DME to ICU Troubleshooting - No Spark on 3.6L conversion

[356graham]

Hi Folks - I'm looking for some validation on my troubleshooting approach and tentative conclusion.

Symptom is no starting / no spark. The failure occurred as I was backing out of the garage (thankfully) - engine simply shut down and now doesn't start.

Car is a '72 Targa with a 3.6L conversion. Relatively new to me. We all like pictures, so here we go.




So, following some of the other threads here plus the diagnostic approach in the factory workshop manual I've confirmed the following without the use of Bosch Hammer tool or oscilloscope.

1. Fuel is flowing - plenty of gas smell with cranking
2. DME relay is engaging and clicking as I crank. Replaced the DME relay with a new spare I have, and same condition.
3. No spark verified
4. Measured resistance of the speed sensor (can't see the actual signal without a scope). Resistance
5. Verified 12v across both coils.

So, I'm now looking at the DME to ICU connection to see if the DME is putting out the spark signals and if the ICU is translating those signals to the coils.

This is my DME



And this is the DME to ICU connection. ICU is Bosch 0 227 200 100 (apparently NLA)


The workshop manual shows that the DME to ICU signal is a very low level - about 0.25 volts peak.



And the output of the ICU to the coils is a full 12v+ swing as the coil is energized then clamped to create the spark.

At the DME out to ICU in pins (pins 2 and 7 on the ICU) I measure:
Steady state 0.13 VDC no cranking
When cranking 0.1 VAC
Based on the workshop manual showing 0.25V peak, I would expect to see about 0.1VAC on my DMM, right?

Now to the ICU out (pins 1 and 6), I measure:
Steady state 7.1VDC no cranking (I expected to see 12V here) and
When cranking 0.6VAC (I expected to see at least 7-8VAC)

My conclusion is that the (very rare) ICU has failed.

I would greatly appreciate the wisdom and experience of anyone who can validate or provide any guidance on other places to look for the failure>

Thanks!

[Joe Bob]

The DME relay clicks when cranking? That's not good. It should click once when enegized. Continous clicking isn't kosher.

When mine did that it was an intermittent short from the fuse box to the DME power.

[356graham]

Thanks, Joe Bob.

I had looked through this thread 3.6L into '74 911 Conversion - Troubleshooting No Spark
and saw the statement from Ingo (seems to know what he's talking about) that says;

"Originally Posted by ischmitz
If the DME relay clicks when you crank that is a good sign the DME is "seeing" the crank sensor fine."

Maybe my interpretation of "clicks when you crank" is incorrect. Maybe should only click once??

I did check the power into the DME and found it to be solid and spot on, but will check again.

Appreciate the help.

[Joe Bob]

Ingo is an friend of mine. You can email him direct.....you have a pm.

[Tippy]

You can measure the speed reference sensor at the DME plug.

[356graham]

Thanks, Tippy. Speed sensor resistance at the DME connector measures fine. I don't have access to a scope to see the waveform.

Anyone in San Diego with an Oscilloscope that I can borrow for a little while? I can trade for beer.

[356graham]

In addition to restistance for the speed sensor, I just measured VAC while cranking and it shows a good 1.2 VAC. Again, with a scope I could see the actual signal, but this points to a good signal coming from the speed sensor into the DME.

[Tippy]

You measure TDC sensor ohms too? I have no idea if it can cause no spark, just throwing it out there.

[ischmitz]

Hi Jeff,

looks like you have a NA 993 engine. It uses a single ignitor module (two-channel version) - it can control the coils independently (multi-spark).

The ignitor isn't rare. You can find them on eBay all day long. The ignitor (you call it ICU) is nothing but an electronic switch that drives the two coils based on input from the DME. Search for item number 300524778397 on evil bay for example. But there should be many more. Bosch put these into lots of engines around the mid-90's

Regarding the DME relay it has two stages: One clicks when the ignition switch is turned to ON and the second clicks when you crank (turns on fuel pump). This is in a stock car. Depending on how the conversion was done it could be different in your car. However, the fact that you see ignition signals on the DME output tells you all is well and the fault is downstream.

Make sure the power to the coils is present and the wiring isn't loose. I don't think these ignitors are prone to failure. With a conversion the first suspect is always wiring. Next I would suspect the coils before the ignitor. I don't have the wiring diagram in front of me. However, think of the ignitor as a switch between the coil and GND. The other side of the coil is at +12V. Once the switch closes (for the dwell time, controlled by the DME) the coil energizes and when the switch opens the magnetic field breaks down and a spark results. So I would expect +12V at both sides of the coil primary with the ignition ON and no cranking. Measure both sides of the coil wit the ignition ON. Then unplug the ignitor and measure again. If there is any change the ignitor is short and trash.

Ingo

[Joeaksa]

My 3.6 conversion had the same issue. Would crank fine, fuel but no fire.

Turned out that the wire to the fusebox was on the wrong terminal. They moved it over one position and it started right up.

[356graham]

Thanks, Ingo. I performed the test you suggested - measure both sides of the coil with ignition on and modules plugged versus unplugged. That was a good idea. The ignitor is fine, and I've discovered that the +12v supply to the coils is the problem.

Now I'm back to tracing wires to find the culprit.

Appreciate the help!

[356graham]

Thanks all who helped. Found the problem and learned a lesson.

One of the POs had kludged an in-line fuse in the supply to the coils which became corroded and no longer was able to supply sufficient current. Tracing the wires back, I found the improvised fuse situation and it became a really easy fix.

Here's the lesson learned:

In my first post, I stated that there was +12V to the coils - which there was the first time I measured. That was ignition on and no cranking. The corroded fuse connection allowed the voltage to show under a no-load condition.

I should have measured again on the supply side of the coil while cranking. It would have shown that the supply wasn't a solid +12 but rather a compromised connection.

I'm happy again. It's a beautiful day and I drove to work with the Targa top off.

Thanks again for the help and support.

[Joe Bob]

Mine fuch'd this AM....found the culprit. Damn fuse had corrosion on each end. I licked it...it worked. I hate these old freaking fuses.