M104 280 (320also?) 1994 Cannot read HFM codes from Pole 8 of 16 Pole diagnostic socket

[ray_hennig]

Hello All,

This concerns a new-to-us 1994 280. I'm trying to resolve a number of issues related to very strange throttle response.

I tried reading codes from the diagnostic socket, using my blink LED device. It worked fine on Pole 6 (Airbag), 1 blink, no errors.

On Pole 8, nothing. Not even 1 blink.

I've done lots of diagnostics on my 1991 300-24 and my tester has saved the day many times.

But on this later car I'm baffled.

Any ideas, anyone?

Upper harness replaced by MB with late Delphi harness.

Throttle body state unknown.

As for the engine ECU, don't know. Does lack of reponse indicate a problem with the ECU.

As said, baffled at the moment.

Best to all.

RayH

 

[ray_hennig]

Some other thoughts:

I'm not sure how this car would perform without the ECU. It starts very promtly, hot or cold. Cold idle is smooth at abour 1K RPM. Things go wrong as it warms up.

If I start from cold and leave it running it will idle when but, when it warms to around 70C, it starts to misfire and possibly stall. Then it will start right up!

I tried a new, cheap OVP relay and couldn't read codes. Didn't try leaving it to warm and idle.

So, how is '94 technology supposed to work if the ECU's not getting power?

I'm pretty clued up with my '91 300-24 with separate ignition and fuel and that, I'm pleased to say, is a delight. Wish I could say the same for the '94!

RayH

 

[kth286]

critical for this engine in my experience is the condition of the plug extenders buried down in hot conditions.
after plug leads have been pulled off a few times, the wire inside the rubber extenders can start to distort and it
can go open circuit when engine gets hot again.
I would be renewing the 6 plug extenders.

 

[ray_hennig]

critical for this engine in my experience is the condition of the plug extenders buried down in hot conditions.
after plug leads have been pulled off a few times, the wire inside the rubber extenders can start to distort and it
can go open circuit when engine gets hot again.
I would be renewing the 6 plug extenders.

Great, thank you, I have logged that.

I final thing I note is that the throttle butterfly does not open fully when I manually pull up the linkage by the engine. It closes fully and we cleaned it but it only opens to about 60%. Is this right???

Thanks again.

RayH

 

[carat 3.6]

I cant explain why you wouldn't get any response from pin 8, but see what you get on 14. There are 3 main issues with the engine you have, you've already crossed one out in the engine loom being replaced. The others are the throttle body wiring harness, and the ignition coils.

The throttle body wont fully open using the cable if your car has cruise control or ASR, this is managed electronically by it's own ecu. The throttle body harness is known to degrade so you need to carefully slit the protective sheath and examine the wires within, if you see cracking or bare copper it needs a rebuild. This is probably what your issue is going to be.

If the ignition coils are bosch with a large blue label throw them away, that type are know for causing trouble. If the HT leads leads are stiff, or the plug extenders under each coil look old, replace them.

 

[ray_hennig]

I cant explain why you wouldn't get any response from pin 8, but see what you get on 14. There are 3 main issues with the engine you have, you've already crossed one out in the engine loom being replaced. The others are the throttle body wiring harness, and the ignition coils.

The throttle body wont fully open using the cable if your car has cruise control or ASR, this is managed electronically by it's own ecu. The throttle body harness is known to degrade so you need to carefully slit the protective sheath and examine the wires within, if you see cracking or bare copper it needs a rebuild. This is probably what your issue is going to be.

If the ignition coils are bosch with a large blue label throw them away, that type are know for causing trouble. If the HT leads leads are stiff, or the plug extenders under each coil look old, replace them.

Thank you for that. I'll address your comments indicidually:

Pin 14: Not populated. I only have connections to 1, 6, 8, 10 and 16. 1 & 16 are earth and 12v. I get a single flash (no faults) on 6 and no response at all on 8 and 10.

Throttle body harness: Yes, we'll check that out when I can access it.

Ignition coils: Again, we'll get to those in the next session.

The car has no cruise control or ASR so I expect the throttle to open 100% but it doesn't.

Lots to do and all your suggestions logged.

Thanks again.

RayH

 

[carat 3.6]

The car has no cruise control or ASR so I expect the throttle to open 100% but it doesn't.

If it has none of that, the plug for it should be an oval 8 pin plug sitting just under the edge of the inlet manifold. If it's a round 12 pin plug it's an electronic version fitted.