Auxiliary Fan does not turn on with A/C

Truck is an 89 4.0 auto with factory A/C.

I recently learned that when you switch the A/C on, the auxiliary cooling fan is supposed to turn on as well. Mine does not. I think the fan turns on if the engine is starting to overheat but I am not entirely sure about that.

I have tried following this tutorial but I am stuck.

http://www.lunghd.com/Tech_Articles/Coo ... gnosis.htm

If i supply battery voltage to terminal 4 of the relay connector the fan spins. So, the motor works.

Next is says "With fan relay removed turn the ignition switch to the RUN position.

# Check continuity between terminal 5 of the relay connector (Fig. 43) and the body ground con- nections on the fender panel back from the relay con- nector (Fig. 42). If continuity exists proceed to next step. If continuity is not found repair open."

When I perform this test I get an open circuit. If you trace the wire that goes to terminal 5 it is the coolant temp sensor/switch. Isn't that supposed to be open until a certain temp? Or is it the other way around?

Also in the factory service manual pdf that I have for an 88. The procedures for testing the fan are nearly word for word but the relay pin diagram is different! 4 and 5 swapped.

(The pin id that i was using in the test is the non factory identification.)

Another question: If the A/C is low on freon does this prevent the cooling fan from running???

:help:

Even if the compressor won't run(low freon) the fan will still come on. When you switch the A/C on the fan will come on regardless of the A/C working or not. I would get the right wire diagram for your truck and see which wire tells the fan to come on when you turn the A/C on.

Bad Zener diode in the wiring (small orange thing buried in there). I wired in the aux cooling fan a few months ago. If you want to figure out whether the fan works unplug the temp switch in the radiator and jump the pins in the harness. It should come right on.

Bad Zener diode in the wiring (small orange thing buried in there). I wired in the aux cooling fan a few months ago. If you want to figure out whether the fan works unplug the temp switch in the radiator and jump the pins in the harness. It should come right on.

I think that you are right about the diode. Do you know which kind I need and where the best place to get it from is?

Not sure about a new one but I probably have 1 or 2 extras floating around. It took me 3 tries at the jy to get everything I needed to add the cooling fan. Be warned you have to physically cut the old one out of the harness. If I can find one of the old ones you're welcome to it as long as you pay shipping (prob $2-3 as it's pretty small).

Send me a PM if you want me to dig for it. Did you jump the coolant temp switch to see if the fan works?

Not sure about a new one but I probably have 1 or 2 extras floating around. It took me 3 tries at the jy to get everything I needed to add the cooling fan. Be warned you have to physically cut the old one out of the harness. If I can find one of the old ones you're welcome to it as long as you pay shipping (prob $2-3 as it's pretty small).

Send me a PM if you want me to dig for it. Did you jump the coolant temp switch to see if the fan works?

Thanks for the offer! I will try and see if I can get one locally first, if not I will let you know.

I did jump the switch and the fan came on. So if the switch is good, the fan should work when the engine begins to overheat...

I pulled the diode out of the wiring and tested for resistance and it's got infinite resistance both ways. Failed open? Sounds like the culprit. But what would cause it to fail?

I had the same problem in my 88 mj, turns out the diode was bad - I went to napa and bought one for some other application that I installed and so far so good.

I pulled the diode out of the wiring and tested for resistance and it's got infinite resistance both ways. Failed open? Sounds like the culprit. But what would cause it to fail?

 

If I remember correctly you need to check voltage on the legs of the diode. Apply +12V to the line running from the compressor. You should see +12V at the relay and nothing at the temp switch. Now apply +12V to the temp switch. You should see +12V at the relay and none at the compressor (sounds like you did b/c the fan came on). If you see anything other than this the diode is bad.

What causes them to fail? Old age, too much dirt, etc.

I went to the local pick n pull to try and get another diode. Both of the cherokees had different wiring than on mine. I saw what you meant about the diode being a little orange thing, unfortunately my truck doesn't have that. It's got a diode that looks almost like this one.

http://www.napaonline.com/Search/Detail ... 0217876969

One end has a bullet type connector that goes over the threads and the other end was soldered. It was heat shrinked and everything, looked factory to me. Don't think someone messed with it before.

But Why is my wiring different? The cherokees both had A/C.

:dunno:

The small orange diode is the only one I've ever seen...perhaps yours has been replaced once already? :dunno: The '87 and '90 manuals only call out one diode, though they don't elaborate on what it looks like. I will say that if the diode has a removable connector it should be listed in the FSM. My '87 electrical manual has a call out and sketch of every connector in the truck (which is why I kept it for the '90).

Theoretically any 12V diode with 2 inputs and 1 output should work. It doesn't carry any current, just triggers the relay.

The small orange diode is the only one I've ever seen...perhaps yours has been replaced once already? :dunno: The '87 and '90 manuals only call out one diode, though they don't elaborate on what it looks like. I will say that if the diode has a removable connector it should be listed in the FSM. My '87 electrical manual has a call out and sketch of every connector in the truck (which is why I kept it for the '90).

