A/C fuse keeps blowing

[the911Tprocess]

I have a 72 911 with original A/C. When I turn on the A/C fan, it takes about 15 minutes before the fuse blows. If I run the A/C fan at a lower speed, it takes longer for the fuse the blow, if it is run at the highest speed then it takes less time.
How can I trouble shoot this, is it the a/c fan or?

Any advice is appreciated.

[mthomas58]

Is the fuse blowing (break in metal) or overheating, melting/shrinking and losing contact.

I recently had a problem with the latter after adding freon.....turns out the cause was the system being overcharged. Had system evacuated and recharged with the correct amount of R-12 and no problems since.

[the911Tprocess]

Hi,
I think the fuse is overheating since it takes some time before it blows, it does not look like a short. A short would cause the fuse to blow right away just after the system is turned on.

I believe the wire from the power source over heats and then blows the fuse since it takes about 15 to 20 min of operation to blow the fuse. Good thing the fuse blows or the wire would fry.

I guess I'll have to check the current draw from the fan motor or the compressor. What do you think?

[niner11]

Mark, I just wanted to clarify that having too much freon or 134a in a system should not have anything to do with fuses blowing or not blowing. Once the a/c clutch is engaged it is doing its job regardless of the the amount of freon in the system. If I'm wrong.....please someone correct me but I wouldnt want somebody to find this in the archives and think that it was true. Chuck

[mthomas58]

Check the condition of your block, fuse posts, wires and contacts for corrosion as resistance = heat.

Lots of content here.

Good luck!

New A/C Problem - melting fuses???

[dshepp806]

could be the "load" building heat over time thru too much current draw...

Definately rule out the possibility of what mthomas58 suggested.....(by following his advice, that is)...double check this first then proceed further.

Best,

[mthomas58]

Originally Posted by niner11

Mark, I just wanted to clarify that having too much freon or 134a in a system should not have anything to do with fuses blowing or not blowing. Once the a/c clutch is engaged it is doing its job regardless of the the amount of freon in the system. If I'm wrong.....please someone correct me but I wouldnt want somebody to find this in the archives and think that it was true. Chuck

Chuck, I understand what you are saying and this is still somewhat of a gray area in my mind . In my case, my fuse was not blowing....it was overheating and then melting/failing with the metal part of the fuse remaining in tact. The Porsche tech as the dealer said that an overcharged system will absolutely cause the circuit to run hotter than if charged with the proper amount of coolant. He stated that is is normal for the fuse block to get warm when running the A/C full tilt this time of year......but is should not "overheat" to the point of melting fuses (plastic type vs the OEM ceramic....which I have changed all mine to). BTW, this is an older/experienced tech and one of two who specializes in the older model cars.

All I can say is that ever since the evac and recharge at the dealership I have had ZERO overheating probs.

[mthomas58]

Originally Posted by niner11

Mark, I just wanted to clarify that having too much freon or 134a in a system should not have anything to do with fuses blowing or not blowing. Once the a/c clutch is engaged it is doing its job regardless of the the amount of freon in the system. If I'm wrong.....please someone correct me but I wouldnt want somebody to find this in the archives and think that it was true. Chuck

Chuck, it is certainly not my intent to mislead anyone here....... I think the final post in the thread by our resident A/C guru Jim Sims addresses the issue in question:

Per Jim:

"The compressor clutch is an on/off device, it's current draw isn't modulated by compressor load or pressure. There is nothing electrical in the mechanical part of the compressor beyond the clutch. If there is too much load the clutch may merely slip which will make frictional heat which can heat up the clutch coil but this will increase resistance and cause current to decrease. Clutch slipping can also be caused by an incorrectly set magnetic gap.

Compressor was running continuously or too long (with clutch slipping insufficient compression was occuring) and thereby heating the circuit too long or all the time. Correcting the refrigerant charge level likely solved the slipping problem but perhaps other problems remain:

Running voltage is too high (car's voltage regulator failing?) and this causes excess current draw for fixed resistance items like the compressor clutch coil.

Clutch coil has a internal damage (shorting) and resistance is too low causing it to draw excessive current.

Clutch magnetic gap is excessive (or amp-turns in coil reduced due to shorting causing less magnetic field to be produced) thereby preventing sufficient clamping to occur causing the clutch to slip under high load."

- Thanks Jim, you're the Man!!!!!!!!!!!!!!!!!!!!

[Barrpete]

Originally Posted by mthomas58

Running voltage is too high (car's voltage regulator failing?) and this causes excess current draw for fixed resistance items like the compressor clutch coil.

This was my problem exactly. Voltage regulator was bad. Replaced the VR & no more melting fuses.