Captain_Slow

Last winter I finally solved the problem with the S4 cooling fans going into failure mode (One fan working on high, then slows while system tries to start the problem fan, then powering up the good fan to high again.). The problem turned out to be an oxidized fuse connection. Deoxit fixed the problem. This was also the last time I pressed the AC switch and confirmed that both cooling fans would run on high. The AC compressor would run but not cool air at the center vent below about 67 degrees on a warm day.

Today I wanted to see if the AC compressor would run. It did not, and I'm assuming the pressure switch is sensing low pressure and not letting it run. But I still expected both fans to run on high. Is it normal for the fans to not run, even when AC is switched on, if the pressure switch is not letting the compressor run?

fraggle

I believe the fans should kick in. Debugging the AC circuit is simple compared to what you were doing with the fans before.

Captain_Slow

Fraggle - could it be the AC switch itself? Sometimes when I push it in to turn ON it doesn't stay in, and when pushing it in to turn OFF it stays in until I mess with it, pushing from different angles, a few times. However, when I push it in and it stays I do hear my new HVAC recirculation actuator and door flap doing there thing and redirecting the air.

Could it have anything to do with that multi-pin connector I had to modify (near the freeze switch) to connect the blower motor I got from Roger?

PS: I hope your car is recovering well.

dr bob

The control head relay [signal] for the compressor signals the fan controller next to the passenger seat that AC is on and it should look at refrigerant pressure as a trigger for fan control.

Edit: [delete original: I would start with some basic diagnosis of the AC control to the compressor. The little control head relay is a DPDT configuration that uses both "A" contacts (normally open) only. It's possible but less than likely that it would cut off fan control signal from the second contact. Worth checking out though.]

Replaced with: The fan controller receives a signal from AC button in the console, the same signal that triggers the AC clutch relay to engage. The fan controller signal routes directly from the AC console button, and does not pass through the control head.

I wrote a simple diagnosis for the clutch side of that, but the one Dwayne wrote is a little friendlier. Mine was documentation of that relay modification that lives with my car in the logbook, while Dwayne's is written as an aid to folks who need to fix their cars.

Captain_Slow

Dr. Bob - Thank you very much. Amazing how much knowledge you and many others can bring to help us understand our cars.

MainePorsche

There is no "11 relay". The device in position XI is a spike suppresor.

The power to operate the A/C compressor clutch comes thru fuse #17 to Relay X, the blower relay. If the A/C blower works, the fuse and this relay are good - if the blower doesn't work, neither will the compressor.

From the relay, power goes to the A/C switch on the dash.

From the A/C switch, power goes to the A/C control head. There is a small relay in the control head that controls the power to the compressor clutch - this relay can fail.

The power then goes to the antifreeze switch on the A/C evaporator. If the evaporator gets below freezing, the switch opens, cutting off the compressor. Remove the plastic shield at the base of the windshield under the hood (careful - it breaks easily)and find the switch near the center of the windshield. Check for 12 vdc on both connections on the switch (ignition on, A/C on, air distribution lever on). If there is power on both, everything so far is OK. If there is power on one terminal, tha freeze switch is bad. If there is no power on either, fuse #17, the fan relay or the A/C controller relay may be bad. If the blower works, but there is no power here, the controller relay is the usual suspect.

From the freeze switch, power goes back to the central power panel, and goes to both the supressor in Position XI and out to the low pressure switch on the A/C receiver/dryer forward of the radiator. From there, power goes to the compressor clutch.

Jon,
How are you ? Sorry you're still with AC headaches.
Above is the power path for the AC. At each check point you should voltage approximating alternator/battery voltage. If voltage exists but is very low (ie 4V), then that check point is faulty. Also AC refrigerant pressures must be good so as to have a reliable test of the low pressure switch check point. Hope it is helpful, and Best Wishes.

MainePorsche

Jon,
Also make sure the AC switch electrical connector is snapped in well. Did you do any work through the driver's side console carpet recently ?

