My heater blower motor fuse keeps blowing every time I select the a/c on.

I found that if I disconnect the harness connector for the a/c control switch (the multi-position lever selector for Max, Normal, Bi-level, and Vent), I could turn the ignition on and the fuse wouldn't blow. I ran 12v directly to the blower fan and it ran fine.

Suspecting it might have something to do with the a/c control switch (the round one with three little springs inside), I pulled it apart and cleaned the inside out. Unfortunately, I'm still blowing fuses.

During troubleshooting, I checked the plug going to the a/c compressor clutch and noticed that both leads were grounded. I'm not a mechanic, but I don't think that's supposed to happen.

Aside from a bad a/c control switch, is there anything else that would likely cause a ground/short like this? It's hard to trace the green and black wires as they disappear into the injector harness toward the firewall.

Any suggestions on where to go from here? Thanks in advance.

 

Probably has nothing to do with the switch in the dash. It's virtually impossible for that to fail in such a way as to cause the observed symptom.

More likely, it's the diode across the terminals that plug into the compressor.

Unplug that plug from the compressor. Does the fuse still blow?
Yes: it's something other than the clutch. Leave it unplugged and continue to the next step.
No: clutch is bad, replace it.

Unplug the harness above the pass side valve cover with the green wires in it. Does the fuse still blow?
Yes: continue on.
No: check the diode, that's probably what's bad

Look VERY carefully at the plug you just disconnected. How bad burned up is it? Is it to the point that the green wires can touch the black wire? (if there is one... this detail varies from car to car)

Cut the black wire if present, right next to the connector, but far enough back to avoid any burned-up section. Put a nice heavy-duty yellow ring terminal for about a 5/16" stud on it, and ground it directly to the firewall, under the same screw with the ground strap from the pass side head.

Are the red wires in the above plug wasted too? Whether they are or not, cut them both, and put a pair of the BIG MEATY HEAVY-DUTY yellow slide terminal on them, preferably insulated if you can find them. NOT, the little weenie ones that are just like the blue and red ones; look for the BIG THICK MEATY HEAVY-DUTY kind. If you can't find em insulated, tape em up real good after you pug them together.

Tell us what you find and we'll tell you what to do about it.

 

Thanks. I replaced the compressor yesterday, so the clutch is new. It did the same with the old compressor too.

I spliced the green and black wires for the compressor clutch about half way back into the harness where they are still in good shape. I am still getting a ground on both wires.


When I unplug the compressor clutch, I can run the fans okay, but if I select anything that calls for the a/c compressor, it blows the fuse.


Where is this harness. Is it back near the blower fan? Where is the diode?

 

It is:



It is:

 

I'm not seeing any harness above the passenger side valve cover other than the two plugs that go to the compressor (one to the clutch the other to the sensor in the compressor).

I cut the wires going to the clutch and checked them individually. They are both grounded, so wouldn't that rule out the plug or diode on the clutch plug?

 

There is a harness with a connector above the pass side valve cover; near the firewall. It has at least 2 green wires and one big fat red one, and maybe a black one.

Why on earth did you do THAT??? That's about the single WORST thing you can do to your car. Don't do any more of that unless you ABSOLUTELY HAVE TO.

No... if they're still hooked to the clutch, then the clutch itself is a very low resistance between the green wire and ground, and your meter wouldn't be able to tell the difference between that and a short.

Put your dykes away and go back to the troubleshooting instructions I gave you. They will take you RIGHT STRAIGHT TO the source of the trouble. It's just electricity, the simplest stuff there is; and in a car, which is about the 2nd simplest form of electricity that there is, right behind a flashlight. It's not hard to figure this stuff out as long as you use logic and reason, and don't go creating extra hassles for yourself by doing non-commonsensical things like needlessly cutting wires.

