Hi, I have a 1998 Caravan and the enginecooling fans wont go on. Ive replaced the relay and the temp sensor. I have checked the temp sensor with a ohm meter to make sure the resistance changes when the engine warms up, and it does. Th sensor is working fine. 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.

Doesanyone have any ideas why the computer module isnt sending a signal to the relay? Could the A/C problem be causing thecooling fansnot to come on? Could there be a problem with the computer module?

 

Read this article:

Cooling fans inoperative

 

Thanks for posting the article, but I dont think that helps me much. By taking off the connector to the temp sensor I prove that the fans work, the relay works, and that the PCM is sending a signal to the relay telling it to turn the fan on. I have put an ohm meter on the temp sensor and watched the resistance change as the engine temp went up, so I know temp sensor is working. Ive think I have narrowed the problem down to the PCM not sending a signal to the relay when the temp sensor is hooked up. The question is "why is the PCM not sending a signal?"
It seems that all the components are working.


If the A/C pressure switch was bad could that keep the PCM from turning on the fan even when the A/C is not being used?

 

You don't know that the PCM isn't providing ground to the relay until you test it under running conditions. If you had a scanner with live data, you could tell exactly what the ECT is reading and whether the PCM provided ground to the relay. If you don't have that equipment, simply testing a coolant sensor under no-load conditions isn't a fail-safe test. I suggest you backprobe the ECT and the relay-to-PCM-ground wire under actual running conditions. That's the only way you'll know for sure that the PCM is getting the right temp reading and is or is not providing ground to the relay coil. I understand you're frustrated, but replacing the PCM is an expensive way to go right now. Resistance related sensors can act differently under load than under the power of a DVOM.

 

It seems that the problem is probably the communication between the temp sensor and the PMC or a bad PCM. Im not sure if I understand exactly how the temp sensor works. I assume the PCM reads the amount of resistance the temp sensor is at depending how hot the engine is. Then the PCM sends a signal to the water temp gauge on the dash to show hot hot the engine is, but also sends voltage to the fan relay when the engine reaches a certain temp to turn the fans on.

My dashboard temp gauge works fine. How can the PCM be reading the resistance of the temp sensor for the dash gauge but not read the resistance to turn the fans on?

 

The PCM provides a 5v reference signal to the sensor. On a cold engine the resistance of the sensor is very high, so you get no voltage flow to ground. As the coolant warms, resistance decreases and more voltage flows to ground--but only IF the ground is in good condition. If it's corroded, the voltage will remain high.

You got a P0118 code. In order to set that, the PCM has to see 4.9 volts on the sensor input for more than 3 seconds. But it doesn't do this in a vacuum. It's also looking at throttle position, intake air temp, trans fluid temp, etc. In other words, the PCM knew the engine was warmed up when it saw high voltge. So, you either have a bad connection between the black wire at the sensor connector and the connector at the PCM. Or you have an open in the reference signal wire. Since you have temp gauge operation on the dash, you can rule out an open in the signal wire. But you cannot rule out a bad connection to ground.

Pulling the connector only confirms that the relay is working. You basically put the PCM into a default emergency state. It saw extremely high voltage--assumed that the sensor had failed or the ground wire had broken, and opted to be on the safe side and turn on the fans to prevent overheating. It doesn't tell us exactly how accurate the sensor is or what condition the wiring is.

You need to read the voltage on the reference wire under running conditions and compare the readings to the ACTUAL coolant temperature.

Here are some values:

At a coolant temp of:

120°, voltage should be 4v
140°, voltage should be 3.6v
160°, voltage should be 3.20 v
180°, voltage should be 2.8v
210°, voltage should be 2.2v
220°, voltage should be 2.0v

If you see where I am heading here, you'll see that the difference between an engine that has JUST reached operating temp (180-190) and one that is overheating is only .8 of a volt.
I doubt that you can detect that difference in the needle on your dash. That's why I've been suggesting that you doing an actual voltage test instead of making an assumption that the sensor is good simply because the resistance changes. (An ohm test is NOT considered a relaible test--well, not as reliable as a voltage test, since voltage is what the PCM reads, not ohms.)

If you do see 2 volts and the coolant temp IS 220, the PCM should provide GROUND (not power) to the fan relay.

You're making a lot of assumptions about the condition of your wiring and I'm afraid you're heading right for a new PCM without doing some basic more advanced electrical testing. Just trying to save you some money here. It still may be a bad PCM, but I'd sure want to see some voltage readings to confirm that first.

 

Thank you, I will test the voltages and post my results.

 

Hi To All: Brand New To The Forum:

I Have The Same Problem,Did You Come Up W/A Solution? Thanks:John.

 

A couple of my posts seem to be missing from this thread were I had posted my results. Anyway, I hooked a volt meterup to the temp. sensor and watched the voltage drop as the engine heated up. Eventually after about 15 minutes the fans did go on butI dont remember what the voltage was at that point. So, it seems that the engine in my van never got hot enough during normal driving to turn the cooling fans on.

 

Have you try hooking up a DRB 3 and actuating the fans. I don't know if any other scanner would work.