LS 4040H AC Issue

Dandolore · Oct 4, 2022

Hello I'm having trouble locating an Issue with my AC unit. AC runs for about 5-10 minutes when cold before blowing the 10amp AIR CON fuse. And when the unit is all warmed up AC doesn't run cold for more then 5 minutes. I've chased all the grounds and made sure there is good contact. The ground wire running from the compressor had 2 spots with exposed wire. One was repaired the other was to close to the AC clutch and I used liquid tape. I was told the the refrigerant levels are appropriate. So far I've taken off the cab roof to see if there was any visible issues. I've ordered order replacement HVAC relay, Thermostat, and resistor(because I'm already there).

Anyone have similar experience and can guide me a little better?

RustyA · Oct 4, 2022

So each time the ac quits after 5 minutes or so the same fuse is blown? If that's the circuit that provides power to ac clutch I'm guessing you have a resistance problem between the switch and the ac clutch. Very likely in the area you had bare wires?

alexpops · Oct 4, 2022

You sure it's a ground and not a hot wire? Ground to ground won't cause a short, but hot to ground will. Were the rubbed wires intact? Lose to much copper & it could cause resistance/voltage drop issues that increase amperage & blow fuses. I'd run the chafed wire, see if there are other damaged spots & repair replace as feasible. If the wire is ok, check resistance across the clutch coil against specs when hot and cold. You may have overheated the coil & developed an intermittent short that only shows up when things heat up, expand, and make contact via burnt coil insulation.

Dandolore · Oct 4, 2022

alexpops said:
You sure it's a ground and not a hot wire? Ground to ground won't cause a short, but hot to ground will. Were the rubbed wires intact? Lose to much copper & it could cause resistance/voltage drop issues that increase amperage & blow fuses. I'd run the chafed wire, see if there are other damaged spots & repair replace as feasible. If the wire is ok, check resistance across the clutch coil against specs when hot and cold. You may have overheated the coil & developed an intermittent short that only shows up when things heat up, expand, and make contact via burnt coil insulation.

I've pulled as much wire to view hot and ground. I'm not seeing any other compromised areas. I haven't unravel the wire cover/conduit to see if there is compromised wires covered up. I'll check the coil resistance on the clutch and get back with you. >< hopefully I can check it without removing the whole compressor. But I doubt it ><. Thanks @alexpops .

Dandolore · Oct 5, 2022

alexpops said:
You sure it's a ground and not a hot wire? Ground to ground won't cause a short, but hot to ground will. Were the rubbed wires intact? Lose to much copper & it could cause resistance/voltage drop issues that increase amperage & blow fuses. I'd run the chafed wire, see if there are other damaged spots & repair replace as feasible. If the wire is ok, check resistance across the clutch coil against specs when hot and cold. You may have overheated the coil & developed an intermittent short that only shows up when things heat up, expand, and make contact via burnt coil insulation.

So the Ohms read 3.4 cold and bounce over 5ohms hot

alexpops · Oct 6, 2022

NO AC on mine, BUT ... Per my service manual (4100 series), field coil resistance should be between 3-3.5 ohms. The troubleshooting guide (see attached) indicates a field coil short outside that range, & recommends replacing the compressor. Do you have a shop manual for your 4000 series to check the specs on your set-up? Also indicates a potential wire/relay defect (which you have already discovered some).

Dandolore · Oct 6, 2022

alexpops said:
NO AC on mine, BUT ... Per my service manual (4100 series), field coil resistance should be between 3-3.5 ohms. The troubleshooting guide (see attached) indicates a field coil short outside that range, & recommends replacing the compressor. Do you have a shop manual for your 4000 series to check the specs on your set-up? Also indicates a potential wire/relay defect (which you have already discovered some).

I do have a manual. I just removed the compressor. I'm attempting to find a replacement that's not $600 before shipping. Or a possible universal AC compressor if that's a thing.

alexpops · Oct 6, 2022

If you have a good "parts" guy, he can cross reference & find one that will work. Was much more common in the 70's, but there are still a few old school guys around that will do more than look up a vin number & say that they don't stock the part. Try a NAPA unless you have an old hot rodder supplier near you. After NAPA, Advance Auto, any independent, & Autozone is last on my list. If you get a cross referenced part number, check RockAuto they have ok pricing & "universal" AC parts, but you need to know what you're looking for if it isn't in their year/make/model database.