BigAl64

I'm a month into new ownership of a 1990 C4, now 110K miles. Overall it is in great shape, but I'm working through some hopefully minor issues. Car runs extremely well.

Seat belt light/Airbag light stays on, I did the clock repair, and that fixed the spoiler light.

DME had knock sensor codes. Replaced the sensors, and connectors, cleared the codes. No issues now.

I ended up tracing the seatbelt light to a bad Gong relay, replacement is on the way. (dcAuto.com)

1) Airbag light stays on. I understand that the code needs to be cleared. It was cleared by a shop (Eurosport in Arvada, CO), code came back with left front crash sensor. I cleaned the contacts, no avail. I asked the shop to swap the two crash sensors, and see if the code follows, new code (I don't have it with me now) shows up, so something goofy with the controller. I do note that the Airbag controller sends a signal through the Gong Relay to the Clock and the central informer. So it is possible that the Gong Relay replacement may solve this.

2) When I go over a bump, Check Engine light flashes intermittently. No codes are stored.

3) When I lock the differential, Check Engine light flashes intermittently, at I think the same rate as the lock light (I can't look at both at the same time), but looks like the same frequency.

So, I looked at the schematic, to see if any thing is common to the above three problems. Nothing runs through the same connector. But, I did find that the Locking Differential Control Button (Sheet 4, F39 in schematic), the Instrument Cluster D-100 (Sheet 6, C52 in schematic, houses the check engine light), and the ground for the Airbag Controller (ground is at J37) all have the same ground, MP XV.

MP XV is located at position 7bQ. One can see it from the trunk, but to access it you need to remove the left two gauges.

So this morning I removed all the grounds wires at MP XV, cleaned them up, and reseated them. No avail.

The Check Engine light signal goes directly from the DME to the cluster (and the Diagnostic Connector), but I guess I could pull the seat and make sure there is good signal integrity from the connector to the gauge, and that the connector is seated well. Seems like a longshot but simple to do.

I did note that in the records the ABS pump was replaced a year ago (4K miles ago).

Any other ideas?

Thanks,

Alex

VA964

Your descriptions sound very much like mine. I have had my 1990 C4 for 1 1/2 year. Milage now 99k+. had the car serviced recently at an independent Porsche shop where they switched out the gauge containing the CEL(the change was to chase down a PDAS light issue). On way back from shop the CEL flashes whenever I go over a bump or over a patch of rough road. Last few days of driving in some back roads have same symptom. The airbag light also stays lit.
I'm not anywhere as handy as you sound being able to chase down the wiring. This is my 1st Porsche. If you or anyone else do have suggestions I would love to hear them.
Peter

Makmov

well as far as the check engine light goes there isn't a whole lot tied to it. Mainly just the O2 sensor. It was really pretty basic.

It does sound more likely a loose connection somewhere.

A lot of Air Bag light issue have to do with the clock solder connections and people have recommended pulling the clock and resoldering all the connection on both sides of the board. However, if you have already done that...

Another common problem with the seatbelt airbag lights is a broken or loose wire in the seat belt wiring. IIRC there is a Porsche bulletin on this and recommend replacing the seat belt recepticals. However, most report the problem comes back shortly. It seems the wire gets broken under the seat and carpet somewhere.

The gong relay has been another noted problem.

wellcraft290

the front shock sensor fixed my airbag light and will tell you it is an expensive part. Roughly $2,400 luckily found a used one and have a spare as well now and they need one time bolts that are like 30 bucks.

BigAl64

As the original poster, I have worked through some of the issues.

With regard to the Airbag light, it did turn out to be the clock. I resoldered it, cleared the codes (thanks to the OBD1 scantool!), and everything is good.

With regard to the check engine light, I pulled the seat, checked the connector, and everything seems to be good. I checked the part number of the DME, (I don't have it with me), and mine was a 911 part number, not 964, indicated early 1990 build. What I could recall, the early DME's donot support a check engine light, but the harness and the light were present. Based on that, I just pulled the light out of the dash. I'm assuming there is a short somewhere, but I kind of lost interest in finding it once I found out that that function was not in the DME.

The only issue I have now is that when I hit a sharp bump, the PDAS light in the lower dash flashes, which I think is normal of a car with 115K miles and the original suspension.....my suspension is shot, hopefully will remedy that this spring.

Alex

VA964

Alex: thanks for the update. Interesting that 1st of all the replacement gauge my shop put in was from a different model than my original(because the PDAS light is a different figure). When I mentioned the CEL light to the shop, they then told me they had foundthe CEL light in my original gauge removed. His recommendation was to remove the light from the replacement gauge as well. When I have time I'll explore the solutions you found.
Thanks