 

I suggest pulling out your climate control panel and inspect the wiring in the plug-in for damage. The 14ga brown or yellow (cavity D of the 5-wire connector) wire supplies power to everything in that panel and I've seen over time where it can literally melt the connector and arc to the wire next to it.
If that all appears OK, you may need to check for a short to ground. To do this, you need a multimeter but its hard to describe the process in this format-hate to start replacing parts too tho. Its also a possibility your blower motor relay is shorted out internally. The 4-wire connector mentioned earlier is another thing to inspect-it is located above or behind your passenger side valve cover. You'll see a 12ga red, 14ga black, 14 Lt green and 14ga dark green wires. Again, over time, that red wire feeds power to the blower relay and will melt the connector shorting out against the other wires. You need to find this connector and inspect it for damage. GM sometimes wraps it up in a black duct tape type of wrap which hybnow will be all brittle with age and years of under hood heat. In order check:

1.) AC 4-wire harness connector
2.) Replace (its cheap) blower relay
3.) Inspect climate control panel connector

Let us know what you find.

 

Has nothing to do with the blower motor. Fuse only blows when he turns on the compressor.

Not the blower motor, relay, or harness.

It's most likely the diode across the terminals connector that plugs into the A/C clutch. And most especially, since that just got worked on (he says his compressor is new); if I was the betting kind (I'm not) my money would be on th ediode itself not being bad at all, but rather, it's twisted up or whatever, such that its leads are touching.

See my signature for a mental approach to troubleshooting that works almost every time.

 

Has nothing to do with the blower motor. Fuse only blows when he turns on the compressor.

Not the blower motor, relay, or harness.

I'm not saying it isn't the diode however, for its leads to be touching, it would have to be broken for that to happen. For you to suggest it can't be anything other than a bad diode is absurd. If he had this problem with his old compressor as he stated, the diode may not be bad at all. If the climate control harness has nothing to do with this, where then does the power come from to run his compressor....assuming his diode is not to blame?

 

Thanks, guys. I think it's either the a/c control switch in the cockpit or there's a short somewhere in the wires going to the a/c clutch. I was able to pull off the connector for the a/c switch in the cockpit and bypass it (jumped the two terminals on the left like the switch would do). I also pulled off the blower fan switch and supplied 12v to it, jumpering another wire to get the high fans to work. With the fans working okay like this, I simply ran 12v to the a/c clutch solenoid that is powered with the ignition switch. So, now I have high fans and a/c and it's not blowing the fuse.

So, it's got to be related to the fact that when I check the plug going into the a/c clutch, both wires are grounded. I previously cut into those wires further back in the large harness along the valve cover to see if the short was caused by the exposed wires going to the clutch up front. The short still existed. I have since re-joined those wires.

At least I've got a/c now and I can take my time troubleshooting. I'd like to get a new a/c control switch but I think they are hard to come by.

Also, Formula1LE said the 4-wire connector "is another thing to inspect-it is located above or behind your passenger side valve cover. You'll see a 12ga red, 14ga black, 14 Lt green and 14ga dark green wires. Again, over time, that red wire feeds power to the blower relay and will melt the connector shorting out against the other wires." It does show signs of melting, but it doesn't appear to be shorted between any of the terminals.

 

UPDATE:

From the FSM:

With this in mind, I disconnected the harnesses to the Pressure Cycling Switch on the receiver/dryer, the A/C High Pressure Switch on the compressor, and the A/C Compressor Clutch (everything that could have caused the fuse to blow).

Then, I reconnected the harness connectors to the A/C Control Head and the Heater Blower Fan Switch inside the car. With the key on, I first checked for voltage at the A/C Pressure Cycling Switch on the receiver/dryer (there was 12V). I plugged that back in. Then I moved on to the A/C High Pressure Switch. There was 12v coming from that harness connector too. But this is where it got interesting.

When I measured the switch itself (with the harness unplugged), there was continuity between the two pin terminals on the A/C High Pressure Switch, as I expected. But there was also continuity between each pin and ground. That's why it was blowing the fuses -- It was a bad A/C High Pressure Switch.

Luckily, O'Rielly's had the switch and harness connector for $30. I had to uncharge the system to replace this plug-type switch, which is held in with a C-clip. When I pulled it off the compressor, I confirmed that the switch had in fact failed -- It was shorting to the outside of the housing.

I guess after 23 years and probably my fifth A/C Compressor, it was time to replace it. This is not something that comes with the new compressor. You normally just transfer the original switch over to the replacement compressor. So, this was an original part that had failed. Sure glad that's over with.

Thanks to both of you for your help and suggestions.

