1999 V70 Radiator Fan only runs when AC is on

[abskee]

My car has started overheating in stop and go traffic, especially if I'm driving on the freeway and then come to a stop for a few minutes. I assumed it was just a bad radiator fan or relay, but if I turn on the AC the radiator fan comes on as expected (it turns on maybe a minute after the AC compressor kicks in). The relay checks out too, I have 80k-ohms between either input pin and ground, and grounding the input pins on the relay turns the fan on both high and low speed. The temperature sensor also seems fine, I replaced it a few years ago (it had failed, so the fan was always on), but I get reasonable readings on from the engine temp gauge on the dash and from the OBD scan tool. Any ideas on what's going on? Does anyone know at what temperature the fan should turn on?

To recap:

• Radiator fan never turns on due to heat. The radiator just boils eventually.

• Fan turns on when the AC is on, so the fans, relay, and wiring seem fine.

• I do get data from the engine temp sensor, and it seems reasonable, although I haven't pulled it out to test the accuracy yet.

[abscate]

I did a series of observations in stop and go and got fan on at 105C and off at 87 C

[Chuck W]

Have you checked that ECT connector for corrosion? How about the relay input connector?

Could be bad wiring from the ECU to the fan relay.

[abskee]

Okay, dove into it a little more, I'm thinking my temp sensor is reading low enough to cause trouble, but just not low enough that I noticed an issue.

I unplugged the temp sensor and put a 150-ohm resistor across the contacts that go to the ECU The car turned the fans right on and the engine temp read 250 degrees F, which is about right for that resistance in a working temp sensor, so it seems like the ECU is reading and turning the fans on fine.

I ran the car until it overheated and the coolant starts boiling when the engine is reading like 180F, with the dashboard gauge just above 3 o'clock. So either water has failed and started boiling at 180F, or my engine temp sensor failed and it's reading like 50 degrees low. I tried cleaning the connectors at the sensor, no difference, so it's got to be that sensor. I've just never seen one fail and be a little low, they seem to always be completely shot.

I'm gonna swap the sensor out with a proper Volvo one this time and see how it goes. I'll report back.

[loveMy850]

That's pretty much the norm on the volvo. they don't need a whole lot of fan.

[abscate]

Okay, dove into it a little more, I'm thinking my temp sensor is reading low enough to cause trouble, but just not low enough that I noticed an issue.

I unplugged the temp sensor and put a 150-ohm resistor across the contacts that go to the ECU The car turned the fans right on and the engine temp read 250 degrees F, which is about right for that resistance in a working temp sensor, so it seems like the ECU is reading and turning the fans on fine.

I ran the car until it overheated and the coolant starts boiling when the engine is reading like 180F, with the dashboard gauge just above 3 o'clock. So either water has failed and started boiling at 180F, or my engine temp sensor failed and it's reading like 50 degrees low. I tried cleaning the connectors at the sensor, no difference, so it's got to be that sensor. I've just never seen one fail and be a little low, they seem to always be completely shot.

I'm gonna swap the sensor out with a proper Volvo one this time and see how it goes. I'll report back.

x

If you used a cheap aftermarket part first time, this is probably the cause.

The resistance vs temperature chart for the 1999 ME7 cars is logged in my T5 maintenance record if you can't find it here in this forum. It is paginated a bit funny, the columns need to be shifted over one notch , so look it over and ask questions here