Electrical problem 940

[Laird Scooby]

Originally Posted by aardvarkash10

Hi guys - I've been lurking so I don't get in the way.

Lights and other load on would lower the voltage across the electrical system. It seems like the ECU is responding to that lower voltage, or more importantly, misreading the no-load voltage.

I'm not well enough versed in the black majic of the ECU itself to identify where and why this is happening (if it is the case). I also don't have the schematic in front of me atm, so I can't trace what may be impacting the ECU externally.

If the fault is occuring on a known good ECU, that indicates the problem is external to the ECU. The cut spaghetti behind hte dash is a warning sign as well.

Any thoughts out there?

I'm still waiting for Ammo to confirm that after trying another ECU from a car that ran fine that the donor car ran fine after Ash - i have some other thoughts, notably the one i've put in as an EDIT on my post above yours.

We've already discounted this theory about the ECU reading the no load voltage by running it with the alternator disabled if you recall?

I'm not going to explain my theory yet as to what's happening but with extra load on the alternator, the ECU will see a voltage drop and increase the AICV opening to cope, meaning more air is getting in but the Lambda feedback is telling it that it's already rich enough so it doesn't enrich the mixture, just adds air.
As it also does it cold before the Lambda is meant to be online, it's using default values during the warm up. That points to my theory again.

Have a read of my EDIT in the post above and you'll see where i'm going.

[Ammorack]

Originally Posted by Laird Scooby

I'm still waiting for Ammo to confirm that after trying another ECU from a car that ran fine that the donor car ran fine after Ash - i have some other thoughts, notably the one i've put in as an EDIT on my post above yours.

We've already discounted this theory about the ECU reading the no load voltage by running it with the alternator disabled if you recall?

I'm not going to explain my theory yet as to what's happening but with extra load on the alternator, the ECU will see a voltage drop and increase the AICV opening to cope, meaning more air is getting in but the Lambda feedback is telling it that it's already rich enough so it doesn't enrich the mixture, just adds air.
As it also does it cold before the Lambda is meant to be online, it's using default values during the warm up. That points to my theory again.

Have a read of my EDIT in the post above and you'll see where i'm going.

It ran fine in donor car. No problem with that car. Nothing wrong. But my Volvo.

[Laird Scooby]

Originally Posted by Ammorack

It ran fine in donor car. No problem with that car. Nothing wrong. But my Volvo.

Was that after you tried it in your car?

Also what about the Orange wire test?

[Ammorack]

Originally Posted by Laird Scooby

Was that after you tried it in your car?

Also what about the Orange wire test?

It ran bad the entire time. So ecu is not the problem. I have not tried to unplug orange wire from tps yet

[Laird Scooby]

Originally Posted by Ammorack

It ran bad the entire time. So ecu is not the problem. I have not tried to unplug orange wire from tps yet

I don't think you understand what i'm asking. When you removed the spare ECU from your car and refitted it in the donor car, did the donor car run ok?

[Laird Scooby]

Originally Posted by Ammorack

It ran bad the entire time. So ecu is not the problem.

We don't yet KNOW this!

Originally Posted by Laird Scooby

The 946 (fuel ECU) has a reputation for doing what your car is doing. It also has a reputation for being fine in one car, you remove it and the fault happens, even before you fit it back in that car or any other.

It can happen!

https://www.volvoclub.org.uk/faq/Eng...alling_Bad_ECU

https://www.volvoclub.org.uk/faq/Eng...CUFailureModes

I can't remember where i found the information on the -946 ECU but i remember seeing it somewhere.

We had a very similar problem with a similar aged 940 with a B230FD some months back - longest thread in the world! That turned out to be the ECU and the people that repaired it said that what had failed was very common on them. They wouldn't say what though.

In between whiles, the owner of that had bought a secondhand ECU that also failed but had allegedly been running fine in the donor car.

What i'm saying is we can't rule the ECU out yet.

Once you've tried the orange wire test, either refit the orange wire or make a note of exactly where it came from on the TPS. Then remove the red/white wire from the TPS and see if the idle quality improves.

Refit both in their correct place after.

[Ammorack]

Originally Posted by Laird Scooby

I don't think you understand what i'm asking. When you removed the spare ECU from your car and refitted it in the donor car, did the donor car run ok?

Aha. I don’t know that.no idea if that donor car even ran or not

[Ammorack]

Originally Posted by Laird Scooby

We don't yet KNOW this!





It can happen!

https://www.volvoclub.org.uk/faq/Eng...alling_Bad_ECU

https://www.volvoclub.org.uk/faq/Eng...CUFailureModes

I can't remember where i found the information on the -946 ECU but i remember seeing it somewhere.

We had a very similar problem with a similar aged 940 with a B230FD some months back - longest thread in the world! That turned out to be the ECU and the people that repaired it said that what had failed was very common on them. They wouldn't say what though.

In between whiles, the owner of that had bought a secondhand ECU that also failed but had allegedly been running fine in the donor car.

What i'm saying is we can't rule the ECU out yet.

Once you've tried the orange wire test, either refit the orange wire or make a note of exactly where it came from on the TPS. Then remove the red/white wire from the TPS and see if the idle quality improves.

Refit both in their correct place after.

How can I do some measurements on the ecu to find out if it’s bad or not?

[Laird Scooby]

Originally Posted by Ammorack

Aha. I don’t know that.no idea if that donor car even ran or not

In that case, you have no idea if the ECU had gone on it already before you tried it - it could well have the same fault and from everything i can find, it tends to be age related.

Originally Posted by Ammorack

How can I do some measurements on the ecu to find out if it’s bad or not?

Have you done the tests with the Orange and the Red/White wires on the TPS?

Also have you got a code reader box on the inner wing near the suspension turret?

[Ammorack]

Originally Posted by Laird Scooby

In that case, you have no idea if the ECU had gone on it already before you tried it - it could well have the same fault and from everything i can find, it tends to be age related.





Have you done the tests with the Orange and the Red/White wires on the TPS?

Also have you got a code reader box on the inner wing near the suspension turret?

Yes I have that OBD1 I guess it’s called. I have faultcodes for missing or absent o2 sensor. It’s a new o2 sensor from Bosch. 120$

I haven’t tried the tps test yet. I have some errands to do now :/