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Coolant Temp Gauge Dancing


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98 V70r. On startup, sometimes coolant temp gauge shoots up and down in ways that indicate physically impossible temperature changes. At same time, orange lights on climate control flash for about 30 seconds. Needle eventually settles down at normal spot after a couple minutes. Most other tkmes needle and ACC work fine Where do I start 'diagnostics' instead of my usual 'random parts replacement'?

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do you have a CEL? this sounds like your ECT sensor.

the CC lights are telling you theres a fault, try gently spraying air into the opening which is in between the right portion of dash switches and the CC

most of the time the lights point to the blower motor and or resistor but can be a number of damper motors among other things

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The climate control is probably also mad at the ECT sensor - it uses it to determine water temp. for heat too. I'm surprised if your car is doing that it hasn't thrown a CEL light yet.

I've never had a CEL for a bad ECT. I've replaced it on 3 cars due to fluctuating temp gauge as well.

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i'd change the ECT fairly soon - if the needle is dancing, that means it's also reporting erratic readings to the ECU and in extreme cases it will cause the car to not start if the ECT dies completely.

now the obligatory VS suggestion when changing the ECT: "it would be a good time to replace the thermostat & thermostat gasket as well"

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http://volvospeed.com/volvo_repairs_how_tos/engine_repairs/ect_sensor_coolant_replacement.html - Just in case :P :lol:

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My 1993 850 GLT had the same symptomes a while ago. First the needle began to jump up and dowon and the lights on the AC began to blink, after a day or two the Lambda light (I think it's the same as CEL) came on and the blower motor stopped working altoghether. There were some stored codes that helped me indentify the engine temp sensor as the culprit, but I don't remeber exactly which ones.

During the repair I found all the following things were shot:

- engine (coolant) temp sensor

- air blower motor power stage (resistor?)

- engine ECU (after replacing both above items and reseting the codes, the CEL came back almost immediately and one of the codes reappered. I cleared it twice or three times, but until I replaced the ECU for a known good unit, the CEL and code alwayas came back)

When replacing the engine temp sensor I found the insulation on the wires was striped where they rubbed on one of the metal parts around there. I'm not sure if that can be the reason for the engine ECU failure, or how it was all connected to the blower motor power stage failure. It could have just been a coincidence. But if it wasn't a coincidence I could have avoided the ECU failure if I replaced the sensor immediately. So just out of caution, try to check your temp sensor out as soon as possible.

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Guest JTSPO

I've never had a CEL for a bad ECT. I've replaced it on 3 cars due to fluctuating temp gauge as well.

It could but not always. It throws a code when the engine runs rough because the wrong ratio. But it is possible no code was caused. Keep the bad ECT and it will give you a code in the long run.

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