Main coolant fan not working

[pauleyman]

Ok I'm stumped. Ac fan works as it should. I tested every relay by swapping to ac slot so all the relays are good. Jumpered main fan (lo) and (hi) under the relay and it comes on so fan is good and wiring to fan is good. Scan tool reads coolant temp fine so sensor and harness are good. Temp hits 200 plus and fan never comes on. It overheated yesterday. Only thing left is wiring to relay to trigger it and ecu i think. Maybe a ground somewhere? This is all after a head swap. Several days after head swap it popped a radiator but that was probably because it was overheating and son didnt notice. Bring on the suggestions before I tear apart something unnecessarily.

 

[luv2rallye]

No. What I'm saying is I think the 2g tech manual is wrong and has 1g stuff in it.


I'm honestly not sure

Well it doesn't appear to match either 1g or 2g. This 2g tech manual condenser fan shows 3 connected wires (but the 2g actual circuit diagrams shows 4 connected wires) - where the 1g one only has 2 wires.

Also the 1g actual circuit diagrams shows a series resistor for condenser fan low speed (which gets bypassed via a relay for high speed), where this 2g tech manual shows the condenser fan high speed is gotten by it's 3rd wire (a 2nd fan winding).

Maybe this 2g tech manual was the way they initially planned on doing it but then they changed their mind and never updated the manual?

 

[chrysler kid]

The charm website I usually use is down

I can tell you the 3rd fan relay was only used on automatic Dsm 2g cars

There are two different sensors, technically three on the housing

The big fat one on the back of the housing is for the ECU readings, the tiny pronged one facing the lower neck is for the dash gauge reading, and the third one (I can’t see the repair manual) is the thermostatic switch for the fan

 

[luv2rallye]

The thermostatic switch for the fan is only on 1g's that I know. Unless I'm wrong about which 2g year has what (2ga vs 2gb?), my 1999 factory diagrams show 2g MT turbos have 4 fan relays and 2g AT turbos have 5. The 420A has 3 relays whether MT or AT. No one seems to have the complete diagrams that show the cooling system for the 2ga (95-96).

 

[chrysler kid]

Oops definitely got my car knowledge confused. There is no thermo switch for the fan

It is controlled by the ecu, however with the temp sensor unplugged the ecu should automatically trigger the radiator fan to come on with the car in the “on” or “run” position

I broke the connector harness on my 2g a few weeks ago and the radiator fan would run on high constantly (and the car ran like crap on a cold start).

So unplug the sensor and start the car and the ecu should trigger the radiator fan immediately. If it does not we can pretty much narrow it down to the ecu not sending out the signal to the relay. With the key to run you should be able to probe the fan relays and fuse for the 12v signal. Next step would be to probe the ecu (pin 21) for continuity to the relay or fuse (I don’t have a clear copy
Of the fsm

The 95/96 have the same wiring but the colors of the wires to the fan were different. The connector pin out is still the same though

 

[pauleyman]

Oops definitely got my car knowledge confused. There is no thermo switch for the fan

It is controlled by the ecu, however with the temp sensor unplugged the ecu should automatically trigger the radiator fan to come on with the car in the “on” or “run” position

I broke the connector harness on my 2g a few weeks ago and the radiator fan would run on high constantly (and the car ran like crap on a cold start).

So unplug the sensor and start the car and the ecu should trigger the radiator fan immediately. If it does not we can pretty much narrow it down to the ecu not sending out the signal to the relay. With the key to run you should be able to probe the fan relays and fuse for the 12v signal. Next step would be to probe the ecu (pin 21) for continuity to the relay or fuse (I don’t have a clear copy
Of the fsm

The 95/96 have the same wiring but the colors of the wires to the fan were different. The connector pin out is still the same though

Thanks for the info. I was headed there soon. Car is doing fine for now. I borrowed an ecu also to test. Ill update when it's fixed correctly.