 

That's kind of where I was going with my logic-start @ the climate control power in (brown) and go out from there. The climate control ultimately powers the AC compressor via the light green 14ga wire. That pressure switch is wired in series between the AC low pressure switch 12V goes in from one side of the pressure switch (light green) then out on the other wire (dark green) from said cutoff switch to AC compressor. So, power for the compressor can only come through that switch as long as the high side doesn't exceed approx. 240? psi in which case the contacts should open effectively shutting the compressor off. So if you had continuity between either switch contact and ground, the switch indeed is bad. That switch (two wire variety) only serves to transfer voltage. Good job on the detective work! So get your system pulled down, charge and test it all again to make sure non other issues exist.

 

Thanks. I started where you suggested -- at the AC Control Head and then the big flat plug harness above the blower fan. When those components were ruled out, I decided to trace the power source from the Control Head to the AC Clutch to see where the problem was along the series circuit.

With regard to the diode mentioned above, is that part of the AC Clutch plug or does it live in the compressor itself. I may order a new plug if there's a diode in it.

I recharged the system and it seems to be blowing nice and cold now. Thanks again.

 

I've ran into this same issue recently and everything works fine as long as the high pressure switch on the compressor is unplugged. I measured voltage on that harness unplugged and I get 12v on one wire but not the other. Is that correct?

 

All the switch does is complete the circuit on its way to the compressor clutch solenoid. Normally that switch is closed and will only open when the pressure in the compressor is too high.

I'm not sure how you could be running with the switch unplugged and still have power to the clutch unless you jumpered the harness at the connector and just bypassed the switch. Maybe a prior owner jumpered it upstream of the connector and you haven't noticed. If they did this, the a/c clutch would have still worked (without the high-pressure protection) and may have later stopped working when the high pressure switch finally shorted to ground like mine did. In this case, if you unplugged the switch, it would have removed the ground/short and allowed the current to flow on to the clutch solenoid.

In any case, there should not be any grounding from either pin of that switch. No part of that switch should be grounding on any part of the compressor body. The only continuity that should exist is between the two pins themselves. The switch's job is to allow current to flow through from one pin to the other unless the pressure is too high. In that case, the switch will open and stop the flow of current to the clutch solenoid.

You can check the switch with an ohm meter -- the type that gives a tone when there is continuity. With one lead connected to ground on the chassis or battery, you should not have continuity (you should hear no tone) when you touch either pin on the switch with the other lead. If you do, the switch is bad and you will have to uncharge the a/c system to replace it. As I said above, there should normally only be continuity between one pin on the switch and the other pin so that current can flow through to the clutch.

Just follow the voltage: From the A/C Control Head, voltage is applied to the A/C Compressor Clutch through the Pressure Cycling Switch (on the receiver/dryer) and the A/C High Pressure Switch (in the compressor). Any shorts/grounds along the way will result in the clutch coil not engaging and a blown fuse when you call for a/c.

 

I actually got it figured out. It was also my high pressure switch. It wasn't grounding out but something was going on with it. Got that replaced also with a new compressor since it was time.

 

Well, hopefully this is not such an old thread I cannot get some help!! I have a 1990 Chevy Cheyenne with 5.7 liter engine, and the compressor high side low-pressure switch got broken. It was a yellow single wire switch, and the auto parts stores can only get a "upgrade switch" with two wires to it, and meaning I have to put a new pigtail with two wires on...my quandary is where the two wires now will go...??? HELP!!!!!!!

 

The downstream wire for the switch on my Z28 went back into the harness running down the right-side valve cover, so I don't know where it ultimately wound up; nor did I try to find out, once I determined it was the switch that was bad. Do you have a wiring diagram for your A/C?

The switch will not be the expensive part. The problem is that you will need to depressurize the freon system to replace it, and then recharge it. IIRC the switch was held in with a C-clip.

Maybe you can have the parts guy show you the whole replacement compressor for your truck to see if the pressure switch has only one lead or two.

 

Do anyone know what order the wires go in the 4 terminal blower motor relay? I have orange purple black and red. 3 goes straight across and one on the side.

 

https://drive.google.com/file/d/0B-W...ew?usp=sharing

 

Thanks! Worked like magic.