Gauge fuse blows when turning on AC (1 Viewer)

[Mr Cimarron]

Here's the story. I have had working AC post diesel conversion so I know it has worked. Since then I have replaced the AC compressor with a new one due to the old one seizing and destroying itself. Today I got the AC charged back up but when I push the AC "ON" button it blows the fuse for the dash gauges. So here is what I have troubleshooted so far. The following tests were all done turning the key without starting the truck.
AC button pushed on. Blows gauge fuse immediately. Replace fuse.
AC button off. HI/LOW fan buttons pushed. Fuse is good.
AC button pushed on w/ compressor disconnected. Fuse is good.
AC button pushed on w/ AC pressure switch disconnected. Fuse is good.
AC button pushed on w/ compressor ground wire disconnected. Fuse is good.
AC button pushed on after reversing the two wires at the compressor. Gauge fuse blow immediately. Replace fuse.

So I am stumped. I have a multimeter but am a real dumb s*** when it comes to knowing how to use it properly. Electrical has never been my specialty. I am in the process of looking through the EWD but I am hoping smarter people may have some knowledge on this.

 

[LandCruiserPhil]

Fuse size you are blowing?

 

[Mr Cimarron]

Fuse size you are blowing?

7.5 was in there to start. The shop blew a 10 and 15 before they decided to stop. I personally have only been using 7.5s when testing.

 

[blkprj80]

My AC has two wires at the compressor for the clutch. One is grounded right at the compressor housing and the other goes to the wiring harness. If you put 12V on the side that comes from the wiring harness, the clutch should click and engage. It's very simple.
Like Phil asks, what value fuse are you blowing? < edit saw your reply, don't go any higher than 10, and more better stay with the 7.5A.
I'm wondering if you have a shorted AC clutch electromagnet or a wire is pinched under the pulley somewhere. Can you measure the resistance? It should be a low number I believe.

 

[LandCruiserPhil]

I would also start to think the clutch electro magnet was bad(going to ground) but not blowing the fuse with the ground disconnected voids that. Just to be sure with the ground disconnected from the compressor and cleared(cap off) voltage applied to the compressor DOES NOT blow the fuse?

If the above is in fact accurate if when you ground the ground wire from the compressor and the fuse does blow it would indicate the load exceeded the fuse size. If the electro magnet is isolated it could be shorting internally. If a Toyota compressor you should be able to look up the resistance of the electro magnet to determine if it is bad.

 

[blkprj80]

I just measured my coil with an ohmmeter. I got 4.1 ohms, and 0.8 with my leads shorted, so the coil is around 3.3 ohms. So it should draw around 3.6 Amps through the coil if all is well.

It's looking like the coil is internally shorted, not to ground or chassis though. That sucks...

 

[LandCruiserPhil]

I just measured my coil with an ohmmeter. I got 4.1 ohms, and 0.8 with my leads shorted, so the coil is around 3.3 ohms. So it should draw around 3.6 Amps through the coil if all is well.

It's looking like the coil is internally shorted, not to ground or chassis though. That sucks...

↑This↑

FSM does not show test resistance. FSM test procedure - Hit it with 12volt and if it does not engage replace

 

[Mr Cimarron]

I have been able to engage clutch by jumping straight form the battery if that matters.

 

[blkprj80]

Yes, that matters a lot. Can you measure the resistance of the coil?

How are you jumping straight from the battery? Piece of wire from + battery to the single pin connector for the clutch while the other side ring terminal is connected to ground/chassis?

 

[LandCruiserPhil]

I have been able to engage clutch by jumping straight form the battery if that matters.

Something is not adding up.

Confirm with the compressor grounded and 12 volt to the compressor fuse blows

blkprj80 got this, not to make things more confusing Ill watch

 

[Mr Cimarron]

Yes, that matters a lot. Can you measure the resistance of the coil?

How are you jumping straight from the battery? Piece of wire from + battery to the single pin connector for the clutch while the other side ring terminal is connected to ground/chassis?