 

[luv2rallye]

It is controlled by the ecu, however with the temp sensor unplugged the ecu should automatically trigger the radiator fan to come on with the car in the “on” or “run” position

I don't know if that's true. With it unplugged the sensor circuit would be "open" (which is a high resistance) which is what the 2 wire engine coolant temp sensor would be at with engine temp normal (sensor actually is 2.1k-2.7k ohms @ 68*F). When engine is very hot, the sensor resistance goes way lower (260-360 ohms @ 176*F). So shorting the temp sensor harness would turn the fan on - not unplugging it. Temp sensor resistance decreases with increasing temp. Here's what turns the 2g fans on: https://www.dsmtuners.com/attachments/fan-operating-mode-2g-jpg.86862/

 

[chrysler kid]

I don't know if that's true. With it unplugged the sensor circuit would be "open" (which is a high resistance) which is what the 2 wire engine coolant temp sensor would be at with engine temp normal (sensor actually is 2.1k-2.7k ohms @ 68*F). When engine is very hot, the sensor resistance goes way lower (260-360 ohms @ 176*F). So shorting the temp sensor harness would turn the fan on - not unplugging it. Temp sensor resistance decreases with increasing temp. Here's what turns the 2g fans on: https://www.dsmtuners.com/attachments/fan-operating-mode-2g-jpg.86862/

With the sensor unplugged the ecu can’t see the sensor or any resistance so the ecu has to assume the engine is overheating to prevent damage. (I assume this because the fan instantly would run high power under the hood)

I broke the harness to the sensor when I removed the intercooler piping, drove the car a few times with it like that. The ecu was adjusting timing and fuel as if the car was overheating, ran like crap at cold start but evened out as the motor warmed up

 

[luv2rallye]

With the sensor unplugged the ecu can’t see the sensor or any resistance so the ecu has to assume the engine is overheating to prevent damage. (I assume this because the fan instantly would run high power under the hood)

That's a nice logical assumption (and maybe the way it should work) but it's not the way it's designed. If you look at the 2g turbo circuit diagrams you'll see the Engine Coolant Temp Sensor ECU input (ECU pin 83) is internally tied high to +5v. The sensor only connects this to the ECU's internal ground (pin 92) through the sensor's variable resistance (which makes pin 83 a voltage divider based on temp). So if it's unplugged, that ECU's input is pulled up to +5v - nearly the SAME as if the sensor had high resistance which it would have at normal temps. So the ECU can't tell the difference between sensor unplugged and sensor plugged in with engine at normal temp (either way the voltage on pin 83 is above a predetermined threshold voltage used to keep the fan off). Only when pin 83 is grounded or the sensor has extremely low resistance (which is when engine is at a very high temp), will the ECU see a low voltage on pin 83 and turn on the fan(s).

I broke the harness to the sensor when I removed the intercooler piping, drove the car a few times with it like that. The ecu was adjusting timing and fuel as if the car was overheating, ran like crap at cold start but evened out as the motor warmed up

Yes because the ECU thinks the engine is at normal temp when it was cold - so incorrect mixture. I doubt your fan was even on unless you had your AC switch on (or maybe you had some other issue or re-wired the fans). You still should have gotten a P0115 CEL however.

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[pauleyman]

That's a nice logical assumption (and maybe the way it should work) but it's not the way it's designed. If you look at the 2g turbo circuit diagrams you'll see the Engine Coolant Temp Sensor ECU input (ECU pin 83) is internally tied high to +5v. The sensor only connects this to the ECU's internal ground (pin 92) through the sensor's variable resistance (which makes pin 83 a voltage divider based on temp). So if it's unplugged, that ECU's input is pulled up to +5v - nearly the SAME as if the sensor had high resistance which it would have at normal temps. So the ECU can't tell the difference between sensor unplugged and sensor plugged in with engine at normal temp (either way the voltage on pin 83 is above a predetermined threshold voltage used to turn the fan on). Only when pin 83 is grounded or the sensor has extremely low resistance (which is when engine is at a very high temp), will the ECU see a low voltage on pin 83 and turn on the fan(s).

Yes because the ECU thinks the engine is at normal temp when it was cold - so incorrect mixture. I doubt your fan was even on unless you had your AC switch on (or maybe you had some other issue). You still should have gotten a P0115 CEL however.

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This makes perfect sense. My son has the car. When I get-together it I will report back. Betting I'm going to be replacing/repairing a wire.