RichS 1986928

86.0 5spd -

My AC doesn't get cold - I've gone through all the troubleshooting for
vents, flaps, heater valve etc. New compressor, drier, expansion valve and
recharge with no leaks found.

I'm suspecting the Aux Fan for lack of much else.

My understanding is that the Aux fan should come on when :
1. Temp is too high - or -
2. AC is on

(opening the hood should always turn it off)

My Aux fan comes on when the Temp is high (around 3/4 on gauge)
but does not come on when the AC is on. It also doesn't turn off
with the hood open.

What is the most likely cause of the AC circuit not triggering the fan?

Thanks

Hilton

Not sure about earlier cars, but on my S4 the fan is activated by the over-pressure switch saying pressures are too high in the AC system - this is separate from the temp sensor in the coolant circuit which can also activate the fan.

However the S4 has a very different fan setup, so your system might be different - but I'd check the pressure sensor mounted next to the drier. If you bridge the pins on it, the fan should come on.

dprantl

No, the fan should not automatically come on with the A/C compressor, unless the system has been modified from stock. The fan turns on if the coolant temp is high. I'm not sure, but it may also come on if the freon temp is high enough too.

Are you having A/C cooling problems even when the car is driving faster than 30mph? If so, your auxillary fan will not help.

Dan
'91 928GT S/C 475hp/460lb.ft

JHowell37

This may seem like a silly question, but is your compressor actually turning on?

RichS 1986928

Yes, compressor engages fine.

Thanks for that input.

The AC seems to start pretty cold but doesn't maintain it very well. I'm just about out of
ideas.

Moving at 30+ mph doesn't seem to help the problem. If I am at a stop and the car is hot, the
Aux fan is usually not running.. it only kicks on when the engine temp is really hot. So I was wondering
if the AC should kick it on when I'm not moving.

SharkSkin

I chased my tail for a long time on an AC issue, turned out to be the new expansion valve I had put in was not doing its job.

What are the AC system pressures @ what ambient? The symptoms in my case were a high suction pressure combined with evidence the expansion valve was operating, just not operating correctly.

RichS 1986928

I'm not sure of the pressures. I have a small gauge that came with the R-134A recharge kit.. but
that's probably not the correct tool?

This might have to wait until warmer weather to test it out again...

dprantl

I'm not sure I understand. Forget about the fan for a second. If you are constantly driving above 30mph on the highway, does your A/C cool well and not get any worse with time?

Dan
'91 928GT S/C 475hp/460lb.ft

RichS 1986928

No, the AC does not cool well at a constant 30mph. It starts out ok, then gets worse as the engine
heats up.

SharkSkin

Without knowing the High/Low system pressures it's a crapshoot. The single gauge you describe is really of no use. Also there is a specific procedure when hooking up the correct gauge manifold for the job so you don't contaminate the system. Best to find someone who knows AC to help you out.

How exactly was the recharge performed? How much oil(and what type oil) was added in the process?

dprantl

In that case, your problem is not with the fan, but with the system. To test the pressures you need an A/C gauge manifold. You can purchase this for <$100. There are ones specific to R134A and R12. What you do is connect this assembly to the low and high side of your system, then turn the car and A/C on. The high side fitting on your car sticks directly up from the receiver drier on the pass. side in front of the condenser. The low side fitting is on the large A/C pipe directly under the top radiator hose and jump post. You should get high pressure and low pressure readings holding the RPM at ~2k for a minute or so.

For now, a quick test would be to feel the large and small pipes that go into the expansion valve under the plastic cover behind the firewall. The smaller one should be warm/hot, the larger one should be freezing. The temperature differential is what is important.

Dan
'91 928GT S/C 475hp/460lb.ft

RichS 1986928

Ok, thanks, I guess this will have to wait until warmer weather... that's right around the corner here.

The recharge was done at a small general auto shop. I don't know the details only that they drained
it and recharged it.... and said it was good to go.

JHowell37

As others have said, you need to know the pressures being generated to diagnose the system. This isn't a fan issue. The stock electric fan your car uses is a joke and probably doesn't do much anyway. Your problem does sound like the expansion valve.

RichS 1986928

I've replaced the stock electric fan with one from 928motorsports.
The expansion valve is also brand new.. so I hope it's working.

It seems odd to me that this fan wouldn't come on when the car is sitting still and the
AC is on Full.

I'll check the pressures as soon as I can.

thanks for the help.

WallyP

Rich,

First, go to our web site. Click anywhere to enter the site.
Click the + on 928 Tips/Links
Click the + on Wally's World
Click on The HVAC System

This is a tutorial on the 928 HVAC system. It will give you a good general knowledge of the system.

There are two possibilities for your system:
- The refrigeration system is not functioning correctly.
- The HVAC control system is not functioning correctly.

Refrigeration System
You can get a good idea of the functioning of the refrigeration system with no tool other than your eyes and hands.

CAUTION: Some of the things that you will be touching may be very hot. CAUTION: You may be working around moving belts and other parts. Be very careful!!

With the engine running and the A/C on, look at the compressor clutch. The center of the pulley should be turning with the outer part of the pulley. If not, the clutch is not engaging.

If the clutch is engaging, carefully feel of the A/C tubes in front of the radiator. These should be hot - be careful! If they are hot, the compressor is working. Feel of the larger A/C tubes at the rear of the engine compartment. These tubes should be cold. If the small tubes are hot and the larger tubes are cold, the refrigeration system is probably working.

With the engine and the HVAC system running, move the condition lever, and see if the air flow changes (floor, dash outlets, defrost). If not, the vacuum control system may have leakage problems.

Move the temp control lever from full cold to full hot. Does the air get noticeably hotter?

Move the temp control lever back to full cold. Does the air get noticeably colder? Did the fan noise change as the system went from outside air to recirculate?

If the air temp did not change much, pull the console side cover off of the driver's side. (Usually two Phillips screws, then push forward and down.) Look at the side of the HVAC box and locate the temp control motor. This will be a black plastic box mounted vertically. Look for the arm and lever coming out of the top of the box. Move the air temp lever from cold to hot wile watching the arm. The arm should move to move the flaps inside the box to move air flow from the A/C evaporator to the heater core.

Some possibilities:

- The rubber seal at the back of the engine compartment is missing, letting very hot air into the HVAC air inlet.
-The heater control valve is not stopping the coolant flow.
- There are vacuum leaks in the air flow actuators. (Among other problems, this will result in the heater valve not being controlled.)
- The temp control sensor string is broken, usually at the outside air temp sensor under the left headlamp in the alternator cooling hose.
- The motor mounts have failed, dropping the engine and letting hot air into the console.

There are other things that can be wrong, but these will give you a start. You CAN fix the system.

When you reply to messages on here, be verbose. The more info that we have, the better our guesses will be!