I'm reposting this message since I didn't realize that this forum was available so thanks to Lazy K for letting me know.

I have been doing some AC work on my 1990 F250 351 and had to replace the compressor and dryer. The problem I found when trying to recharge the system was the compressor would not turn on. Using a volt meter the electrical connection showed 0 volts when I had the dial in the cab turned all the way on. I can't recharge without the compressor cutting on so any tips on what to check next would be appreciated. Also, I did check the electrical connection on the dryer and it was showing 11 volts. One other thing was when I turned the dial in the cab all the way up the blower did blow air out the of vents like it should.

 

The compressor won't engage until there is about 45psi ststic pressure in the system. Once the compressor starts, the pressure on the low side will drop and the compressor will disengage.
Install a jumper in the cycling switch connector and see if the compressor will engage. If it engages and you had 45psi in the system, the cycling switch has probably failed.

Also, if you used the aluminum R134a retrofit fittings commonly available at the chain stores, the valve in the service port may not be getting depressed to allow the refrigerant to flow into the system. Those fittings are problematic, I would suggest brass retrofit fittings from a "real" parts store like NAPA.

Are you using a manifold gauge set?
Did you pull a hard vacuum on the system before attempting to charge it with refrigerant?
Why did the compressor need to be replaced?

 

I was retrofitting the system to 134 and I since the ac had never worked on this truck when I bought it, I decided to learn something new. I bought a vacum pump and gauge set and replaced the orifice tube, dryer, and compressor. When I went to try and recharge the system my service line on the gauge set to the can tap had a leak and I couldn't fill the system. I just went at lunch today and replaced the service line. Hopefully that will solve that problem. What I was a little worried about was when I checked the compressor voltage on my Chevy truck (I know don't hate me) it would read 13 volts when I cut the ac on to high. I guess that since that system is working then the low pressure switch had high enough pressure to send signal to the compressor. Is this a fail safe for the compressor? Makes sense if it is. Also since I had a crappy service line my gauge set couldn't get the system high enough in pressure to send signal to the compressor to cut on?

 

The primary job of the cycling switch is to keep the evaporator temperature just above freezing. It stops the compressor at a certain Low Side pressure to prevent the evaporator form getting cold enough to freeze condensation.
A side benefit of this is that if the refrigerant leaks out the pressure, of course, drops and it prevents the compressor from engaging. This prevents the compressor from running when the lubricant in the system can't circulate.

The leak in the service line wouldn't necessarily prevent you from pressurizing the system, unless it was a big leak. It would, however, prevent you from pulling a full vacuum on the system.

Did you replace the old compressor "just because..." or did it fail? This is important to know. If it grenaded internally there is debris and contaminated oil in the condenser, evaporator and lines that must be removed by flushing. If you don't, you won't be very happy. Did you see any metal shavings or other debris on the old orifice tube?

I would still suspect the retrofit fittings if you used the aluminum ones from a "retrofit kit".

Since you are new to AC repair, spend some time reading the FAQ section on this forum:
Automotive AC Information Forum - ACKITS.COM
There's a lot of good DIY AC system info there that can help prevent mistakes or overlooked details. Some homework can prevent an expensive "learning experience".

 

FWIW, if you're charging into a vacuum, that first can of refrigerant will practically jump into an evacuated system. It also helps if you warm the can of refrigerant in a bucket of warm (not hot - about 100F is good) water. You don't even have to worry about the compressor running at that point.

Steve

 

I'm glad to say I've fixed my truck. Thanks for y'alls help and I'll replay a little bit of my saga cause I'm sure y'all will get a chuckle from the been there done that senario.

Ok...so I'm fired up thinking that my pressure cycling switch is bad and after reading the posts I remembered that the switch must see a solid pressure before kicking in the compressor. Ok...no problem just add the first can of 134. Pulled vacuum and starting adding can. Nothing happening. Damn...didn't open valves correctly. Easy fix...keep on keeping on. Then I see strange oil dripping on the ground under truck...Damn...smell oil...not motor oil....sure enough PAG compressor oil. Fitting on the back of compressor not tightened all the way. One helluva tight fit to get the hose line to compress onto the new compressor stem. Fixed that but used a can of 134 in the process. Ok...keep on keeping on. Pulled vacuum again, valves opened right, can of 134 in warm water, compressor lines tight, start adding 134 again. See pressure building on gauges then boom....loud hiss....134 sprays out and on truck engine...Damn now what....Liquid line from evaporator has completely blowed off the flared evaporator tube....guess the spring clamp and o-rings were wore out. This one cripples me cause I have to take a trip to the ole parts store. After a day or two locating the store that didn't have to order the part its back to work. Replaced line and remembered all my training (I'm a quick learner...hard way...but quick) and like magic my truck takes first can, compressor kicks in, I charge to proper pressure and bam cold, cold air.

So for my first AC repair I had my ups and downs but its a good feeling of accomplishment and knowing that I saved several hundred dolllars, even with my screw ups.

Thanks guys and now I just need somebody elses vehicle to work on.

Lex

 

 

Cheers to ya!

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