Yes, I'm using jumper wire from compressor to positive on battery and other wire to chassis. Same location where the ground terminal is secured to bolt on compressor mount for ground. How/where should I measure resistance with my multimeter? On the connector that the compressor plugs into? Sorry I'm a dummy with the multimeter.

 

[blkprj80]

Yeah, measure resistance (ohms setting) from the connector from the compressor to chassis. You want to measure the coil in the compressor, not the electronics in the dash, so you want to disconnect the wire coming from the fender.

 

[Mr Cimarron]

Yeah, measure resistance (ohms setting) from the connector from the compressor to chassis. You want to measure the coil in the compressor, not the electronics in the dash, so you want to disconnect the wire coming from the fender.

I got 3.8 if I measured correctly. If I did I will chalk that up for knowing something new.
On another note I ending up destroying the crappy pigtail bullet connector that came with the compressor by taking it off so many times. I cut off the wire to add some better connectors and while I was at it I opened up the sheathing to check the thingy that goes between the two wires. Looks pretty fried. I investigated this after the last compressor bit the dust and noticed it was actually split in half. Any connection here to anything?

 

[blkprj80]

Hehe... That was a back-EMF diode that they usually put across a coil so it doesn't kick back twice the voltage when it shuts off.
When you reversed the wires, you killed this diode, and likely shorted it.
Cut it out of the circuit and try your compressor again. Chances are you won't fry your AC amplifier, but if you're afraid, don't try it.
The diode will be a typical 1N4001 (or 1N400x) rectifier diode. Put the cathode (stripe) on whichever wire will be +12V.

Good job measuring the resistance! (need a "sparky" icon here...)

Is this some aftermarket compressor/clutch since you swapped in a diesel? I'm not sure Mr. T even uses a diode, so it may not even be needed at that location. I'll look at the EWD and see if there are any answers.

Edit: EWD doesn't show a diode across the magnetic clutch. That doesn't mean it isn't there, just that it isn't shown. Chances are Mr. T put the diode in the amplifier module. Not sure.

 

[Mr Cimarron]

Hehe... That was a back-EMF diode that they usually put across a coil so it doesn't kick back twice the voltage when it shuts off.
When you reversed the wires, you killed this diode, and likely shorted it.
Cut it out of the circuit and try your compressor again. Chances are you won't fry your AC amplifier, but if you're afraid, don't try it.
The diode will be a typical 1N4001 (or 1N400x) rectifier diode. Put the cathode (stripe) on whichever wire will be +12V.

Good job measuring the resistance! (need a "sparky" icon here...)

Is this some aftermarket compressor/clutch since you swapped in a diesel? I'm not sure Mr. T even uses a diode, so it may not even be needed at that location. I'll look at the EWD and see if there are any answers.

Edit: EWD doesn't show a diode across the magnetic clutch. That doesn't mean it isn't there, just that it isn't shown. Chances are Mr. T put the diode in the amplifier module. Not sure.

Yes it's a non OEM Dodge compressor I got at O'Rielly's when the last one blew up on the beach. I'll get another diode before I try again. I'm sure our engineer at work has one I can steal. I sold my spare amplifier so I don't want to do anything to the one I have. It's a good stopping point for the evening anyway. I'll try again tomorrow evening. Thanks for all your help. I really appreciate it.

 

[Mr Cimarron]

WE HAVE A/C!!!!!
Replaced the diode, made a new pigtail, and it works like a charm. At least for now. Thank you @blkprj80 and @LandCruiserPhil for walking me through the trouble shooting. It's always a better day when you learned something new. Thanks again.

 

[TrickyT]

7.5 was in there to start. The shop blew a 10 and 15 before they decided to stop. I personally have only been using 7.5s when testing.

Glad you got this sorted out. BTW, the EWD shows the gauge fuse should be 10A.

 

[Mr Cimarron]

Glad you got this sorted out. BTW, the EWD shows the gauge fuse should be 10A.

Good catch. I'm guessing I used a 7.5 at some point in the past when I was out of 10's in the garage.