Captain_Slow

Hi Craig,
Great to hear from you. I've not seen you here for awhile. Thanks for all this info. But...wow..at least my sunroof and windows work

I've just started to turn my attention to the AC. It worked when I bought the car two years ago. It worked but would only blow about 67 degree air about a year ago. Now the compressor doesn't come on and neither do the engine cooling fans (they work perfectly for regular cooling, but don't come on HIGH when pressing the AC button).

I'd like to think I can just jump the compressor relay and suck in some R12 on the low side until the pressure switch will let the compressor and fans run (Dr. Bob's post sounds like he's saying the fan controller looks for good pressure or will not turn on the fans). But before I do anything I will put a gauge on the high and low fittings and see if there's any pressure...if not, I'll evacuate the system for 45 minutes to boil off any water and give it a fresh fill. Thinking I'll put my wife and however many cans of freon are needed on a scale and stop adding when she..I mean the cans lose enough weight Do you know how many pounds/kgs of freon the system takes?

But there could be other problems introduced by me when I replaced the HVAC blower. I was working around much of what you described:

1. My new blower works...but it's using a higher amperage relay supplied by Roger and harness that draws power from the jump post and has it's own inline fuse. Now that I think about it, I think the original blower relay is triggering the higher current relay.

2. There is a multi pin connector that, if I recall correctly, was on the backside of the fan resistor pack (the notorious one with all the coils in it). I know the freeze switch is in this area. I had to make modifications to wiring pins to include Roger's blower harness. That all went well and the blower works well. I can't remember clearly if the freeze switch wiring was involved in this pin connector. Getting all the wiring under the cover was difficult. I'll revisit the installation instructions and look for wires going to the freeze switch.

Possibly related: I recall not seeing a voltage I was supposed to see at two black wires. The blower instructions say one will have voltage and the other none, and to use the one with voltage and let the other dangle (or vice versa..will check). I found weak voltage on one wire and nothing on the other and chose the wire with the weak voltage. Greg Brown told me I must have chosen wisely because the blower would not work if I hadn't. I'm thinking this may have something to do with what you described about the voltages at the freeze switch. A problem here may explain why I wasn't observing the voltages stated in the blower installation instructions.

I'm pretty sure the original blower relay is triggering the new higher amperage relay that now controls the more powerful blower. The blower works.

I'm going to keep it simple to start and assume it's low pressure related. If not, then on to the voltage checks you described.

Thanks for more great information. How are you doing and how is your fan controller setup doing?

GregBBRD

You might be hunting for something you can't find....

The fans do not run at "HIGH" speed with the A/C....only low speed.

The fans are controlled by the high pressure switch located to the left (vehicle left) of the dryer.

Without freon....no pressure....no fans.

dr bob

Happy to help. I only remember this stuff because I wrote it down way back when. I crawled through circuit diagrams, and eventually decoded the internals of the control head and the support circuitry in my quest to have working AC. Black car in Los Angeles would be almost useless without good AC.

Greg B has done the same thing, and taken it a giant step forward by assembling the complete system electrically as a test fixture. He can plug a suspect component into his set-up and tell you quickly if it's functional. It also lets him prove the components he rebuilds/upgrades (HVAC controllers, fan controllers and final-stages, etc) before they are returned to you.

Captain_Slow

Greg - You said so much with so few words Thanks!

So, here's one more question: With the vent position set to center, does pushing the AC button automatically activate the recirculation flap to the recirculation position?

Captain_Slow

This is impressive...

Alan

Greg on an '88 the fan activation signal to the fan controller is directly from the AC switch - it doesn't matter if the compressor is actually activated or not, the fans will do their thing either way.

Alan

Captain_Slow

This is not good news...

Back to square one. Would it matter that I have an early 88? There's at least one thing on my car that is like an 87 - my car has the large resonators.

What is the most likely reason power isn't getting to both fans when I press the AC switch?

dr bob

Searching will find a slew of threads on cooling fan troubleshooting for S4+ cars. Each fan has a dedicated feeder from the battery positive post. Make sure they are clean and tight. Each fan has its own fuse. Make sure there's power on both sides of both fuses while the fans are trying to run. Each fan has a feeder from the control unit. Make sure both are plugged into the fan connectors. If only one fan runs, swap these connections and see if the "problem" moves with the connectors. If not, the fan itself is yourproblem.