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Old 07-13-2021, 11:51 PM   #1 (permalink)
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I need a bit of scrappy electrical help to get my AC running again

So, things have happened and I am often employed in a pretty hot place, out in the elements, which is medium physical. For the first time ever, I genuinely want my AC to work, and it hasn’t for about a year and a half.

Not sure how to describe what my diagnosis process has been so far. I did a basic resting pressure test yesterday, 100psi at 80*f but after driving it and getting the engine warmed up. It may be low on refrigerant but not low enough for that to be my main issue. Basically I have traced it, so far, to what is marked as the condenser fan fuse, a 20a circuit in the underhood fuse and relay box. It feeds the load side of not only the condenser fan relay, but the AC clutch relay as well. My own paranoia led me to swap relays while I (wastefully) idled the car on my one known good circuit for the cooling fan. No surprise as I got a click from all of them in any position, and all of them ran the cooling fan when requested by the ECM. I pulled the dash fuse for the AC, a 7.5a and it visually checked out. The same paranoia prevailed, so I ohm’d it out and it had continuity.

It had a 30a fuse in the underhood box for the condenser fan already (bought it this way), which was blown. Key on, engine off, a fresh and correct 20a fuse brought the condenser fan online when the blower was turned on and the ac button was activated. However, start the car, and the 20a instantly blew. Car still running, I pulled the 20a and inserted another which again blew the moment it made contact.

I went against my own advice and put in another 30a, curious as the AC worked for a year and a half on that 30a and the car didn’t burn down. Fuse inserted, voila, I got condenser fan and the ac clutch engaged. Compressor seemed to spin with no noise. But that lasted about, eh, 20 seconds.

At this point, if something which is supposed to be nerfed at 20a blows a 30a fuse that quickly, something bad is afoot.

I basically have three possibilities:

Condenser fan is drawing too much current; I’ll have to hand spin the blades tomorrow as a basic check to see how freely it spins. Given that it ran KOEO quite well, I’m fairly confident that it isn’t the problem.

Compressor clutch coil is drawing too much current: this seems borderline. It should kinda act normally until the engine warms up and pours heat into the circuit, and it has to cycle again and again, building up its own heat.

A short to ground in either one of those circuits. This seems the most likely, and not having isolated either circuit, I suspect the compressor clutch circuit given that engaging it is what leads to the swiftest death of the fuses.

Given that I don’t have a scope with amp clamp, any relay jumper tester things (the relays are fine), and I don’t have a Power Probe, how do I go about testing this further? The equipment I do have is the ol’ “scope on a rope” test light, and a fairly basic autoranging multimeter.

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Last edited by jcp123; 07-14-2021 at 12:31 AM..
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Old 07-14-2021, 08:50 AM   #2 (permalink)
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Given that the circuit hat a 30 amp fuse rather than the 20 it was designed for would seem to indicate a previous problem. See if you can obtain a schematic diagram of the circuit.

Might try disconnecting the fan and clutch separately to see which blows the 20 amp fuse and or removing the relay separately.
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Old 07-14-2021, 08:00 PM   #3 (permalink)
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Sounds like A/C compressor clutch is shorted.
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Old 07-15-2021, 01:12 AM   #4 (permalink)
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Quote:
Originally Posted by nemo View Post
Given that the circuit hat a 30 amp fuse rather than the 20 it was designed for would seem to indicate a previous problem. See if you can obtain a schematic diagram of the circuit.

Might try disconnecting the fan and clutch separately to see which blows the 20 amp fuse and or removing the relay separately.
Yeah the 30a fuse being in there from purchase definitely gave me reason to believe something was already going on. Isolating the loads is a great idea. I’m a bit embarrassed I didn’t think of that one.
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Old 07-15-2021, 01:16 AM   #5 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Sounds like A/C compressor clutch is shorted.
I donít have much reason to disagree. But Iím not that smart about electrical stuff. My understanding is that a failing clutch coil makes for intermittent, but not constant fuse blows, but obviously if thereís a short it doesnít take much time at all to see a fuse getting blown.
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Old 07-15-2021, 02:44 AM   #6 (permalink)
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A/C compressor clutches are supposed to fail open, so you just don't have the A/C come on but everything else still works. But that doesn't always happen.
If I remember correctly a typical A/C clutch coil gives around 10 ohms of resistance. It it's blowing a 30 amp fuse right away then it's got to be less than 1 ohm.
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Old 08-12-2021, 11:36 PM   #7 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
A/C compressor clutches are supposed to fail open, so you just don't have the A/C come on but everything else still works. But that doesn't always happen.
If I remember correctly a typical A/C clutch coil gives around 10 ohms of resistance. It it's blowing a 30 amp fuse right away then it's got to be less than 1 ohm.
Iím lazy so havenít followed up, but as both the condenser (not the radiator!) fan and the clutch are both on the same circuit, I have to narrow it down to one. And Iím not that smart when it comes to electrical.
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Old 08-13-2021, 12:13 AM   #8 (permalink)
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It's easy, get some fuses, unplug all of the suspected bad components, tuen on the A/C, then keep plugging things in till the fuse blows.
If a fuse blows with everything unplugged then you really have problems.

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1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.
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