It's been very hot here the past few days - mid 90's. I have made a lot of progress with the A/C (R12), but I'm having trouble getting the temp switch to kick on the aux fan. The fan works (jumped the wires on the temp switch on the drier and fan runs). The temp switch works as well - tested it today by putting the switch in some water and warmed the water - fan kicked on when the water temp reached 52 degrees C - just like it is supposed to. I don't see how it is possible that the A/C refrigerant coolant temp is not reaching 52 degrees C, but I don't know why else the fan isn't kicking on when the car is idling.

With the York compressor running, idle slows down and clutch stops momentarily, squeals, and if left alone the car will stall and clutch gets very hot! With the fan running, the car idles better and clutch doesn't squeal - obviously the fan is lowering the high head pressure but is there something obvious I'm missing? The hose going from the compressor to the condensor is very hot and the one going from compressor to the evap is cold - system cools great but I don't want to burn the compressor or clutch!

Ryan

1978 300D

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Is your fan clutch good too? That's the main cooling device...

Where you pulled the switch, is it blocked by anything?

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

You are talking about the main engine fan, right? It runs and my car does not overheat - even with A/C on and in this 90 degree heat, car runs a little over 80 degrees C on the dash gauge.

How would I check the clutch?

What do you mean 'blocked'? The temp switch just screws right into the drier - nothing blocking it.

ryan

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

The electric fan you speak of has been part of my: “Things to do for summer operations” checklist for years now.

Every year, when the temp hits and holds 80+ during the days, I unplug those two wires from the dryer switch and place a 15amp blade type fuse across them. I just leave the wires from the switch tucked out of the way. Not sure if that’s the right size fuse for the fan or not, I just had one handy one year and started using it. It winters in the glove box. Same fuse. Same fan now, for the past 5+ years.
All summer long - When the key is on - The fan is on.
Does not seem to ill effect glow time and starting either.
I go back to the OEM set up around September/October.

I’m sure the pros here will find issue with doing this, but it’s worked fine for me for many a hot N-TX summer. Never been over 100c in any type of driving. I leave the A/C on Auto-HI and the dial at 65 too. (But then, I’m running 134 now. Not as efficient as R12, I hear.)

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1980 300D - Veggie Burner !

52C is quite hot. It's very possible that you're not seeing that temperature at the receiver with R12, expecially if you're a bit low on refridgerant. The fact that you're not seeing a high engine temperature would seem to confirm this.

Get an infrered thermometer and check.

Concur. It does not take much of a refrigerant loss before the temp at the drier won't get hot enough to actuate the switch.

See I haven't had the switch off, so I was just throwing that idea out there. I thought it may have had a sensor that sits within that could have been caked up.

Just run the car to normal operating temps, then turn it off with the hood open. The fan should stop pretty much instantly. If it continues to turn, it's bum.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Ok, I bought an infrared thermometer today and some R12 manifold gauges and did some checking. I am now beginning to think it might be a bad fan clutch. The temperature at the drier switch really struggles to get to 52C so the aux fan doesn't come on much - when it does the fan works fine. The high pressure is between 250-275 psi and low side is around 30 psi, with idle at 1500-2000 rpm (that was with the aux fan running).

While idling with the A/C on, the idle is slowing down a lot and the A/C clutch stops periodically - just for a second or so at a time. If the aux fan does not come on the car may stall. Could this have something to do with the fan clutch, or something else?

Again, if I jump the drier switch and let the fan run all the time the A/C clutch seems fine and the car idles fine.

Ahy thoughts? Thanks,

ryan

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Do you have a high-pressure cutout switch? If so, it could be bad, and opening at too low a pressure. Slower idle is probably fine, but the AC clutch disengaging is not.

Yes, its a new pressure switch - except my understanding is that it is only a low pressure switch on this model. It cuts off the compressor when the refrigerant is low and prevents the compressor from drawing a vacuum on the system.

ryan

1978 300D

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

It certainly was equipped with a low pressure switch only, but it would surprise me greatly if the replacement switch did not include a high-pressure cutout. The switch is on the high-side, right?

It's possible that you have too much refridgerant in the system and it's tripping the high pressure cutout. Of course, without looking at the chart (that I don't have) and knowing the test parameters (that you don't give), I can't tell. Or that the switch is bad.

Edited to add: A '78 has a York compressor, right? Then it may have a low pressure switch on the low-side line, in which a low charge will cause the cycling. It probably also cycles in normal operation, but not often.