dcbailey

I completed my first valve adjustment this past Sunday, replacing distributor caps, rotors, plugs, wires, 02 sensor, and filters along the way. After letting the fuel filter fill up, she started right up and purred like a kitten. I took her out for a spin and found performance to be reasonably good. As I had been off the road for a while, I took her in Monday to get a safety inspection done; the technician tried attaching his monitoring probe to a couple of the upper plug wires, then called me over to ask about the distributor, as he could get no signal from them. I suggested he try the lower wires, and that worked fine. Of course this made me wonder what was going on, and knowing that the car could run on 1 distributor, I did some checks on returning home (it passed the safety inspection with flying colors).

At home, I pulled the center coil wire from each distributor in turn and found that the front coil would not start the car when linked to either distributor. The rear coil would start the car on both distributors (moving the coil wire from distributor to distributor).

I reversed the wires from the coil to verify that it wasn't a wire problem, and again the rear coil fired up on both distributors in turn.

I replaced the front coil, but have the same problem. Tonight I intend to swap the coils and ignition connectors to see if it is truly a bad coil or not.

Two questions: am I doing any damage to any components or the engine by running it using one working coil (I'm not getting any pinging that I can detect, although that doesn't rule it out), and after switching coils/ignition connectors, if the coil is good, but the connector isn't, what would be the next step?

My understanding is that the black wire (connected to the positive terminal on the coil) is 12v power and that the green wire (to the negative terminal) goes to the ECU via the wiring harness and DME.

I know little about troubleshooting electrics/electronics, although I do have a good digital ohm meter.

Regards,
Clay

sml

bad move. suggest you pull the electrical plug at the bottom of each ignition control module.

Bob Rouleau

YES! Damage. Could be a known problem with the distributor on the 964. Fix it fast. Been a while but if i remember, there are rubber belts in the distributor. One goes bad. Causes serious problems if not fixed.

Ducks964

Did you pull the distributer caps off and see if one of the rotors move freely? This would be a broken distributor belt.

jimq

you dont want it to be stopped on one plug and keeps firing! Like said before pull caps and see if you can spin one its self.

Shamus964

This doesn't appear to be a distributor belt issue unless I'm missing something (which is entirely possible). The best way to check the distributor belt is to see if a rotor is freewheeling or not.

This sounds like you've got a dead coil or connection. DO NOT disconnect the high voltage wires on the distributors - it's like grabbing a lightning bolt if you get it wrong. Rather, unclip the low voltage connections at the coils (rectangular). You should be able to start and run on either coil. If you can't, then it's between the coil and the distributor methinks.

I had the same situation on my car and just disconnecting and reconnecting the low voltage clips resolved the issue, which must have been a small amount of corrosion.

dcbailey

No, the distributor belt is complete and functional, both rotors moving together. I did sever the clip on the replaced coil, but it still didn't work. Assuming the belt is still good, will running on one set of plugs (lower, I think) cause any engine problem? My main concern was pinging, but it takes off very quickly as it is, and I don't hear anything.

Shamus964

Have you swapped the low voltage leads at the coil to see if the front coil works while attached to the other lead? My guess is you've got corrosion issues at that point or perhaps a dead ignitor.

I don't think anything will be damaged but it will lack the power and clean emissions of the system operating as it should. LONG term, maybe the cat would be damaged? Hard to say.

rarebear

can you measure resistance on the various leads ? I reckon there is a broken wire somewhere....

elbeee964

From what you describe, I'd posit you'd suffer higher likelihood of pinging...
The management system would then attempt to retard ignition timing to compensate as best it could. Yes?
So, possible sluggish/retarded running till you get that coil and/or lead changed out.
Using the highest octane you can find would minimize the problem until you fix that coil.
(Frankly, I'd park it till then.)

This is one of those times when (I'd say) preventive maintenance is called for: change out both coils - and their leads, to be sure.

Have the spark plug wires been changed out? Distributor belt? Them, too, as they're getting on 20-freekin' years old.
(Egad! Time to throw $700-1,000 dollars in ignition parts(!) at your car?)

Good luck, Mr. Phelps.

dcbailey

I checked continuity on the coil ignition cables to the distributor, but will see if I can measure the resistance. The spark plug wires and distributor caps/rotors were changed out during the valve adjustment. I was able to run the car using either distributor from the known good coil, using either coil cable, so I think they're probably fine. Will be checking the igniters, base on your input. How unlikely would it be for the ECU to only work with one coil?

Thanks,
Clay

rarebear

also check earthing on several ignition parts.....

dcbailey

It turned out to be the ignitor for that coil. I wasn't sure what it did, so looked it up. Here's Danno's explanation from the 944 forum:
"It's an amplifier to switch a high-current load using a small-current drive signal. Basic darlington transistor circuit. The DME sends out a low-power signal (200-500ma) that switches on & off the transistor in the ignition-amplifier (also known as ignitor). The transitor then switches on & off a high-current (10-15amps) signal to charge and dump the ignition coil.

Another reason to relocate the ignition box is that the high-current and high-frequency also tends to cause interference in the other low-power circuits in the DME box.

With the newer cars the entire ignitor & coil assembly is situated directly over the individual plugs themselves to keep the length of the highest interference wires (the high-voltage spark-plug wires themselves) to a minimum. "

It's described as the Ignition Control Module on Pelican, and is also referred to as the ignition main stage. Here's the parts number for any future reference: 928 602 706 01.

Thanks everyone for your assistance.

Regards,
Clay

falcor

Thank you for postion up the solution to your problem. Makes it much easier to diagnose the same problem for others in the future.