Hey guys, I was chatting with ya'll about my '79 with a 350 a while back, anyways I converted it over to a serpentine system from a 91 camaro (with 91 components) to get A/C. Works great but here's one weird problem. I knew I needed an idle up solenoid to compensate for the compressor being engaged so I went out and got one and tuned it to about 550 - 600 RPM with compressor on, and my idle screw to a little over 500 RPM without the compressor. Now here is the problem:

I can drive it around all day without the A/C on and I will get no warning lights, but if I drive around with A/C at city speeds for about 15 minutes, when I come to a stop light the red "idiot" light comes on. If I rev it up it will go away but it takes about 650 RPM to make it stop.

My first reaction was to just turn the compressor off, which works momentarily and then the light start coming on intermittently again. The only thing to make it go away from this point forward is to put in neutral, hold the brake, and give it some gas.

The belt is routed correctly, seemed to be no problem with the 91 A/C compressor??

Im stumped, anyone have any ideas?

 

How many miles on the engine?
Either your engine is badly worn internally, having excessive bearing clearances, which results in having low oil pressure (your light going on).
Or, your oil sending unit is going goofy.
Oil pressure and engine rpms' go hand-in-hand. As rpm's go up, so does oil pressure. As rpm's go down, so does oil pressure. Both high and low pressures have a certain limit, with a healthy engine. Oil pressure doesn't start at 0 at low rpm and climb to infinity. But if engine wear is your cause of the oil light, it's because you're not getting enough oil pressure at idle.
You can replace the old sending unit with the hopes that that's the problem. Or you can hook up a mechanical gauge, which is more accurate than an electrical warning device. That's why gauges are a nice thing to have, you can see what the engine is doing before an idiot light goes on.
By the way, speed up your idle to at least 750, or even a little higher. Where you have it set now is too low to have with an automatic trans.

 

Not even sure, they guy I bought it from pulled it out of an old Buick. I do need the gauge so I can see whats going on, is there on that can just pop in place of the sending unit I have? Cause I bought one but then found out that it used a tube test the pressure and I dont want a tube of oil coming into my cab with a new dash!

So 750 RPM idle with and without A/C? I'll give it a try, just dont want to have to do an overhaul yet on this sucker!

Thanks

 

750 is a good idle with A/C off, and your step up solenoid should compensate for the rpm drop when the A/C switches on.
Where is your oil sending unit now, left side of block or behind the intake manifold? You can install a gauge at either spot, either by replacing the idiot light sending unit, or connecting both with a "T", or keeping the idiot one where it is and connecting the gauge to the other hole.
A mechanical gauge isn't going to be a problem as long as it's installed correctly. And you want to use COPPER tubing. DON'T use the cheapo plastic tube that comes with most gauges. Those can kink, crack, melt, or break, and then you'll have a mess.

 

Yeah it does look cheap, I think it's on the left side of the block unless that is the water temp (cant find that thing! But I know it works cause I overheated a while back with bad thermostat)

Let me try bumping the idle up and I'll post back, it may help.

 

I have a 79, 350 myself, and the oil pressure gauge location is at the back of the intake in the block, and the water temp guage is in the side of the block, hope this helps ya.