1988 300 with ac. I had this issue last summer but never really found a fix. Ac works great. Normally the gauge will sit at about the n in normal. When idling with ac on it will get close to the m. Fan clutch in new replaced that last year. Any ideas?

 

Flush ur cooling system. How old is the thermostat? What temp is the stat? Does the rpm go down when u turn it on?

 

Our 300 does about the same, it's an old work truck and I think the radiator is restricted with years of build up. Boys like to puddle jump so I did hose it down and that helped.
I've thought to drain it and flush with chemical but haven't got that far yet.

 

Cooling system was also flushed last year. Thermostat was replaced at the same time. 180 temp. Yes the rpm does drop when you first turn it on. And changes when compressor cycles if I remember correctly

 

How many amps is ur alternator? What does the rpm drop to? And how long does the compressor cycle. Does it just over heat with the ac on at idle or with it driving also? Do u have a shroud?

 

It's a stock alt rebuilt. 60 amps I think it is. It's the old 2g. Compressor cycles quicker than it should. Every 15 to 20 seconds or so. It only overheats with ac at idle. As soon as you get moving again it'll drop back down to normal operating temperature. That's what made me believe it was the fan clutch. I have a shroud but it's a little broken. I forgot about that actually. That may be the ptoblem. But I'd still like to figure out why the rpm drops with the compressor

 

AC is taxing on the alternator and if is 60 amps alternator, it is really weak. What are the volts before and after the rpm drop? When was the last time the ac was serviced? Could just could be low on refrigerant. Duct tape that shroud.

 

On max ac fan on high. It drops from 14.5 to about 12. I filled refrigerant a couple of months ago. It has a leak I just haven't done anything about it

 

Did it work good after the refill? I think it is just low on fluid. Find and fix the leaks. Finding the leaks is hard to do on the ac system. Mine don`t work either but I live in Alaska but wish it did on some days. My truck is black so it gets hot.

 

No since I've had this truck the ac always makes it overheat at idle. Never found a fix for it. Maybe a better fan clutch? And the shroud too. I haven't bothered to fix the leak because it will last through the summer. And it'll need recharging by next year. The ac system was converted from r12. But it's still got everything there from the old system. I haven't changed anything