AC circuit causes power surge??

[Wishmeluck]

84 Bronco, 351w...I'm not sure if this has been happening all along or if I just noticed it because I was tinkering around in the dark last night. But, when I move the ac/heater selector switch back and forth, anytime it switches off the ac circuit (only off) when the compressor is cycling, the rig has a blip of a power surge causing the dash lights (blinkers, brake) to flash, gauge needles to slightly jump, and I can hear the fuel pump start to prime (I switched to electric with a primer relay)...all for just a split nano-second, and then everything works fine (its not even blowing fuses). It also doesnt do it when the compressor kicks on and off by itself once the selector lever is in the ac mode or mix/defrost, ONLY when the selector is turning off the ac system and the compressor is cycling.

I unplugged the pressure switch and compressor leaving just the heating elements/blower motor and no surge moving the selector. Plugged back in the pressure switch, and all was still ok (obviosly the circuit wasnt complete but no short on power side at least). Plugged back in the compressor to complete the circuit and the surge came back again when sliding the lever to any position turned off the ac circuit.

I checked for loose grounds, bad connections, exposed wires, etc and didnt see any. Anyone have any ideas on other things to check?? Would a bad pressure switch or compressor cause a power surge like this? What am I missing??

 

[L\Bronco]

Probably nothing.
When you move the selector and the A/C clutch comes on it draws about 5-8 amps, that will always cause a slight fluctuation in the electrical system. Is there a symptom? Or you just notice a surge?
Likely normal.
Cheers

 

[Tiha]

I agree, maybe a weak alternator? Weak battery? bad battery connection. Something like that. Being able to see the lights or gauges blip when it comes on is normal, but shouldn't be very drastic.

What happens when you hit the brake pedal with the headlights on? or flip the blower motor to high speed?

 

[Wishmeluck]

"What happens when you hit the brake pedal with the headlights on? or flip the blower motor to high speed?" - they work like normal with minimul fluctuation to voltage but no other power issues.

I understand when you turn things on/off you'll see things up/down tick a little. Heres the breakdown:

- With the engine running and the ac compressor cycling, if I move the climate control selector switch to a position that cuts power to the ac compressor it causes the right interior blinker bulb on the dash to flash, and all the guages to jump a slight bit. The blinker bulb flashing seems odd to me but I can live with it.

- When the key is in run mode with the engine not running and ac compressor not cycling, when I move the ac selector switch anywhere that cuts power from the ac compressor it causes the right blinker bulb to flash, the tach to jump, and the fuel pump to prime the lines. This isnt just when I initially turn the key to the "run" position...its literally anytime I move the ac selector switch to any position that cuts off power to the ac circuit. Maybe Im crazy, but it seems to me like the climate control slider/switch should have anything to do with tach movement or fuel pump priming when the engine isnt running and the key isnt being moved to trigger power to the primer relay switch.

Its not raining today (miraculously) so Ill go out and check my grounds again and do some more testing.

Thanks

 

[L\Bronco]

"What happens when you hit the brake pedal with the headlights on? or flip the blower motor to high speed?" - they work like normal with minimul fluctuation to voltage but no other power issues.

I understand when you turn things on/off you'll see things up/down tick a little. Heres the breakdown:

- With the engine running and the ac compressor cycling, if I move the climate control selector switch to a position that cuts power to the ac compressor it causes the right interior blinker bulb on the dash to flash, and all the guages to jump a slight bit. The blinker bulb flashing seems odd to me but I can live with it.

- When the key is in run mode with the engine not running and ac compressor not cycling, when I move the ac selector switch anywhere that cuts power from the ac compressor it causes the right blinker bulb to flash, the tach to jump, and the fuel pump to prime the lines. This isnt just when I initially turn the key to the "run" position...its literally anytime I move the ac selector switch to any position that cuts off power to the ac circuit. Maybe Im crazy, but it seems to me like the climate control slider/switch should have anything to do with tach movement or fuel pump priming when the engine isnt running and the key isnt being moved to trigger power to the primer relay switch.

Its not raining today (miraculously) so Ill go out and check my grounds again and do some more testing.

Thanks

Definitely not “nothing” I’ll look at the wiring diagrams for a potential cause when I get home tomorrow. (At the Phoenix nationals last weekend)
Sounds like a short inside the heater controls on the circuit that energizes the A/C (I just need to see the map so I can formulate a theory)
Cheers