99 V70 T5 - Coolant temp sensor interchange?

[j-dawg]

I think my ECT may be boned. Even though it has been so for many moons, I'd like to fix it now because I want instant gratification. Given how much fruitless labor I've put into this car recently, I could really use a quick boost.

The "local" (35 miles) Volvo dealer has one in stock for an 850. I'm pretty sure the location for the sensor changed in 99; did the sensor itself change as well? The parts guy said it didn't, but websites are not showing the 850/98 S70 sensor as working in a 99.

How about S80s? There are four 99-01 S80s at the Waco Pick'n'Pull, and probably at least one has a coolant temp sensor. Same issue, though - none of the parts sites shows interchange.

Everything I read about aftermarket sensors suggests that they're going to not work right off the bat, so I won't bother unless it's <$15.

[Ozark Lee]

The location is the same but the connector is different between the 850s and a '99. Since the ECU is a totally different version (ME7 vs Motronic 4.X) the sensor may have different resistance characteristics as well.

...Lee

[abscate]

1999 ECT specs from Alldata

[j-dawg]

nice! I'll check that before I order a new one, thanks

[abscate]

Little different from what cn90 measured on the 1998 new sensor ...data below

Most Engine Coolant Sensor works by NTC principle: as temp increases, the resistance (measure in Ohm) decreased:
For example, I measured the brand-new Volvo ECT and got these values
* -20C: 16,000 Ohms (16 kOhms)
* 0C: 7,300 Ohms
* 20C: 2,600 Ohms
* 80C: 300 Ohms
* 90C (Operating Temp): 200 Ohms
* 100C: 150 Ohms

[jreed]

A while back I measured FAE and SWF ECT's for a '97 855 and plotted up the values. The aftermarket FAE had failed and was reading higher across the entire temperature range:
https://www.matthewsvolvosite.com/forums ... =1&t=61920

As cn90 and abscate posted, for the 850's the specified nominal value at about 90C (194F) is about 200 Ohms. I summarized the values for the 850 in the table below.


Table of resistance values versus temperature for the ECT in the Volvo 850:

ECT resistance at various temperatures (-20C to +100C) for Volvo 850

For the '99, the resistance values are significantly lower (for example, at 20C it is supposed to read 1150 Ohms compared to 2600 Ohms for the 850's). If these values are correct, then putting an 850 ECT in a '99 S70 will cause the engine computer to think that the engine temperature is significantly colder than it really is... this will make the engine inject excessive fuel to compensate for the supposed lower temperatures.