Engine Cooling Fans Not Working

[alydar66]

Hi, I have a 1998 Caravan and the engine cooling fans wont go on. Ive replaced the relay and the temp sensor. When I pull the connector off the temp sensor the fans go on so I know the fans and fuses are fine. For some reason it seems the computer module isnt sending a signal to the relay to turn the fan on. I get the trouble codes P1491(radiator fan relay circuit fault) and P0118(engine coolant temp circuit high input). I am also having a problem with the air conditioning not working. When I do the self diagnostic test on the A/C the results are that it fails the cool-down test.

Does anyone have any ideas why the computer module isnt sending a signal to the relay? Could the A/C problem be causing the cooling fans not to come on? Thanks for any help.

[tempfixit]

Originally Posted by alydar66

Hi, I have a 1998 Caravan and the engine cooling fans wont go on. Ive replaced the relay and the temp sensor. When I pull the connector off the temp sensor the fans go on so I know the fans and fuses are fine. For some reason it seems the computer module isnt sending a signal to the relay to turn the fan on. I get the trouble codes P1491(radiator fan relay circuit fault) and P0118(engine coolant temp circuit high input). I am also having a problem with the air conditioning not working. When I do the self diagnostic test on the A/C the results are that it fails the cool-down test.

Does anyone have any ideas why the computer module isnt sending a signal to the relay? Could the A/C problem be causing the cooling fans not to come on? Thanks for any help.

Have you tryed servicing the A/C with a gauge set? Your system may be low on freon.

[alydar66]

Could being low on freon cause the PCM not to send a signal to the fan relay?

[RIP]

Originally Posted by alydar66

Could being low on freon cause the PCM not to send a signal to the fan relay?

Good thought but no. I've been driving around with no freon all winter and the fans come on as designed.

[jpb53]

You say the fans won't come on yet when you disconnect the coolant temp sensor they do. PCM is turning on the fans when you do this. If the PCM does not know what the coolant temp is then the fans are turned on to protect the engine from overheating. Is the van overheating or is just an A/C problem? If the fans aren't coming on it will fail the cool down test. When you push th A/C button on the dash the PCM gets a signal from the control head. If it does not get a signal from the pressure transducer the A/C will not come on. Low freon or high head pressure are the 2 extremes that will cause this.

[2000izusu]

Originally Posted by jpb53

You say the fans won't come on yet when you disconnect the coolant temp sensor they do. PCM is turning on the fans when you do this.

good point! could be a bad new cts.

[BackSpacer]

well, I might as well chime in. I have a 1999 voyager that is having the same problem as the creator of this thread. Fans will not come on. I changed the relay and they still wouldn't come on. I turn on the a/c (which is out of freon, but the switch should still turn on the fans right?) and no fans. I then changed the coolant temp sensor and still nothing. If I short out the green and gray wires on the connector to the relay, they will come one, so I know the fans work. If I unplug the wires off of the coolant sensor, they will come on also. what am i missing? Please help

[jpb53]

A/C pressure transducer is for the A/C system. It is a dual purpose switch.If the pressure is too low(low on freon) pressure too high ( high head pressure) the A/C will not come on. This is to protect the system from damage. If the compressor seizes or start to come apart you can have a very expensive A/C system repair. So after what I just stated if there is no freon in your system when you turn it on the compressor cluth should not engage and the fans should not come on. If I remember right the fans should not come on until the coolant temp reaches about 220 degrees. Is your van overheating?

[alydar66]

No the engine is not overheating but im afraid it might this summer with the 90 degree temps. Im not to concerned with the a/c not working, it probably is low on freon. I was just wondering if the a/c not working properly would cause the fans not to come on. I cant imagine they would design an engine were the cooling fans would not go on if you were low on freon.

I think ive narrowed the problem down to the communication between the temp sensor and the PCM.

