Captain_Slow

Dr. Bob - I'm familiar with how the fan system works (thanks to Theo). I posted a few of the threads on troubleshooting them. I'm happy to say they work fine after cleaning grounds and applying Deoxit to the fuse panel and all fan connections. Both sides cycle on and off together on low after the engine reaches operating temp. I've never observed or heard them running on high....probably because I haven't used the AC since fixing the fan problem.

The last time I pushed the AC switch to test the AC vent temps was about last September or October on a warm day. The vent temp dropped, but not below 67 degrees (outside temp was about 80). The compressor was running and so were the fans.

A few days ago when I pressed the AC switch the compressor didn't run and neither did the fans. Could be low pressure won't let the compressor run, and some other problem with the fan activation. Or, the fans are also looking at low pressure and not coming on. The coincidence of neither the fans or the compressor coming on has me crossing my fingers and hoping Greg is correct.

Alan says fans on 88 and onward should come on when the AC button is pressed no matter what. But my car is an early 88. Again, crossing my fingers that I might happen to have a pre-88 AC system, as it also has pre-88 exhaust (large resonators).

Alan

Unfortunately an '87 is the same... seems like a head unit/switch issue... Do the compressor/fans run in DEF mode - if so it may just be the connections to the AC switch.

Alan

Captain_Slow

Alan - Then here's a puzzler for you:

This morning I confirmed again that compressor and fans are not working with AC switched on, engine idling. Turned off the AC and turned off the engine.

Connected a can of 134 via gauged hose to the low side port.

Started the engine and switched on the AC (still no fans and no compressor).

Opened the valve on the can of 134 and heard flow. Within 3-5 seconds the fans started and so did the compressor.

Put my hand up to the center vent - blowing coldish.

Continued to fill to 35 psi on the gauge.

Took it for a 30 minute drive with digital temperature probe in the center vent. Temperature varied between 63.0 and 64.9. I had the slider set to high (65), but the interior never got cooler than 77 (still relatively comfortable due to lower humidity).

NOTES: My center mixing flap actuator needs to be replaced. I have the constant tapping behind the vent. I have the replacement actuator - this winter's project. My car is an early 88, built in July 87.

Just the fans and compressor coming on made my day.

Captain_Slow

My plan is to buy a vacuum pump and gauges and do what Dr. Bob suggested in this AC Fill Method Thread:

ROG100

87 to 90 fans run at low speed with AC on not high speed.
91 to 95 - with recirculation mode selected, fans run at high speed, whether the AC button is pressed or not.

Alan

What was done when the car was converted to R134a?

It should not work like that in a stock (R12 for your year) config.

If you look at the clutch activation circuit you will see it goes nowhere else after the pressure switch.

Alan

Alan

The only difference I see in configuration for S4+ is that late '93 - '95 models have the 3 way pressure switch (for R134a). This causes high speed operation when the refrigerant high side pressure is high enough - but not otherwise.

Alan

Captain_Slow

Unfortunately, I don't know anything about what was done during the conversion to 134. I could only vaguely remember the previous owner saying it was converted to 134, then doubted my memory when seeing the R12 fitting near the dryer. He bought the car in 2005 so it may have been converted before he owned it. The only obvious evidence I see that it was converted is the 134 adapter on the low side port fitting.

ROG100

Do you agree with this Alan?

87 to 92 (early 93 R12 USA/CAN) fans run at low speed (low stage) with AC on.
93 (R134a) to 95 - with recirculation mode selected, fans run at high speed, whether the AC button is pressed or not.

The reason I need to clarify is that this is an easy test to see if both fans are working - providing AC is good.

Captain_Slow

Roger - My situation may not be related, but now that AC system is charged to 35 psig on the low side at 82 degrees my fans DO run on low when I push the AC button.

However, my experience today supports what Greg Brown stated in an earlier post above. My fans and compressor were not functioning when I pushed the AC button. Just seconds after I started to add 134 to the low side port the compressor and fans came on together. It appears that Greg's post is accurate. It appears the fan controller next to the passenger seat does look for sufficient pressure before starting the fans.

GregBBRD

Yes. That's what the high pressure switch (looks like an oil pressure sender) by the dryer is for....turns the fans on....low speed. Sends the pressure signal to the fan controller. (The pressure sender is comviently not in the A/C diagram, but in the fan diagram, if memory serves me right.)

High speed (both fans) only happens from temperature signal from radiator.

