940 no spark. Power stage issues? Or something else?

[Ltom]

Hi All,

I've got a '98 940 LPT auto with Bosch ignition system.

Breakdown
Complete and sudden loss of power driving in town. Car would not restart. All dash lights came on, engine stopped, had to put it in neutral and coast to a safe stop.

Symptoms leading up to breakdown

• Occasionally it would not start when hot. Engine would crank just fine but not catch. Always started immediately the second time.
• Occasional and short-lived sputtering or intermittent loss of power, usually soon after starting. Lasted for about 5 seconds each time, then performance was normal.

Diagnostics

• Timing belt is fine, cam is turning on crank.
• No spark at plugs. Confirmed by failure to catch with ether (start-pilot) and removing one HT lead.
• Assuming functional CPS. Rev counter registers the cranking and using a multimeter at the CPS connector I can see an AC voltage when cranking, also there is continuity.
• Radio suppressor OK. I removed and dismantled this part, all solder joints look okay, it looks new inside.
• No fuses have blown, fuel pump relay functions.
• Coil resistance is as expected both low and high side.

So that brought me to the power stage:

As I understand it the power stage cable has five pins:
1. Red/white cable to primary winding on coil
2. GND
3. Shield for pin 5(GND)
4. 12V supply (ignition on)
5. Signal input from ignition control unit

The coil has two (primary) wires, blue and red/white. I understand that the blue is the 12V supply, the red/white is selectively grounded by the power stage to induce a change in current in the coil.

If I test the voltage between pin5 and pin2, when I crank I can see a variable voltage (I don't have an oscilloscope) so I assume the ignition module is sending pulses and the CPS is okay.

If I unplug the red/white connector at the coil, it has continuity to ground all the time, resistance is about 2 kOhms.

My questions are:

Is there a way to test if my problems are caused by the power stage?

What else might be causing my issues?

If you got this far thanks for your time.

[RollingThunder]

Have you checked that the coil king lead is firmly connected at both ends and undamaged. Have you removed the distributor cap and checked the rotor arm and cap interior for damage?

Do you know anyone local to you with another 940 that could help - a temporary swap of crank position sensor, coil, power stage, etc could help you positively identify the culprit.

[Forrest]

Have you checked the fuel relay behind the ash tray? I believe this module contains two relays, one of which powers the fuel pump and the other of which goes to the radio suppression relay which nominally powers the injectors. However, one or other of those black RSR relays under the bonnet usually provides power to the coil. You will need the wiring diagram for your exact year/model to confirm this.

There’s a lot of info in the FAQ about no start conditions but again some of it is model specific.

[lynns hubby]

As per rolling thunders post. Remove dizzy cap and check rotor arm.
I had one a few years back that just died and no spark. Turned out to be the rotor arm had burnt through. New arm and happy days.

[Ltom]

Thank you for your replies so far.

• King lead seems fine and connections are okay.
• The double relay on the fuseboard is fine, that was the first thing I checked.

I haven't checked the distributor, tomorrow I'll take the cap off and have a look, it's dark now.

[irishcarfan]

I see you've checked it, but I couldn't get our 98 Celebration to start last summer '22 and it turned out to be the CPS. The car had been off the road for about ten years and the fuel pump was also gone.

Sure you've done it but don't forget to check your earths.

I was getting ready to inspect the ECU in the driver's footwell had it not been the CPS. I can't remember the exact order of it now, does the Radio Suppression relay send a signal to the ECU, which grounds the injectors?

[Ltom]

Right, I installed new power stage, new CPS (old one had damage), new leads, and new king lead. Same problem.

Now I'm stumped, I had convinced myself it must be the power stage.

EDIT

I get a spark from the coil but not at the plugs which I suppose implies the distributor.

[Ltom]

I took the distributor cap off and found the remnants of a fiber washer, and lots of black marks on the contacts and rotor. I cleaned it up with brake cleaner and emery paper and replaced. I then used a spare king lead to check the spark to each plug which looks good.

So now I have a spark at each cylinder but still no start.

The plugs were wet with fuel so I thought perhaps the engine was flooded.

I removed all the (wet) plugs and let them dry out.

After re-installing them it almost started, I heard a cylinder or two catch, but then it was back to square one. I'll try again but now I'm suspecting a fuel issue.

Any ideas? Is it possible the fuel mix is too rich?

[Rversteeg]

I get a spark from the coil but not at the plugs which I suppose implies the distributor.

In that case it can hardly be anything else than the distributor cap or the rotor. Just replace them, they are wear items anyway, especially the cap does not live forever. I did not think mine was too old, when a few weeks ago a stranded with a hardly visible crack in the dizzy cap. The engine only ran quite badly on 2 or 3 cylinders. Taking the HT lead of the non-functioning 4th cylinder off actually improved the 3 cylinder running. I guess with the lead on, the spark of cylinder no.3 got partly diverted to no. 4.

[Ltom]

Well I would agree, except for now I get a spark at each plug but it still doesn't start.