Delco distributor, I was
rather surprised to find perished bushings on
the centrifugal advance limiter pins. The
UK distributor does not use bushings and
the window is just large enough to allow full
centrifugal advance. Older American nonHEI Delco distributors did use a limiter bushing, but not
the HEI distributors. The shape of the top plate, and the
strength of the springs determine the maximum advance
and bushings are not needed for US HEI distributors.
This distributor looks to have been rebuilt or recurved
at some time. I can’t tell if the bushings were original
to the distributor or a later addition. When I looked
at my own distributor, I found that it did not have any
bushings, and there were no apparent persihed remains
inside the distributor. So, it is unclear to me whether
bushings were fitted originally.
and with vacuum advance. I might have thought that
this would have the most gentle advance curve, and
it does. But the “late” UK distributor has the most
aggressive curve, and it also uses vacuum advance!
Is the lower compression that significant a factor in
determining the best advance curve? I don’t know. I do
know that one can use a seat of the pants technique to
determine the “optimal” advance curve. In general, you
want as much advance as you can get without causing
pinking, not even a little pinging. This is hard to do
with vacuum advance-equipped distributors because
the total advance depends on the manifold vacuum
which is RPM, throttle, and load dependent.
On a US TR7 Delco distributor without any bushing,
I measured 15.14 degrees of maximum distributor
Given that the “UK-Late” curve is used on a low advance (30.28 degrees at the crank). If the distributor
compression engine like the US engines, it would appear can actually reach this amount of advance with the
to be safe to use a UK-type vacuum advance module original weights, top plate, and springs, then this 30
on a US distributor originally equipped with a vacuum degrees of advance in combination with the 10 degrees
retard unit. The advantage of this is that the engine of basic advance probably adds up to too much advance
really needs more advance in partial throttle cruising at WOT. I had on hand a new HEI pivot bushing which
situations in order to get the best performance and fuel fits the advance limit pin. This bushing had a thickness
of about 0.031” and an ID of about 0.190” and an OD
of about 0.252”. A length of no more than 0.300 would
allow an E-clip to lock it in place. When measured,
it did little to limit the advance, giving 14.73 degrees
at the distributor. A thicker limit bushing seems to be
needed.
I retrieved the perished remnant of the original bushing
as fitted to the distributor. This had a thickness of 0.050”
to 0.060” and a length of 0.268” Assuming a 0.190
inner diameter, then the outer diameter would be about
0.300”. When I used it for the advance measurements,
I got 10.74 distributor degrees (21.48 crank degrees).
This is in the correct ball park, and probably I was
pushing the advance harder than the centrifugal weights
do, causing a little extra advance. McMaster-Carr has
Teflon tubing (8547K24) available in 1 foot quantities at
a reasonable cost. The OD is 5/16 (0.3125) and the ID
is 3/16 (0.1875) with a wall thickness of 0.0625”. I have
not tested this tubing for its performance, but it is the
closest I have found so far.
As I said, it is not clear to me that a bushing is required.
If you get pinging at WOT, and have to retard the spark
more than a small amount, then maybe you do need
to add a limit bushing to your distributor. Testing the
distributor on a distributor machine would tell you the
whole story. Jeff at Advanced Distributors said he can
service these units and you can tell him the curve you
want, and he can tune the distributor to that specification.
I suspect that something like this had been done to my
test distributor.
Appendix CCC
Calculating coil electrical parameters
It is easy to get a fairly accurate measurement of the
elcctrical parameters of an ignition coil. It is essentially a transformer. By measureing the inductance of the
primary windings, you can calculate the inductance of
the secondary windings if you know the turns ratio of
primary to secondary. Typically the turns ratio is anywhere between 75 and 110 to 1. It is a little harder to
measure the resistance of the primary when it is under
1 ohn, as the HEI style coils are. But a regular digital
volt/ohm meter will suffice.
Figure CCC-1. Schematic of the tuned circuit to measure the coil
inductance
To determine the coil inductance, you create a tuned
circuit by placing a 0.1uF ceramic capacitor in parallel
across the primary coil terminals. Then you feed a
variable frequency sine wave through a 1000 ohm re-
sistor, thru the parallelled coil and capacitor to ground.
