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Story Leavesley 27 Aug 2002 22:19

GS Ignition Problems
Hi gang,

I'm having problems with my 95 PD airhead that some of you experts might be able to help with. The bike has 28k miles and is in good shape. I have modified the bike with new plug wires/caps, Thunderchild diode board with solid mounts, and a high output voltage regulator.

When the bike gets warm, the ignition will cough and skip as if it's running out of gas. One time, the bike stalled and would not restart. There was plenty of fuel in the float bowls, but when I checked the plugs there was no spark. After about 20 minutes the spark was back and the bike ran well.

The consensun on the GS list was that it was a coil problem. However, I replaced the coil with a Dynacoil and there were no changes in the symptoms.

The other possibilities for this problem seem to be the ECU and the Hall trigger unit. Has anyone had similar problems, and what was the cause? I am planning on riding this bike RTW so I need to straighten out this problem.

Thanks for your advice.


John Ferris 28 Aug 2002 00:35

Maybe the ignition control unit. It is under the tank. As far as I have read it can not be fixed, just replaced. It is heat sensitive. It may not be broken just over heating. It uses a heat transfer compound in between it and the heat sink. That compound should be cleansd off every few years and replaced. Clean it off with electrical contact cleaner, both the unit and the heat sink. spray the cleaner on a rag and wipe both off. I used "silicone-base, Heat Sink Compound" from Radio Shack - part # 276-1372
When putting the unit back on the heat sink tighten both screws evenly and carefully. The tabs the screws sit on can be broken off.
That is the only thing I can think of that you didn't say you looked at.
Good luck, John

Story Leavesley 28 Aug 2002 01:21

I have cleaned and reapplied heat sink compound to the back of the ECU (electronic control unit). It was pretty grungy the first time but I keep it clean now.

The next thing I'm going to try is to replace the ECU. Just thought I'd see if anyone else has seen these symptoms before I start swapping expensive parts.


Grant Johnson 28 Aug 2002 01:29

Those symptoms are USUALLY coil or icu. Sounds definitely heat-related. Since you've replaced the coil, I'd go for the ICU.

Note that there are TWO principle causes of ICU failure

- heat, caused by failure of the heat-sink compound, and

-cranking the engine over with the spark plug not grounded. The spark MUST be grounded at ALL times.

Grant Johnson

Seek, and ye shall find.


One world, Two wheels.

Story Leavesley 28 Aug 2002 01:55

Good point Grant. I know not to crank the motor without the cap and spark plugs grounded to the engine. However, I bought the bike used and the symptoms were there when I bought it so I'm not sure of its previous history.

Do you what happens if the engine is cranked with the caps/plugs grounded? Does it completely fry the ECU or can you get intermittent ignition problems like I'm seeing?

I guess my next step it to get a new ECU. Does anyone have any experience with the ECU from Motorrad Elektrik? And does anyone know if an R80G/S ECU is compatible with an R100? Can I swap my G/S ECU with my R100 to try to diagnose the problem?


Grant Johnson 28 Aug 2002 02:06

I assume you mean with the plugs NOT grounded?

It is unfortunately not instant and complete failure. It is however guaranteed ultimate intermittent failure, often just as you describe.

Motorrad Elektrik's unit will be fine, Rick makes good stuff, definitely recommended.

Catch on all replacement icu's for R80G/S is that the original one is no longer available. It was designed slightly differently than the later ones.

Apparently the later (non-kickstart) ones turn off the ignition after the key has been on for a short time in order to prevent frying of the system. This means that if you aren't quick when KICK-STARTING there won't be any spark when you actually kick it. So you have to get ready to kick, turn on the ignition and kick relatively quickly.

I have NOT seen any evidence of this behaviour myself, but haven't seen a late ICU fitted to a kickstart model and tried it, so can't comment either way, but that's the theory. If you don't have a kickstarter it's irrelevant, you have the later version anyway.

ALL R airhead ICU's are now identical and ALL are interchangeable. Apparently there is a Volkswagon one that is also the same. It's just a Bosch ICU.

Grant Johnson

Seek, and ye shall find.


One world, Two wheels.

Story Leavesley 28 Aug 2002 03:50

Thanks Grant.

I just talked to Rick @ Motorrad. As Grant says, he knows his stuff, and here's what he told me about ECUs.

The old style ECU is no longer available. There is a new style ECU that is compatible with old and new bikes. However, you should not use the old style ECU with the new style (post '92) coil. Apparently, the new style coil has more resistance and that will stress the old style ECU. You can use the brown dyna coil with either the old or new style ECU. Rick only carries the old style ECU - you can get the new style from your dealer.

So where does that leave me? Let's see - I have an old style ECU that I carried as a spare for my G/S in South America. I can swap that into my PD since I'm also running the dyna coil (compatible with both old and new). I left the stock coil on the bike so I would have some redundancy if I have problems with the dyna coil. However, I cannot drive the old style ECU with the stock coil. Therefore, if I want redundancy with the coil, I need to get a new style ECU which I can use with both the dyna and stock coil.

I hope that's clear to everyone, 'cause I'm not sure I've got it clear yet!


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