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1991 Volvo 740 AC Help

J

jvluntzel

Member
Joined
Mar 1, 2018
Hello All!

I need your skills and assistance!

I have a 1991 Volvo 740 Non Turbo REX/Regina.

I am working to get my AC working before it gets too warm out.

The system pressure on the low side was like 10 psi, and the AC compressor was not coming on. The 30A fuse was good.

I thought maybe its just low on freon and so I added it (it is already converted to R134) Low side pressure now at 40psi.

Heres the weird part. I jumped 12v to the compressor and it came on, then I disconnected that jumper and connected the factory plug back (I had 12v there as well). Next I checked the sensor on the condensor and it had 12v. So I jumped that and the compressor did not come on....

I checked the sensor and no more 12V, same with the compressor clutch power.. nothing.

I go and examine the 30A fuse and while it is not popped, it is very hot (not melting wires hot but thats not too far off)



Any assistance with this is greatly appreciated.

Thanks!

p.s. comments on "just go to a pro" " take it off" "buy a honda" are appreciated but not needed :-D
 
I haven't worked on 740 AC systems in years, but you should get a wiring diagram and figure it out with a test light.

I use an AC machine to recover, vacuum, and then recharge the correct amount. Pressure should be about 70 psi on both sides static. Then about 30 psi low and 150 high side when the compressor is working.
 
ZVOLV said:
I use an AC machine to recover, vacuum, and then recharge the correct amount. Pressure should be about 70 psi on both sides static. Then about 30 psi low and 150 high side when the compressor is working.
Click to expand...

Exactly. The static pressure measure (system OFF) is always a good tell on the charge (and leak) status. 70 is a good number.

I doubt your system has a usable high side fitting, if it looks anything like my 9 series system. Charging is done via the low side suction port, which (as noted) when running will be much lower than your can of R134a and will take the gas easily.

Your system should be simple: low side pressure switch at accumulator (firewall), and 12V connection to the compressor. I don't know what year the 7's went to the CCOT setup but yours "should be" the same as my 92. Should have an orifice tube right below your exhaust manifold liquid line (small diameter).

Be careful jumping ANYTHING until you have the right static charge in the system. If the gas/oil is unknown, then have the entire system evacuated and refill with R134a and ESTER oil, NOT PAG OIL. Your O-rings will have the chance to survive.

For actual usable advice, see the UK Volvo Club "Maint. Pages 700/900"..... a plethora of info.
 
You controller is bad. 12V is not the same as adequate power to run the compressor. Whenever I have a car doing what yours is doing, replacing the dash panel/controller fixes the problem. The fact you have 12V at the switches all the way to the compressor, yet, it won't run unless jumpered directly from the battery points directly to the controller.
 
In ture TB fashion just add a separate switch for compressor control. Find one that illuminates when turned on. The green wire coming from the oe controller is where the compressor gets its power. Add switch there.
 
15A said:
Also check the ground for the heck of it.

BTDT, so dont ask me how I know. :oops:
Click to expand...

Does anyone have any pics of the grounding in the engine bay and its locations? What would be the best way to check the ground of the AC compressor? Check resistance between it and battery ground?

2 old for this said:
In ture TB fashion just add a separate switch for compressor control. Find one that illuminates when turned on. The green wire coming from the oe controller is where the compressor gets its power. Add switch there.
Click to expand...

I would do that if jumping the compressor made the air cold....but it doesnt. Its still ambient temp. HALP PLEASE
 
I'm running into this same scenario on a 740 myself. My suspicions are the pressure switch or the climate controller. The latter is the #1 issue I've had with them all.
 
Typical TB poster...

He won't listen. It's always the controller. Even with the tutorial with pictures on the Brickyard, he'll scrap the car because of it.
 
fwiw.....got mine straightened out tonight, and it was (drum roll)....the freaking controller. I'll try and act surprised. Those controllers are connected to the alternator (wtf for? no one knows) and can fail causing a back feed that sets off half dozen dummy lights and runability problems.

This will be the 4th controller in 3 yrs.
 
2manyturbos said:
You controller is bad. 12V is not the same as adequate power to run the compressor. Whenever I have a car doing what yours is doing, replacing the dash panel/controller fixes the problem. The fact you have 12V at the switches all the way to the compressor, yet, it won't run unless jumpered directly from the battery points directly to the controller.
Click to expand...

I think the 91 and 92 controllers have a higher failure rate than the 93+

The controller was bad in my 92 at purchase; however without adequate charge , ground on the compressor (check it straight to the battery) the controller argument is moot without the prerequisites. When my NEW controller showed 12v and clutch still wouldn't cycle, the ground was the problem.

OP, tell us what fixes the issue.
 
DET17 said:
I think the 91 and 92 controllers have a higher failure rate than the 93+

The controller was bad in my 92 at purchase; however without adequate charge , ground on the compressor (check it straight to the battery) the controller argument is moot without the prerequisites. When my NEW controller showed 12v and clutch still wouldn't cycle, the ground was the problem.

OP, tell us what fixes the issue.
Click to expand...

Will do. I will be getting a new controller this weekend.

Thanks for your assistance!
 
BIG UPDATE:

We got some frost cold goodness everyone!

Turns out my new (junkyard) climate control module (where you choose fan speed and temp) had poor solder joints. I remelted all the joints and my compressor kicks on as it should! Filled it up with some good R134 and its cold!

Last thing I did was I converted to a later model 940 2 speed electric fan. Instead of using the 3 wire bmw temp sensor, I have the low speed coming on when accessory power is turned on, and the high speed when the AC button is pressed.

Works like a charm :D
 
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