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

 

[Wishmeluck]

Update...I checked and cleaned all the grounds, traced all the wires, and didnt find anything exposed or shorted out that was visually noticable.

I unplugged both the AC pressure switch and compressor and all systems functioned normal.

I then plugged back in the ac pressure switch and jumpered the connections across the ac compressor connector just to complete the circuit, and all systems functioned normal.

Then, I unplugged the pressure switch and jumpered those connections in the connector, and plugged back in the compressor connector and the glitch was plain as day.

With that, I whipped out my trusty voltmeter to test the compressor coils resistance and I got a solid reading of 14. Apparently an ac compressor coil should have a resistance of between 2 and 7 ohms...which would mean my compressor is WAAAAY outta spec. and perhaps thats whats causing the ruckus...which would make logical sense (at least to me).

Can anyone confirm this?

 

[wyo58]

That definitely sounds like a bad compressor clutch coil, the compressor is probably okay. anything under 2 or over 5 means the coil is bad, it might even be back feeding to ground causing your weird stuff in the dash.

 

[L\Bronco]

Update...I checked and cleaned all the grounds, traced all the wires, and didnt find anything exposed or shorted out that was visually noticable.

I unplugged both the AC pressure switch and compressor and all systems functioned normal.

I then plugged back in the ac pressure switch and jumpered the connections across the ac compressor connector just to complete the circuit, and all systems functioned normal.

Then, I unplugged the pressure switch and jumpered those connections in the connector, and plugged back in the compressor connector and the glitch was plain as day.

With that, I whipped out my trusty voltmeter to test the compressor coils resistance and I got a solid reading of 14. Apparently an ac compressor coil should have a resistance of between 2 and 7 ohms...which would mean my compressor is WAAAAY outta spec. and perhaps thats whats causing the ruckus...which would make logical sense (at least to me).

Can anyone confirm this?

Hey Wish
That is a high number for sure, however, high resistance makes low amperage. That doesn't cause your symptoms at all. hook the leads of your ohm meter together and see if you have some meter resistance in there. High resistance will make the clutch weak, and you said it works normally, correct?
It actually sounds like the diode is open across the coil circuit. Its in there to suppress the voltage surge when the clutch coil de-energizes. (-300-600VDC)

It's taped to the harness a foot or so from the clutch plug. (Plastic block 1"X3\4"X3")
unplug the compressor, disconnect the battery, then measure between the clutch plug terminals (truck side). make one measurement, reverse the leads then measure again.
You should get a high number (maybe OL, and a low one). If you get 2 high numbers, the diode is open (or broken wire) and your surge is very likely from the collapsing magnetic field in the coil.
Hope that helps
Cheers

 

[Tiha]

i agree that is a high number.

Also as a rule of thumb, any time a blinker bulb lights up when it is not supposed to, that is a great indication of a bad ground.