The craziest problem with 96 Volvo 850 T5 Topic is solved

[misha]

Did they checked TCU for fault codes?
Erikv11 have good point about this.
TCM,unlike ECU and ABS control unit will not clear the fault codes itself when parts are being replaced.
They will remain stored untill read and cleared even if parts is replaced...AND will not proceed further untill codes are read and cleared.
My point is that they should check TCM and see what it have to say about it.
TCM also needs at least VOL-FCR or special Volvo scan tool to communicate with it.
Generic/universal ones will not work with TCM.

[redbeak1]

Did they checked TCU for fault codes?
Erikv11 have good point about this.
TCM,unlike ECU and ABS control unit will not clear the fault codes itself when parts are being replaced.
They will remain stored untill read and cleared even if parts is replaced...AND will not proceed further untill codes are read and cleared.
My point is that they should check TCM and see what it have to say about it.
TCM also needs at least VOL-FCR or special Volvo scan tool to communicate with it.
Generic/universal ones will not work with TCM.

I drove 400 miles since my last P0120 code including what my dealer mechanic put on. Would the TCM let me go this far without throwing the code again? I will print out all the responses and take it to another dealer when I get into trouble again. I will be pulling a motorcycle trailer this week another 350 miles. I never know when it will leave me stranded. But all these great responses give me hope.

[redbeak1]

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 ?

I agree that doing a resistance test can give very elusive results. I just repaired my 17 year old Kenmore dryer that didn't ignite. I measured ohm resistance on the old gas coils and compared them with brand new ones. They were all in range and very close in results. By replacing the coils it solved the ignition issue. If I had followed the OHM results only, I would still be troubleshooting.

Since my Volvo problem is so sporadic, wouldn't I just be better off replacing the 3 wires from the TPS to ECU since no problem was found with the wiring during the resistance test?

[Clemens]

yeah, give that a shot. you could probably just run a temporary fix before hacking up the wiring loom permanently. if you have a chance, get a spare tps connector off the junkyard.

[redbeak1]

I thought I'd update to help benefit others with a similar P0120 issue. After the great suggestions on this forum, I removed the TCU and ECU. I cleaned the contacts with CRC electronic cleaner, moved the units up and down in the sockets. There is no visible corrosion. Since then I put 2,000 miles on car with no CEL and the P0120 code. I thought I fixed the problem, but the code is back with stalling again.
Would using di-electric grease on the TCU and ECU help? I have read many controversial issues regarding the use of it. Also, I think I will swap the TCU as erikv11 suggested. Do I need to reprogram the TCU if I get a unit on ebay with the exact part numbers? Thanks for helping me with this very tricky problem. I won't give up.

[misha]

It's easier to buy VOL-FCR and to read codes from tcm.
Like i said....they won't go by themselves if not read and cleared unlike to ecu or abs unit.

[redbeak1]

Thank you, I'm learning. I don't have the special Volvo scan tool or the FCR. But through process of elimination, I think you are correct. My only question is how come I was able to go another 2,000 miles before the fault came back? If the TCU won't clear the code internally, how could it allow the car to go so far and long without triggering it again? I will look into where I can get the VOL-FCR software. Thanks all the way from Serbia. I am Hungarian, so we are neighbors!!

[RickHaleParker]

Would using di-electric grease on the TCU and ECU help?

NO!

Dielectric grease is non-conductive. If you get it on the electrical connection mating surfaces of the connection it can induce a electrical problem or make a electrical problem worst. Dielectric grease is for protecting a electrical connection from the elements, not improving or cleaning the connection. Dielectric grease serves the same function as Vaseline on a battery terminal. You apply it after the connection is made and clean it off if the connection is ever broken.

You might try cleaning the connectors again. This time use MAF cleaner and brushes. Small Interdental Brushes work good for brushing the female part of a pin connector.

[redbeak1]

It's easier to buy VOL-FCR and to read codes from tcm.
Like i said....they won't go by themselves if not read and cleared unlike to ecu or abs unit.

Misha, you were spot on. I sent my TCM to G7 computers in New York for them to look at it. They said that their is no hardware issue...only software. So basically they did what you've been saying. I also was able to swap another TCM that I found on ebay for just $50.00. The car has been running OK with that.
This has been an interesting journey. Even my Volvo mechanics apparently didn't clear the TCM. When I asked the service manager they just say that they checked everything.Maybe they did, and their is still a wiring issue that triggers the P0120 code. If the saga continues I will update again.

Instead of buying a VOL-FCR is there a handheld scanner that I can buy that will read and clear the TCM???

I can't thank everyone enough on this forum for helping me out with this very troubling problem. I would have never thought it would turn out to be the TCM!!

[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.

You were 100% correct. The code was stored in the TCU. I sent my TCU to G7 computers. (XeModex doesn't work on the 96 Volvo) No hardware problem was found.

Is there a hand held scanner that would read and clear the TCU? I have a generic one that will not clear the TCU.
Thank you for your great advice for this complex problem.