You measur ethe voltage across the coil terminals as
you vary the frequency of the supply. You will see the
voltage peak somewhere around 2000 Hz. Then use
the formula: L = 1/(2 * Pi * F * root(C))^2. This gives
you the overall inductance of the primary windings.
To get a complete model of the coil, you also need to
measure the inefficiency of the magnetic connection
between the primary and secondary coils. Standard cylindrical coils are a little less efficient that “E-core” coils
because the E-core coil has a closed magnetic loop
which recuded losses. Some of the inductance of the
primary windings does not contribute to induced voltage in the secondary windings. This is measured the
same way as the total inductance, but it is done with
the secondary hot lead shorted to the positive primary
Figure CCC-2. Measuring the turns ratio of the coil
terminal. The peak voltage will be somewhere around
4KHz to 10Khz, This inductance s called the “leakage”
inductance. This is subtracted from the total primary inductance to determine the amount of primary
inductance which does induce voltage in the secondary
Coil Electromagnetic Specifications
R Primary
R Secondary
Turns
L Primary
L Secondary
Original TR7
0.6 ohms
10.0K ohms
106:1
4.06 mH
45.79 H
Peugeot 5048
0.6 ohms
5.0K ohms
75:1
5.60 mH
31.5 H
Delco D525
0.6 ohms
8.25K ohms
100:1
9.46 mH
94.6 H
Chart 1. This shows the parameters of the HEI coils that I found which fit the TR7 electronic
ignition coil mounting frame. There is considerable variation between coils which are physically fairly similar in appearance.
windings. The secondary winding inductance cam
be calculated by multiplying the primary inductances
by the quare of the turns ratio. For a 100:1 ratio, you
would multiply the primary inductance by 10,000 to
get the secondary inductance.
The turns ratio is measured by feeding a sine wve into
the secondary windings and then measure the steppeddown voltage across the primary terminals. The ratio
of the voltage is equivalent to the ratio of the turns
between the primary and secondary windings.
These topics are discussed in-depth and very clearly by
Hugo Holden in his article : http://www.worldphaco.
net/uploads/ELECTRONIC_IGNITION_FOR_TR_
CARS.pdf. His website at http://www.worldphaco.
net is an excellent resource for many electronic issues
pertaining to TRs.
Appendix DDD
Part Interchange Information
PART
Coil
Ignition Module
Variable Reluctor pickup
INTERCHANGE
Peugeot-type: Original Engine Management #5048 (reuses original wiring harness)
Delco D525 (NAPA MPE IC22SB) (needs new wires to module and car)
Delco D1906 (stock type module and very reliable)
NAPA ECH TP45 (probably same manufacturer as the D1906)
Pertronix D2000 (street: about 5.5 amp current limit)
Pertronix D2070 (Race: 7.2 amp current limit)
No exact replacement: reuse original back shell.
Wells DR-102, NAPA MP102, Delco D1910, LX311(Star ring is very close match
(or perfect), the pickup coil wires are correct).
Wells DR-107, NAPA MP107, Delco D1966, LX310 (star ring stator is a perfect
match, the pickup coil wires are reversed).
The permanent magnet is correct on both the DR-107 and DR-103. (Probably any
pickup module magnet with a similar size/shape will work regardless of the configuration of the star ring stator).
You will probably need to cut off the small indexing tab on the pickup coil to be
able to orientate it properly in the shell (the original coil had no tab).
The pickup coil Green wire always goes to the “G” terminal of the ignition module
regardless of which side of the pickup it comes off.
Delco D302 Electronic Distributor Exploded Diagram
The condenser in the ignition coil and module frame
is labelled “DR 3474770 0.2uF”. The dimensions are
0.670” diameter, and 1.130” length. The diameter and
length are critical values because it fits in a recess just
large enough for the original condenser. The length
of the recess is 1.175”. Most generic condensers that
I have found are too long to fit, though the diameter
seems to be quite standard.
Delco D302 Points Distributor exploded Diagram