[BackSpacer]

man, this is weird. My problem is exactly like alydar66. my van is not overheating, the gauge reads just over half way. I have let it sit and idle for over 30 minutes with the motor reving and it never gets any hotter, but the fans still don't come on. maybe nothing is wrong since the van never gets any hotter. maybe all i have is an air conditioner problem. i just assumed that the fans would come on when i pressed the air conditioner button and the light come on on the dash. are fans smart enough to only come on when the compressor is actually turning, or are they just linked to the a/c switch? If mine is working correctly, they must know if the compressor is turning or not. I have changed the coolant temp sensor, the fan relay, and a new thermostat. I may have wasted all of that and nothing is actually wrong but out of freon. the fans on my gmc truck come on about the time the temp gauge starts to lift up and turn on and off a lot. i can't imagine the voyager not coming on, but may be thats the way its supposed to be. out of curiosity, by looking at the temp gauge, about when should the fans come on? mine goes up just a little over half way and the fans still don't come on. also, which wires do you short to make the a/c comp manually come on?

[RIP]

Gauge readings can be an apples and oranges situation from van to van but, I've never seen my temp gauge above half way and I know the fans are coming on.

A way out on a limb theory. The PCM isn't recognizing anything but a maximum temp reading, simulated by disconnecting the temp sensor. the PCM may be bad. Thoughts?

[BackSpacer]

the pcm is really the only thing that is left isn't it? unless one of the new parts I put on is defective. I have even traced all the wiring and checked for continuity. there aren't any codes showing either. would a bad pcm show a code? if i disconnect the fan relay or the coolant temp sensor, i get a code, but if i hook them back up and clear the codes, i don't get anymore codes. i guess i need to take it back to a dealer to get it diagnosed and fixed. the airbag light is on and the horn and cruise control don't work either. i have been told that is a safety recall and it should be fixed free, is that correct. any other thoughts before i take it to a dealer next week? thanks

[RIP]

PCMs can be bad without showing a code. The OBD system only monitors certain parameters, not all. For example, I've read threads where the fix for a no start is a PCM. No mention of codes. Even if a component is designed to throw a code when it fails, they many times don't. Cam and crank sensors are infamous for going bad without throwing a code as they should. Unfortunately I'm on shakey ground in your cases because I don't recall ever reading of failed fans with your particulars. As you said, the only part left is the PCM. I'd sure like to be able to tell you how to prove it but at this point I can't.

> Yes, they should change your clockspring under the recall for free.
> I've read of some dealerships that keep a PCM around for testing. Might ask them.

[jpb53]

Think about it. If you unplug the coolant sensor and the fans come on the PCM it is doing it's job; when you uplug the coolant sensorit is not seeing the coolant sensor signal and to protect the engine it turns the fans. From what you are saying I don't think there is anything wrong with your van. The BCM gives your cluster the gauge inputs and the gauges can be adjusted by using a DRB ( factory scan tool) In other words there are a lot of variables in your temp gauge reading. Most I have seen the fans don't come on until the gauge is 5/8 to 3/4 .I live in southern Arizona and if a cooling fan doesn't come on when the gauge is half-way up I don't worry about it.

[RIP]

Originally Posted by jpb53

Think about it. If you unplug the coolant sensor and the fans come on the PCM it is doing it's job; when you uplug the coolant sensorit is not seeing the coolant sensor signal and to protect the engine it turns the fans.

Ya I know. It's a stretch. The fact the ECT sensor is a variable resistor and the system is a two step multi-speed system has me wondering if...in so many words...the circuit in the PCM that controls the ECT sensor input has failed sensing any resistance except maximum resistance (an open) as it sees when you disconnect the ECT sensor. You reconnect the sensor, the PCM sees nothing but a fixed lower resistance regardless of the sensor's output, and keeps the fans off. Without knowing how that internal PCM circuit works, I can't be sure of anything. Just wanted to throw the theory out there. While in the Air Force I served a number of years in a flight test squadron working avionics on jets. We were pushed to think "out of the box". Hard to shake old habits.

I'd recommend to Alydar66 and BackSpacer that they let the vans idle till either the fans come on or it's obvious they will overheat if left running.


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