1996 850 Low speed radiator fan not turning on [Bad ECT sensor] Topic is solved

Post by **ZionXIX** » 12 Aug 2019, 16:04

Copied from the WDYDTYVT thread.

Background: a few days ago I got to work early let my car idle for a bit. A few minutes later i noticed smoke coming from under the hood and shut the car off and popped the hood. I saw copious amounts of coolant pouring out of the reservoir cap. Dash gauge was normal the whole time.
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Today:

Spent the afternoon trying to diagnose why my low speed fan doesn't seem to work.

Things I'm noticing:
At 206F the overflow tank is filled to the top
At 208F the lid begins leaking
Turning on the AC engages the fan and brings the temps down quick.
The yellow wires for the center relay plug were crammed full of whatever orange insulating rubber material was originally put in there. It wasn't doing its job anymore so I removed them from the plug housing and connected them directly to the relay pins.

Things I've tested:
Low speed and high speed both come on if I ground out the yellow wires on the relay.
The yellow wires correspond to A7 and A22 of the ECU. I am measures 3-5 ohms for both wires up to the ecu.
I tried switching the yellow wires
I swapped out the relay for a spare
I tested the relay outside the car with a meter.

Also changed out the thermostat today. A craftsman T40 ended up doing the trick. The new one is made in Germany where as the previous one is stamped France.

Conclusions:
Theory 1: bad ground --> cleaned as many as I could find a few weeks ago with a brass wire drill brush.
Theory 2: ECU is not grounding the relay properly = bad ECU
Theory 3: I could have 2 bad relays
Theory 4: reservoir cap is not holding enough pressure, not sure what they are supposed to withstand or what temps activate the fan.

Post by **ZionXIX** » 13 Aug 2019, 08:56

Today's diagnostics include using the 850 OBD-II app to troubleshoot the cooling fan issue. I've used this app many times for other issues but I was unaware of the cooling fan testing options.

Under "service" and "advanced" there are 2 options for cooling fan tests under "mototronic" and "MSA 15.7". Neither of these options triggered the cooling fan.

I inspected the ECT sensor wiring and found tons of green gunk and corrosion. I took it apart and clean everything inside and out until it was shiny. Still no change in fan functions. I did try unplugging the sensor while the car was running a and the fan came on immediately.

On my multimeter the sensor is reading infinite ohms. My parts car temp sensor was reading like 1800ohms so I think that gives me my answer. Will probably be ordering a new one.

I still think it's strange the cooling fan would not trigger for the app test.

Post by **ZionXIX** » 13 Aug 2019, 09:54

Now that I sit here and think about it. This may explain why I get such terrible mileage and always run rich. Maybe.

Post by **Chuck W** » 13 Aug 2019, 19:03

That is odd that grounding the trigger wires to the relay switches on the fan, but the app test trigger from the ECU didn't (seeing as you supposedly have good wiring from the ECU to the relay).

What if you ground the trigger wires back at the ECU plug? That will certainly rule out the wiring. Do you have a spare ECU handy? (If you were closer, I have 2 "-072" Turbo/T5 ECUs on the shelf.

Those ECT sensors can wreak havoc if they get corroded up. I fought with mine this past winter.

Oh! One thing to also check is the high pressure switch for the AC (behind the PS headlight). We were having issues with the Low-speed fan coming on in the wife's car, and it turned out to be the high pressure switch. I was able to get the app to trigger the fan relay though on hers.

Post by **abscate** » 14 Aug 2019, 04:17

OEM only on those sensors. Aftermarkets are a cheap crapshoot.

Post by **RickHaleParker** » 14 Aug 2019, 04:33

ZionXIX wrote: ↑13 Aug 2019, 08:56 My parts car temp sensor was reading like 1800ohms so I think that gives me my answer.

ENGINE COOLANT TEMPERATURE SENSOR RESISTANCE TABLE

Temperature ° F ( ° C) Ohms
32 (0) ............................7300
68 (20) ......................... 2800
176 (80) ......................... 300
212 (100) ....................... 150

Post by **ZionXIX** » 14 Aug 2019, 06:22

RickHaleParker wrote: ↑14 Aug 2019, 04:33
ZionXIX wrote: ↑13 Aug 2019, 08:56 My parts car temp sensor was reading like 1800ohms so I think that gives me my answer.
ENGINE COOLANT TEMPERATURE SENSOR RESISTANCE TABLE

Temperature ° F ( ° C) Ohms
32 (0) ............................7300
68 (20) ......................... 2800
176 (80) ......................... 300
212 (100) ....................... 150

Wow, I was getting infinite resistance at what was supposedly 195F. If I can't get any sort if resistance reading, how can the ECU be reporting a temp value?

Post by **abscate** » 14 Aug 2019, 07:10

Yeah, thats strange. I don't think there are any other coolant temperature sensors on these cars.

Post by **RickHaleParker** » 14 Aug 2019, 11:23

ZionXIX wrote: ↑14 Aug 2019, 06:22 Wow, I was getting infinite resistance at what was supposedly 195F. If I can't get any sort if resistance reading, how can the ECU be reporting a temp value?

What is the difference in temperature gauge reading between Sensor hooked up and sensor not hook up? Engine hot.

Post by **ZionXIX** » 14 Aug 2019, 11:54

RickHaleParker wrote: ↑14 Aug 2019, 11:23
ZionXIX wrote: ↑14 Aug 2019, 06:22 Wow, I was getting infinite resistance at what was supposedly 195F. If I can't get any sort if resistance reading, how can the ECU be reporting a temp value?
What is the difference in temperature gauge reading between Sensor hooked up and sensor not hook up? Engine hot.

Well the fan runs on high speed when I unplug the sensor.....so cold engine in this scenario. I just don't understand where the computer is getting its temp value if there is no measurable continuity.