Roger's information about the later GTS vehicles is new, to me....I've not noticed this. And a 928 running both fans on high speed sounds like a jet getting ready for a carrier launch...so I'm surprised I haven't noticed it.

gb

Captain_Slow

Update: Last night I checked the performance of the AC after dark when solar warming wasn't a factor. I also wanted check the low side pressure from a cold start (85 degrees in the garage, engine 85 degrees, and I only have the cheap low side filler hose with gauge). Pressure before starting the engine was 95 psig (assuming a similar equilibrium pressure on the high side). Immediately after starting the engine and pressing the AC switch, low side pressure was just barely 30 psig, rising to 35 psig within just a few minutes. With AC temp slider all the way to the left the center vent temperature steadily dropped throughout a 30-40 minutes drive. Temperature was still slowly dropping when I pulled into the driveway - 44.3 degrees. It felt plenty cold.

Alan

The diagram for the compressor clutch and the cooling fans is the same one for all S4+ models. I may possibly be wrong (because what I have detailed is not 100% conclusive) but I believe the AC switch is directly what turns the fan on at low speed. The high refrigerant pressure switch (or the high temp coolant switch) is what Switches it to high speed operation. on later '93-'95 this changes due to the addition of the 3 way pressure switch - but actually in the end that seems to be functionally ~the same with the exception of the addition of over pressure clutch cutout (a safety factor only - no impact on normal operation).

So Roger - I don't see how what you said can be true. I don't see an easy way (with normal controls) to test for operation between fast and slow fan modes (easiest is probably the pressure switch connector).

To Jon - when the fans came on - how fast were they running? Fast or Slow?

High fan speed all the time will get you nice cold AC - but lots of fan noise... If the fans weren't working in slow mode with the compressor running - the fan would kick in to high speed quite quickly because the R134a pressure will rise fast without sufficient temp extraction...

For R134a the pressure switch is: Clutch on at 2.3 bar, high Refrig pressure on at 18 bar, clutch cutout at 32 bar.
I assume the fans are supposed to be on (low speed*) from 2 bar - 18 bar and on high speed from 18 bar to 32 bar

*Could also be high speed if the coolant gets hot enough to call for high speed operation

Alan

ROG100

Hi Alan,
I am trying to confirm it is true.
My 87 & 88 turn on both the fans at low speed when AC is engaged and AC is working. I accept the AC needs to be working to do this - no AC pressure no fans.
My 93 GTS with 134a in recirculation mode turns on both fans at high speed. What does your 94GTS do?
Looking at the service information for 91 MY where they introduced the recirculation mode on the head unit - it states that if recirc. is engaged fans turn on at low speed.
Anyone with a 91 who can confirm that?

dr bob

There's a set of function charts for the various cooling fan modes in WSM section 19, starting at 19-15. Fans essentially go to medium speed (described poorly as 8 volts on the last chart) when high-side pressure exceeds 3 bar (about 45 PSIG). As high-side pressure moves above 15 Bar (a bit north of 200 PSIG), fan duty increases from the 8 Volt (medium speed) towards high speed (around 12 Volts) by the time pressure reaches 20 Bar (about 275 PSIG). It's pretty easy to hit 275 PSIG on the high side is stop-and-go traffic on a hot day if condenser airflow is limited. If the compressor is actually compressing, or condenser temp is above about 50ºF, the fans should be running at medium (8 Volt) speed if the AC button is depressed.

There is no correlation between the compressor clutch under-pressure safety switch and the fan operation, regardless of year. It is wired in series with the compressor clutch with no other connections.

The dual safety switch (both under- and over-pressure protection) in the high-side loop on the later cars adds another fan function when pressure is above 18 Bar and has not dropped below 14 Bar. This is partially redundant to but independent off the function from the pressure transducer described above.

The signal to the fan controller to indicate AC engaged is from the AC button in the console, and is in fact teh same signal sent to the control head to engage the relay that controls the compressor clutch. (I'll correct my earlier post where I stated that it connected to a relay contact rather than the trigger.) The fan controller does not engage the fans based on high-side pressure at the transducer unless the AC button is depressed, and then without regard for whether the compressor is actually engaged. Knowing that the compressor and fans run from the same signal is a valuable diagnostic aid; if you are trying to diagnose compressor not running, and notice that when it does run the fans come on with it, you've narrowed the search to the AC button and the connections between it and the control head, and the control head to fan controller. No internal control head function nor compressor protection (anti-freeze switch, pressure protection switch, or 14-pin terminal connection) function is included in the cooling fan function.