The craziest problem with 96 Volvo 850 T5 Topic is solved

[redbeak1]

For over a year I've been having the most insane dangerous issue with my very well maintained 96 850 T5.
Code P0120 came up so I changed the TPS. Three months later (800 miles), this time the engine died on the highway with same code. I clear the code and I'm able to drive various miles till the saga is repeated. My local old school Volvo dealer and mechanic changed the throttle body and TPS. They also did the OHM resistance test while moving the wires from the TPS to the ECU. That passed and no codes were found. I also swapped back my original ECU. I have one flashed by IPD. 1000 miles later going at 75 mph it's like turning the ignition key off. Code P0120 . This time I was on I 95 just after the GW bridge in New York. No fun dodging Mac trucks!! So after clearing the code about 5 times I slowly made it back home. Now that it's back at the Volvo dealer they can't find anything after driving it over 100 miles. It's not the ECU,Throttle body, TPS and the wires seems find and passes resistance test.

What can cause such a sporadic problem? I don't trust the car at all. Only for short trips around town.m Can anything else trigger P0120 code?? Thank you for any help

[erikv11]

Hopefully they looked carefully at and cleaned the TOS connector and the wiring there. Actually, check every bit of wiring you can get a look at.

Could be a wiring harness problem. Did the dealer check the circuit from the correct ECU pin to the TPS? Still, if it is so sporadic, would be difficult to track that down.

And remember, P0120 can also be stored in the TCU. I'd try swapping that too, at this point.

[RickHaleParker]

What can cause such a sporadic problem? I don't trust the car at all.

Oxide build up on the connectors. Clean the connectors on the TPS and the ECU with electrical contact cleaner or MAF sensor cleaner. Corrosion under the wiring insulation. Inspect the ends of the wires, look for corrosion that may of worked itself under the insulation.

[redbeak1]

Hopefully they looked carefully at and cleaned the TOS connector and the wiring there. Actually, check every bit of wiring you can get a look at.

Could be a wiring harness problem. Did the dealer check the circuit from the correct ECU pin to the TPS? Still, if it is so sporadic, would be difficult to track that down.

And remember, P0120 can also be stored in the TCU. I'd try swapping that too, at this point.

After I changed the TPS the first time the P0120 code appeared, I cleaned the connector and made sure there was no break in the wire. I also used CRC electrical cleaner on the ECU and TCU. The guys at the dealer have been working on Volvo's for over 30 years. They checked Ohms and wiggled the wires to make sure there is continuity between the connector and ECU. I sometimes do get rodents in the car, but there is no sign of chewing on the thick plastic housing. This Volvo is a very clean sample with only 142,000 miles. One pattern I noticed is that it throws this code mostly when I'm on the highway at above 60 mph. I just got the car back from the dealer after it was with them for almost a week. It didn't throw a code in 200 miles, so no problem was found. So this is what has been done so far. New Throttle body and TPS. ECU was swapped the last time and problem persisted. All connections checked and cleaned. Ohm and voltage checked on three wires. I will be driving a lot this weekend, if it persists, then I need to swap TCU as you suggest. Very difficult problem to find since there is so much time and distance between codes. I love this car, but it is life threatening.

[redbeak1]

What can cause such a sporadic problem? I don't trust the car at all.

Oxide build up on the connectors. Clean the connectors on the TPS and the ECU with electrical contact cleaner or MAF sensor cleaner. Corrosion under the wiring insulation. Inspect the ends of the wires, look for corrosion that may of worked itself under the insulation.

All connectors cleaned with CRC electrical cleaner. Seems like a impossible project to replace the only three wires going from TPS connector to ECU, but looks like the next project. Can't think of anything else. But if it cures this disaster of a problem, I would replace the entire harness if needed!!!

[callahanoffroad]

What can cause such a sporadic problem? I don't trust the car at all.

Oxide build up on the connectors. Clean the connectors on the TPS and the ECU with electrical contact cleaner or MAF sensor cleaner. Corrosion under the wiring insulation. Inspect the ends of the wires, look for corrosion that may of worked itself under the insulation.

I'll agree with this.

A good rule of thumb is this:
Mechanical problems are constant.
Electrical problems can be intermittent.

If you're missing a piston your car will run like crap all the time no matter what. But if your and Module is bad your speedometer may work fine 90% of the time and only be broken 10% of the time. In my old 850 I'd just "reboot" the car to fix the dead speedometer.

It sounds to me like you have an electrical problems. I'd start with cleaning the connectors and then use a multimeter to trace the electrical lines. A thorough visual and touch inspection wouldn't hurt either. You could have a bad grounding, bad connector or bad wire in the loom. Remember electricity is like water it flows from point a to point b. If the plumbing pipe is busted it stops flowing entirely instead of leaking though.

I'd find a friend who has a garage that you can borrow to track down the problem for an afternoon.

[abscate]

I think in the 6 years i have been on MVS I have seen a TPS replacement solve a TPS code - zero times.

Likewise, replaceing ECU has worked twice, not counting reseating bad connections.

These cars don't like bad supply voltages. one of our members just solved a 6 month epic ghost CEL light code by replacing the 12 VDC feed from battery to fuse box.

[redbeak1]

Thank you for all the great responses. It seems like you all agree that the issue is in the wiring connections. I spoke with the mechanic who performed the test at the dealer and he did do a resistance test with ohm meter from ECU pin to TPS connector. They also moved the wires during the test. No problem found. With all the other variables removed such as TPS and Throttle body replaced. ECU swapped. The only thing left is to replace the 3 wires from the TPS connector to the ECU. So my question is, will that help solve this problem? I only have the Haynes manual and it doesn't show wiring diagram. Can anyone tell me which ECU pin I should test along with the TPS connector. I just want to make sure to double check dealer mechanics findings before I dig in. After reading about Volvo's with plastic manifolds going up in flames and newer models braking without warning needlessly, I'm even more motivated to keep this car going!!!

[abscate]

See sticky thread above on wiring diagrams.

Resistance measurement is an art of itself and Is hard to do correctly. Have you removed and sprayed the contact staff with cleaner ?

[RickHaleParker]

Can anyone tell me which ECU pin I should test along with the TPS connector.

ECU pins A15, A16 & A18.
A15 TPS Out
A16 TPS IN
A18 Ground

Check Junction 23/8.

The Electrical Wiring Diagram for the '96 850 is document TP3911202.

7/54 is the TPS.