Fans not running when A/C is ON

I have a 97 Caravan with 3.0L engine, 3spd tranny.

My dual electric cooling fans do NOT come on when th A/C is on.

The fans DO come on when the engine temp rises to over 1/2 on the temp. guage.

Would the relay be the cause?

If the relay starts the fans for high engine temp., then it must be working properly, correct?

So my question is, what part of the AC system tells the fan relay to start the fans?

GT from Ontario, Canada

Forgot to mention - the AC works great when the van is moving.

Last one I had that acted like yours- it was a loose connection at the a/c pressure transducer- located upsidedown on the liquid line near the firewall- All I did was to unplug it and plug iit back in while it was running and the fans came on--- I was going to replace the $64 solid state relay- but it was testing ok-- just that the PCM was not being told to turn on the fans because it was not getting the proper pressure reading from that 3 wire transducer.. so check yours,,,

Both turn on modes arrive at the relay over the same wires from the PCM. Logic says if one mode works then the relay is good. I'd like to say that is a hard and fast rule but, the relay is a solid state "relay". Don't know it's internal functions. brcidd's post is some evidense it may be the case.

Last one I had that acted like yours- it was a loose connection at the a/c pressure transducer- located upsidedown on the liquid line near the firewall- All I did was to unplug it and plug iit back in while it was running and the fans came on--- I was going to replace the $64 solid state relay- but it was testing ok-- just that the PCM was not being told to turn on the fans because it was not getting the proper pressure reading from that 3 wire transducer.. so check yours,,,

I checked the pressure switch, the plug was tight and there is no corrosion.

When I unplugged the switch, the AC clutch shut off immediately. When I plugged it back in, the clutch kicked in, the fans remained off.

Then I unplugged the coolant temp sensor. The fans kicked on at high speed, spinning much faster than what they do when the engine heats up.

Still a big mystery.

I'm kinda thinkin' that maybe the pressure switch isn't working properly.

Is it possible that the pressure switch is working when the AC clutch is activating, but at the same time is malfunctioning because the fans don't activate?

Could it be a PCM problem too?

Thanks,
GT

From checking the current temp where you are it is about 20 deg cel or 68 deg fahrenheit. Fair winter daytime temp where I live. Rad fans will not come on till the A/C pressure on the high side is at least 1724 kpa (250 psi) It is possible that the comp to be engaged and the fans to not be on if the conditions are right. At your temp the A/C is not working very hard.

Thank you, but, I tested my In-Law's 2003 caravan today.

When I turned the A/C on, the fans came on within 10 seconds.

Mine are still not coming on.

If the system charge is low they might not come on. All you stated was the compressor clutch engages. How does it cool?

the one I worked on was cutting out the compressor for high pressure cut-outs- so I figured the pressure transducer was okay- I mean it could tell the pressure was 400+ psi- so it must be working-- but still the fans were inop until I reseated the connector- then they came on.

My problem got worse tonight.

I took the fan relay out. I started engine, disconnected the coolant temp sensor, and the fans came on, as they should.

I disconnected the relay, and the fans stopped.

When I reconnected the relay, the fans worked momentarily, ten stopped. Now they won't work at all.

Some probing with the tester found the following;

1. The connector for the relay is working properly - grey wire is hot, 2 others are ground, the green one goes to the fan.

2. The new problem - the plugs for each fan have ground at both terminals. If I'm not mistaken, I thought that one terminal should be hot at all times, and the other terminal is the ground which is controlled by the relay.

I've done lots of electrical over the years, this system & problem really have me stumped.

A good scan tool will tell you what the PCM is thinking--- it will show you transducer pressure and fan output status- but that is usually a trip to a dealer unless your neighbor is a chrysler mechanic..

brcidd is right. The best way to check the sytemis with a good scan tool and A/c gauges hooked up to the system. You probably burned out your rad fan relay. It is attached to the frame for 2 reasons; heat sink and ground for internals.

I think I got her fixed.

I picked up 3 fan relays from the junk yard, and installed one of them today. I used dielectric grease on the wiring connections, and silver thermal grease on the back of the relay for heat dissipation.

The fans turned on within 60 seconds of activating the AC.

And, when the engine warmed up beyond 1/2 on the guage, the fans activate properly.

Thank you everyone for your assistance.