I hit a bump in the road yesterday on the way to the grocery store and the engine cut out and would not start again.
It's been running well up until that point.
- Battery connections are good ... light, radio, etc, working
- No visible issues under the hood ...
- Daughter board on LH computer retrofitted years ago
It's parked at the store and I plan to do some troubleshooting before towing it.
I will see if I can swap some of the critical relays with on of the non-critical ones.
I can't think of much else try ... wondering if anyone else has any thoughts?

Brian
1988 928S4 5-speed

 

Have you looked at your alternator? I had that happen on a different car; there was not direct contact with the alternator in that case (this was on the 996, so it would be pretty tough given where the alternator sits on that motor) but going over a little dip that put the car in fast, full compression at a reasonable clip was enough to make things bad. The 928's alternator is much more exposed and vulnerable.

Good luck

 

CPS - check the connection from the CPS to the harness.

BTDT.

 

Good suggestion, I'll look at that. The area I was driving was under road construction .. so it's possible.

 

I had my 928 not start after I parked it once. Lights and radio worked fine. It ended up being the ground strap from the battery to the frame had hidden corrosion inside the cable connector and was not strong enough to start the car. You might try running a heavy ground wire to test that out.

 

does the battery have a hold down clamp?
if not then the battery positive terminal can hit the bottom the battery lid ,
this results in instant shutoff, hopefully you didnt damage the computers

 

I had an issue with my 89 where everything worked fine, but the car would not start.

Turned out to be one of the small wires attached to the positive terminal on the battery.

So even if the main terminal is tight, check the others just to be sure.

 

Agree with MrMerlin (Stan) and forgotten one. Bad/ loose battery connection, engine ground strap, or 12v positive where it spade bolts to starter. Twice I’ve experienced bump-in-road condition and it was one of those.

 

Interesting - mine did the same when I went a bit fast over a sleeping policeman. Started again a few minutes later.
Will have to go and check my battery mounting and connectors now

 

For the engine to start several things have to happen in sequence viz:

1. The ignition switch has to energise bus 15 in the central electrics when switched on.
2.The EZK relay [XXII] has to energise to power up the EZK computer- terminal 86 in the relay socket has to show 12 volts
3. The engine has to crank when the key is turned full
3. The CPS has to generate a viable signal as the engine cranks- the unit can fail- common failure is age related disintegration of the male connector
4. The LH relay [XXV] has to energise after the CPS impulse signal fires
5. The fuel pump has to be turned on by the LH unit - the fuel pump relay [XXVI] thus has to energise

Your analysis needs to consider the above steps are viable. Relay operation can be simulated with a jumper wire with a spade connecter at each end- good to keep such in the car at all times. Pulling the relays can be awkward - you really need the appropriate tool for this. Each relay has a live connection at terminal 30 so I disconnect the earth strap before pulling a relay or remove the fuse for the relay being pulled.

If the battery positive post has earthed on the lid you should see a scorch mark on the under side of the lid assuming the battery has not welded itself to the lid in which case nothing will work.

 

Check the underside of the battery box lid and look for a burn mark where the battery jumped up and hit it My 81 did this right after I bought it when going over a railroad track before I realized it didn't have a battery hold down.

 

I did what diagnosis I could in the parking lot. Battery connectors, ground strap, alternator connection, cps connector, hall connector, etc.
I could not pull out any relays ... ordered pliers; will have them tomorrow.
I need to tow the car home where I can do proper diagnostics.
There have been few to suggest the CPS ... is there a procedure to test the CPS?
I understand the hall sensor can be an issue also ... can it be tested?

Thanks again for any help provided.

 

Will it turn over but not fire or will the starter not engage to spin the motor? If it turns over look at the tach and see if the needle is registering the rotation of the engine. If it just sits dead at the bottom, that's CPS. If it jumps up and down slightly then the CPS is seeing the rotation.

 

The Hall sensor is not your issue. The best way to test the CPS is with a scope.

You might find this thread interesting:

https://rennlist.com/forums/928-foru...signments.html

 

A quick CPS test is...the tach isnt bouncing when you crank it...