Greetings,
Here's the problem: Whenever I run the A/C on my 1988 F150 351W at full, the voltage drops to 12VDC or less while at idle. (it will go to about 12.5 or 13 when at 1000RPM or more.) When this happens my turn signals blink about once every two seconds or so. I have replaced the alternator, regulator, battery and blower moter. If I connect the blower motor directly to the battery it only draws 15-20Amps and does not suck the voltage down any. So Im guessing that there is a problem with the power going to the blower through regualar means? Am I making any sense?
I greatly appreciate any ideas/suggestions!
Thanks much!

 

20 amps for a blower on high sounds correct. 20 amps is a lot of current, doesn't take much resistance to drop voltage a noticeable amount. For example, 20 Amps through a tenth of an ohm resistance would give a voltage DROP of 2 volts (V = I x R, so 20 Amps times .1 Ohm = 2 Volts). Every ground connection, every connector pin in the circuit would have to be clean and in good condition. It sounds like the load is dropping the voltage to your dash because there is a small resistance built up some where. It will be under an ohm, so a regular DVM may not find it. A tenth of an ohm is not much. Happy hunting, I don't envy you!