So I removed the massive mechanical fan from my radiator and put on 2 slim electric fans. I welded in a bung and put a temp switch into the radiator to trip the relay for my fans. Works great under normal use.

Now I'm fixing the air conditioning as it's summer and hot. I can pull power from the air conditioning switch circuit to activate the fan relay any time the air conditioning is on and this works well for keeping the system blowing cold.

Here's the issue. The truck is a 94 1500 Blazer with the TBI and the 105amp alternator. With everything running, the alternator can't keep up at idle. New alternator. I don't sit at idle for more than a traffic light or two so it isn't long enough I discharge the battery and die. Once I'm moving at a brisk pace or on the highway, the fans are still running so the alternator isn't putting that surplus power back into the battery and it take a while to offset their constant draw. I know the new trucks have the ECU switch off the fans above a certain engine speed and I know it can be done with megasquirt as well. I also know cruise control won't activate below a certain speed.

I'm wondering if there is a circuit on the TBI cruise control unit that I could link into for either power or ground that A: becomes power or B: goes to ground at the speed that cruise control finds acceptable for use. I figure that would allow me to have the fans only run at a stop or low speed.

 

Why not just upgrade your alternator to a higher output one? I know it was really easy to put a ad244 145amp alt in my first gen. assuming it would be similar in the full size blazer

 

Well, 2 reasons I guess. Top of the list, the one I have is almost new. Put it on just a few months ago. Secondly, there is no need to run the fans once I'm running down the road.