I have been getting an intermittent coolant temp sensor check engine light for the last year and a half or so. I've changed the sensor, first with an aftermarket unit, and just last week with a sensor bought from my local volvo dealer. Still I get the light.

I bought a known good 951 computer to check to see if my computer was dying and causing the problem, but still the light remains.

So this leads me to believe there is something in the wiring that is causing the signal from the sensor for the ecu to fail intermittently. So there a ground associated with the sensor in the wiring that could be corroded and causing it to not ground sufficiently sometimes? Do the two grounds on the intake manifold tie into this system? The connections at the ECU and at the plug for the sensor both look good so I'm kind of at a loss as to why the thing isn't happy. I'd really appreciate some assistance if anyone knows of a cause/solution.

Car is a 1990 240 automatic with LH2.4

"So there a ground associated with the sensor in the wiring that could be corroded and causing it to not ground sufficiently sometimes?"

No, your Coolant Temp Sensor does not use a discrete ground wire. Grounding is accomplished by the sensor body's contact with the tapped hole in the head.

If you have mistakenly used teflon tape or other thread sealant, I believe it could affect the grounding. There should be a copper sealing washer under the sensor head.

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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

This is an issue from when the original wiring harness was assembled.

There are several wires that come up between the second and third runners for the intake manifold, I can't remember exactly which harness it is, but in one of these is a ground wire that is pressed together, and this is a cold joint that is causing your issue.

So, locate the one for the ground wire, and if your black plastic sheathing is still in place, just feel down the plastic for a bulge, and this is where the cold joint is located. Then, cut the plastic about two inches to the left of the bulge, then you can slide the sheath off, after you disconnect the ground wire.

You will need to get this pressed connection real hot and resolder it. I used a small butane torch when I did mine about 15 years ago, and I have not gotten a coolant temperature sending unit fault code since.




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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 292K looking forward to 300K badge (or sticker??). >>You haven't really worked on a car until you draw blood<< :-}

i know this is an old thread. i cant find the ground wire causing the issue. i cut the black sheathing off and i dont see a pressed conection. please help. still have 1-2-3 code and effecting the starting of the vehicle

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Art Benstein near Baltimore

MapQuest really needs to start their directions on #5. Pretty sure I know how to get out of my neighborhood.

Good work!

Hmmm. You're not alone.

Check Engine Light

I don't have the answer for you from my experience. Since you're at the end of the line for parts-swapping and visual inspection troubleshooting methods, you may need some help tracing the trouble by probing the voltage the ECU sees and finds out-of-range. It is the same course of action I recommend to newmex999.

If this happened to me, I would rig a multimeter to the harness wiring at the ECU pin 13 such that I could observe the voltage on that temp sensor circuit until the OBD code finally gets set. How it behaves would steer me to the next step which would logically and rationally explain its behavior. I would then post my findings on the Brickboard for the benefit of others who might run into this.

The ECU grounds are indeed those often discussed, made under the bolts which fasten the fuel rail to the intake manifold. The sensor signal is derived from a 5V source, current limited by a 2.7K ohm resistor in the ECU.


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Art Benstein near Baltimore

You can say any foolish thing to a dog, and the dog will give you a look that says, 'Wow, you're right! I never would've thought of that!' - Dave Barry