A/C & heat cause the engine to surge

[KYplumber]

'91 GMC C1500 5.7L (350). I changed my alternator yesterday and my actuators started working. That kind of excited me because they weren't working before. However, now when I turn on the heat or A/C the engine surges up and down a few hundred rpm (estimate because no rpm gauge.... But it's not extreme). It will continue doing this until I turn off the air. Is this an issue with the blower motor, the air controls, an actuator, or something else? How do I test to find which?

P. S. Whoever owned this truck before me didn't do it any favors. The alternator had been drilled and tapped to accommodate a larger screw. I assume he stripped the original, but it made installing the new one a ton of fun.

 

[Schurkey]

Almost certainly NOT a problem with the HVAC system.

All the usual suggestions:

Verify fuel pressure at prime, idle, and under load.

Connect a scan tool, verify ALL the sensors and computer outputs, fuel trims, etc.

How old are the usual "tune-up" items--cap, rotor, plug wires, spark plugs, fuel filter, air filter, inspect base timing, EGR, throttle-body cleaning including the IAC passage, etc.

 

[KYplumber]

Almost certainly NOT a problem with the HVAC system.

All the usual suggestions:

Verify fuel pressure at prime, idle, and under load.

Connect a scan tool, verify ALL the sensors and computer outputs, fuel trims, etc.

How old are the usual "tune-up" items--cap, rotor, plug wires, spark plugs, fuel filter, air filter, inspect base timing, EGR, throttle-body cleaning including the IAC passage, etc.

The plug wires appear to be new, and since they're just laying however they fell I'd guess they were installed by the car lot "mechanic" (he's not the sharpest knife in the drawer). I meant to change the fuel filter last weekend, but got busy with honey doo's.

I hope this truck is worth the trouble. The body and interior are in good shape. The engine has a lot of power and only 140K on the odometer. But the maintenance has been neglected for who knows how long.

 

[RichLo]

As long as the engine runs fine and its only slightly surging during idle, that is normal. Its the engine driven AC compressor cycling on and off. Every car surges slightly when that clutch engages but with new cars you usually cant tell because of new motor mounts, quiet exhaust, etc. You can pop the hood and watch the compressor engage/disengage to verify that is when its surging.

If you have the AC button pressed it will still cycle the compressor even when your on full heat mode. It takes moisute out of the air when in heat mode... New cars cycle the compressor in winter when its set on defrost to take moisture out of the air as your windshield de-thaws. For better mileage in new cars, switch it to foot mode and/or face mode once the windshield is clear.

 

[Schurkey]

Good call!

The compressor will cycle more frequently when the refrigerant charge is low. Is the A/C still nicely-cold?

 

[KYplumber]

Good call!

The compressor will cycle more frequently when the refrigerant charge is low. Is the A/C still nicely-cold?

When I bought the truck a few weeks ago it had no refrigerant. It had already been converted to 134a so I'm sure it had been charged, just no idea when. I bought a can at Walmart (I know....let the pros do it right) and the compressor started kicking on...."IT'S ALIVE"!
But the one large can may not have been enough because it still short cycles. I'm not trying to use it until I know it's right.

 

[KYplumber]

As long as the engine runs fine and its only slightly surging during idle, that is normal. Its the engine driven AC compressor cycling on and off. Every car surges slightly when that clutch engages but with new cars you usually cant tell because of new motor mounts, quiet exhaust, etc. You can pop the hood and watch the compressor engage/disengage to verify that is when its surging.

If you have the AC button pressed it will still cycle the compressor even when your on full heat mode. It takes moisute out of the air when in heat mode... New cars cycle the compressor in winter when its set on defrost to take moisture out of the air as your windshield de-thaws. For better mileage in new cars, switch it to foot mode and/or face mode once the windshield is clear.

I'll try to make a video.... This is slightly beyond normal... Lol. I was told replacing the failing alternator will fix many things. So it would be awesome if all I needed was more refrigerant.

 

[RichLo]

Good call!

Wow, I feel like I should add this to my resume or something! lol

 

[KYplumber]

Good call!

The compressor will cycle more frequently when the refrigerant charge is low. Is the A/C still nicely-cold?

I meant to ask, is there a scanner you'd suggest over others?

 

[Schurkey]

When it was my money, I bought Snap-On. I'd rather have a used, semi-obsolete professional-grade scan tool than a brand-new consumer-grade tool, or a software-and-cable + laptop computer based tool.

I used a Snappy MTG2500 for near 20 years. Software good from 80 1/2 to 2005. When that croaked, I got a Snappy Solus Pro with software good from 80 1/2 to 2007. You'd need various OBD-1 cables (at least two, and preferably three for GM alone, a couple for Ford, some for Chrysler...you get the idea.) You'd want an OBD2 connector plus a fistful of "Personality Keys". Owner's manuals probably on DVD. Battery and charger, various other cables, and a bigass plastic suitcase to carry it around in.

Another forum member has donated an MT2500 for "community use". Maybe get yourself on the waiting list to borrow it. There's a sticky thread in the Engines forum.