Theoretically any 12V diode with 2 inputs and 1 output should work. It doesn't carry any current, just triggers the relay.

This diode only has one input and one output. I took some pictures. I hope you can see the differences. And there is no little orange diode to be found.

I think I'm going to just add a switch to turn the fan on manually for the time being. A jumper controlled by a switch across the two temp switch wires should work fine right?

That's definitely not OE. A diode should not be spliced inline with the red power lead as it serves no purpose there. That's the power feed for the relay and it's fed by the fusible link from the junction block/solenoid behind the battery. Did you trace the orange wire back? The diode should be on it.

I think I'm going to just add a switch to turn the fan on manually for the time being. A jumper controlled by a switch across the two temp switch wires should work fine right?

Yes, this will work for the time being. Hit the JY and cut the connector off a temp switch. Then you can wire a switch to that pigtail that can be easily removed.

That's definitely not OE. A diode should not be spliced inline with the red power lead as it serves no purpose there. That's the power feed for the relay and it's fed by the fusible link from the junction block/solenoid behind the battery. Did you trace the orange wire back? The diode should be on it.

 

I think I'm going to just add a switch to turn the fan on manually for the time being. A jumper controlled by a switch across the two temp switch wires should work fine right?

 

Yes, this will work for the time being. Hit the JY and cut the connector off a temp switch. Then you can wire a switch to that pigtail that can be easily removed.

I did trace the orange wire back and there isn't a diode on it. It looks like someone totally rewired the fan relay. Which is probably why it's not working properly. I'll take some pics.

The previous owner didn't do any work himself but he had a ton of work done to the truck, and saved every receipt/ work order. I looked through the giant folder of stuff and found something from 1995 that said the a/c fan was not coming on. Apparently the mechanic advised him that it was possibly a relay. But no work was completed. So I don't know where and when someone messed with it, but we know that it has had this problem for a while.

I would really like for the fan to work as designed and have the option of turning it on manually. I will need help re-wiring it though. Are you up to a barrage of questions?

Somebody did some splice and dice funny business...

These four wires continue all the way over to the ac compressor and some sort of switch, like a low pressure switch? And they go to the ac relay.

Is this plug even supposed to be here?

Is this plug even supposed to be here?

That plug was left by the factory for dealer installed A/C. I used one of the pins to provide key on 12V+ for my fan relay (not load, just trigger). This way the fan won't come on if the truck is shut off.

Are you up to a barrage of questions?

Sure. It might be best to PM me and take it offline. Perhaps you can document rewiring the relay and put in the how to section. You can give me honorable mention :)

Is this plug even supposed to be here?

That plug was left by the factory for dealer installed A/C. I used one of the pins to provide key on 12V+ for my fan relay (not load, just trigger). This way the fan won't come on if the truck is shut off.

 

Are you up to a barrage of questions?

Sure. It might be best to PM me and take it offline. Perhaps you can document rewiring the relay and put in the how to section. You can give me honorable mention :)

All right. Let's give this a shot.

Here is a follow up post for future reference since I finally figured this out.

First off a big thank you goes out to 1987Comanche for answering a ton of questions in my attempt to decipher OE wiring diagram

How these Bosch style relays are supposed to work:

I started to type a description of how they work but this was better.

http://www.bugmanweb.com/gsxr/relays.html

As stated in earlier posts, my problem was that when I turned on the a/c in my truck the aux cooling fan did not turn on with it. The cooling fan is supposed to turn on when either the temp switch closes or you switch on the a/c.

I drew up a simple diagram in microsoft paint to help see where all the wires were connected without having to physically trace them every time in the truck. Keep in mind the wiring in my truck is not OE, somebody else wired it when A/C was added.

Here is that diagram.

Ok so our knowledge of relays tell us that pin 30 is constant power, 86 is switched power, 85 is ground, and when 86 is energized, 30 essentially connects with 87.

On my diagram you need to know what the four wires off the connector are. I figured out (by poking around with a volt meter) that orange is power when the truck is running, green is power when you switch on the a/c, and blue is ground.

If you follow the power around through the wiring it actually makes sense (keep in mind that the a/c low pressure switch is normally closed) and in fact the fan should turn on when you switch the a/c on. Because when you turn on the a/c in the cab and power is routed to the a/c clutch it would also be sent to pin 87 of the fan relay which is the power for the fan. Well guess what, it wasn't getting power. I figured out that the diode was bad and it wasn't allowing any power though.

Basically diodes work like a one way valve, they allow current to flow through but not back out.

Now the reason the diode was there to begin with is that if the temperature were to close because the truck was beginning to overheat power would also be sent back to the a/c clutch and turn it on too.

I didn't like the idea of a diode on the main power circuit so I added a diode I got off a cherokee from the junkyard. This diode has two inputs and one output. The middle wire is the output. Test yours for continuity to make sure.

This is what I came up with and I works great so far! I may add a manual in cab switch later on.

Keywords for searches aux auxillary auxiliary cooling fan A/C air conditioning zener