94 S-10 blowing ac/heat fuse-

fuses
airconditioning

#1

Have replaced the relay and it worked for about 45 days without blowing a fuse. Second replacement only lasted a few days. Inspected some of the wiring to compressor, but some of it is unreachable. Do not see any bare wires or anything obvious. Compressor about 3 months old. About to have it checked to see what is causing fuses to blow. Would appreciate comments about what could be causing this problem. Any ideas will be appreciated.


#2

Have replaced the relay and it worked for about 45 days without blowing a fuse. Second replacement only lasted a few days. Inspected some of the wiring to compressor, but some of it is not easily accessible. Do not see any bare wires or anything obvious. Compressor about 3 months old. About to have it checked to see what is causing fuses to blow. Would appreciate comments about what could be causing this problem. When temp controls are set to “vent” or “heat,” the fan will continue to operate. Fuses blow only when a/c settings are used. Any ideas will be appreciated.


#3

I don’t have a wiring schematic for your truck so I’m theorizing a bit.
Chances are both the compressor and the radiator cooling fans are operating through the same fuse and relay.

You could have 2 possibilities.
One is that there is an intermittent short in the compressor clutch coil.

The other, and the one I’m leaning toward, would be that the cooling fan is worn and dragging. When a motor is dragging the current required to operate it will go up, which of course translates into more heat. This in turn can blow a fuse or burn a relay up and it may take a while to do so as this is not a direct short, but a sustained high current draw.

Try spinning the fan blade with your finger and note if it turns easily. It should. The current draw could also be checked with an inductive ammeter.
Will rummage in some of my books this evening and ee if I can find a wiring diagram. Hope that helps.


#4

Beware that most inductive amp clamps only sense AC (and often quite inaccurately if the waveform is anything other than a 50Hz or 60Hz sine wave) since the coil only senses the rate of change of current in the wire, which must be integrated by an electronic circuit. Better amp clamps use hall effect sensors to directly measure the current, but they are expensive - about $80 is the cheapest one I could find.

When they replaced the compressor, did they flush the lines, replace the liquid line filter drier, and pull a good vacuum? If not, debris and acid left over from the old compressor can cause the new compressor to burn out.

Try disconnecting the compressor and protecting the wire terminals from shorts. If the fuse still blows, it’s elsewhere. If it stops blowing, either the compressor is at fault or the load of the compressor is just enough to add to an overload and blow the fuse.


#5

OP, you would be well advised to ignore any of Star’s ramblings about A/C repair; a subject of which he appears to know very little.
Forget this waveform business, A/C current change, micron gauges, and all of that other bilge.
It has zero relevance to the subject and only makes a difficult situation worse.

Star, for your info, I have both a Snap-On low current inductive ammeter and a Snap-0n high current inductive ammeter, both of which work fine and are accurate - on DC current, which as you may or may not know, this truck uses.


#6

No book for a '94. I think it’s loaned out (note to self).

However, I thumbed through some up to '92 schematics with different engine configurations and they all appear similar.

The radiator cooling fan does NOT appear to be in the same circuit as the compressor.
However, the cabin fan is on the same circuit as the compressor and should be a 25 amp fuse. That means it’s a heavy load circuit normally and if the cabin fan were worn and trying to seize up this could cause blown fuses and burnt relays.

Checking the cabin fan for excessive current draw involves the same procedure as the radiator cooling fan. No difference in methods at all.
Normally a cabin fan may draw 3-4 amps when it’s in good condition, but a worn blower motor can easily draw 8-9 or even more if it’s trying to hiccup now and then and seize up.

The blower motors are usually easy to pull. You could do this and note if the squirrel cage turns freely or not, and possibly replace that motor if it has never been done.

Hope that helps.