Gareth H

Advice from the knowledgable please.
I have a 95 manual C2, bought 6 months ago having been in dry storage for 10 years prior to my purchase.
I had done a few hundred miles when it cut out while driving, the symptoms are no spark and pulsing of the DME relay, which also causes the fuel pump to pulse (this has been changed for 2 new relays, and a good feed onto the pump which then works normally)
It has been recovered to a garage who look after my cars, they have checked the obvious and the ECU has been away to be checked and found to be good.
The garage is working through the wiring which is a bit labour intensive.
I wondered whether anybody had experienced similar that may short cut the fault finding?
Thanks
S

orangecurry

I'm thinking you have a ROW specification 993, so you should make that clear on here if that is the case - there's quite a few differences in this area between US and ROW

71-3.0-911

How many miles? From your write up, you changed 2 relays? Which ones? Have you replaced the fuel pump?

You describe cutting out while driving. That is usually indicative of a bad DME relay. Wiring doesn't usually go bad while driving. Once you sort the DME relay and fuel pump, report back.

pp000830

Hi Gareth,
Verify the system voltage the state of charge of the battery and the condition of the leads to the battery.
If you jump the DME relay socket to bypass it, does the fuel pump run normally? This may point to or rule out the fuel pump as the issue.
Another alternative is to pop a known good DME relay open, plug it in and force both of the internal relays closed with a small insulated wedge and see what happens. Does the fuel pump run normally, does the engine start and run?
Andy

jay@EZimmoblock.com

If the ECU and DME relay are confirmed good, and the DME is still pulsing (unclear if the pulsing stopped when you swapped in the new relay) what you're seeing is consistent with loss, or out-of-tolerance crank sensor signal. It's a safety feature - the ECU de-energizes the DME relay if the engine stops rotating. This might help troubleshooting the crank signal.

Gareth H

Thanks guys, (as the above advice from Jay (thanks)) I’ve been pointed in the direction of a shorting crank sensor which causes the relay to pulse, the sensor had been checked, but is now being checked again, not sure if this is also causing the spark issue, but we need to work through this before we’ll know)

jay@EZimmoblock.com

I'm not sure, but it would make sense, that the ECU, in addition to turning off power to the fuel pump on loss of tach signal, Tn, would also stop driving the ignition coils, so could explain both symptoms. Just do a quick check of the Tn signal on the OBD port as noted in my previous link. It's unclear whether your symptoms are intermittent, but if they are, you'll obviously need to look at the signal while the symptoms are present. A scope would be best in this case as the signal may be "present" but dropping out or out of spec in some other way. Keep us posted.

systemsc

Yes, you need to focus on the crank sensor, a DME ECM input, and NOT the Tach signal, which is a DME ECM output. Ideally, one should use an oscilloscope, but a VOM is adequate.
The DME ECM pins are 48 & 49. The resistance should be about 1K ohms, with an AC voltage of about 1-2 volts. The AC voltage is measured while cranking. It's a simple 5 minute test.
If the crank sensor tests OK, then the next step is to check the 12V power and ground pins to the DME ECM.

jay@EZimmoblock.com

Not as a first step.
Removing the seat, grinding off the security bracket to get to ECU pins 48 and 49, as suggested, is not only a lot of work and risky, it doesn't tell you whether the ECU is able to process the sensor signal. A valid tach signal at the OBD port, on the other hand, tests tells you a lot more, with a lot less effort: a) the sensor is good b) it's mounted/gapped properly c) its wiring and connection to the ECU are good and d) the voltage, frequency and timing of the sensor signal meet the requirements the ECU need to energize the DME relay. The tach signal IS an output, but that's a good thing: it enables you to easily test the entire crank sensor signal chain without disassembling a thing.

OP: Start at the OBD port. If the signal is there, you ruled out the crank sensor. If not, THEN work back towards the sensor. If it gets that far, the problem is most likely at the sensor, in which case a replacement may be all you need. Don't remove the seat or mess with the ECU unless absolutely necessary - you could end up with a $5K repair bill.

"Do the easy things first".

systemsc

Should have indicated that the crank sensor can be easily tested by just disconnecting it in the engine compartment, before it gets to the DME ECM.
This approach allows one to properly evaluate the actual signal. As you can see, the crank sensor has a connector before the DME ECM;
https://www.fvd.net/us-en/9966061059...on-sensor.html

Avoid Mickey Mouse troubleshooting approaches!

jay@EZimmoblock.com

You missed the point, Mickey- he already tested the sensor (Post 6).

systemsc

So maybe a questionable testing procedure? Who's doing the test and how was it done? Yes, it will cause "the spark issue"!
Again, avoid using DIY unreliable diagnostic procedures learned on YouTube and elsewhere.

ninjabones

If you’ve ruled out DME relay/fuel pump pathway issues (Bentley manual 20-11 to 20-17) and there’s no spark, and you’ve gone through the basic steps of ignition system troubleshooting (Bentley 28-5 to 28-9), the last step in a’95 is to check/replace the ignition control module (Bosch 0 227 100 200) under the driver seat ($50 part). Just another idea.

Gareth H

Just an update on this, I was directed to the crank sensor by a couple of people who have experienced these symptoms, and despite an oscilloscope showing a good signal, bizarrely this continued after the engine stopped cranking! That is exactly what it was, 5 hour train journey to collect tomorrow, it’s like new car day all over again

71-3.0-911

Thank you for the follow up. Interesting result.