My R134a converted system is running too high on the Hi pressure side - up to 400psi - low side around 45-50psi (low 90's here today). Spray the condenser and the Hi side pressure drops below 300 - approximately 275psi.

But, my auxiliary fan won't come on at all - no matter what the dash temps read.

Question 1 - Does the aux fan run whenever the A/C is on?

I tested the switch on the receiver/drier - jumped the wires and I hear the relay click, but no aux fan. I checked the wires leading to the fan right in front of the condenser and I'm not getting any voltage there. I did get 12v's at the R/D switch. Checked fuse and changed it to be sure...

Question 2 - Could a non-functioning aux fan explain my high Hi side pressures?

Question 3 - Any suggestions on troubleshooting the fan?

I plan to do some more testing tomorrow - haven't checked vent temps yet but it is cooling.

thanks, ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

IMO:
A damaged receiver dryer switch is as common as the sun rise every morning.

Good luck.

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1) It's been a while since I worked on a 123, but I'm sure it works like the later cars...the fan only comes on when high side pressure is over 300psi.

2) Yes, but with these pressures it's likely overcharged. If you charge with 134a, you should use 10% less gas than R12. As a rule of thumb, the high side pressure in PSI can be estimated as ambient temperature in fahrenheit times 3. So at 80F, pressure should be around 240psi. It's not scientific, just a rough estimate. I'm attaching a chart for a W201 as an example. Less is better than more...you don't want to freeze solid in high humidity conditions.

3) Begin by shorting the pressure sensor leads. If the fan comes on, you have a bad sensor. Replacement would require you to evacuate the system, replace the sensor, and recharge afterwards. With luck, it's something else: check fuse, relay, resistor and fan in that order.

If you have to remove 134a gas, you need to have it done at a shop with compliant recycling equipment.

The switch closes based on refrigerant temperature, not pressure.

With R-134a in a W123, freezing is about the last thing you need to worry about.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

The switch controls the relay coil, since you hear the relay click, the switch is probably good, and half the relay is ok. But the relay contacts are separately powered...even if the coil pulls the relay closed, the contacts may be burned. The contact leads are 30 and 87...remove the relay and use a bit of wire to short these connections at the socket. If the fan comes on, the relay is bad.

I really don't think it's overcharged - only 80% R134a was used...

I have checked the fuse, will try the relay test tomorrow.

How would I test the resistor? I will follow Bill's advice and test the fan directly as well.

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

You can swap the aux fan and compressor relays. Evidently, the compressor relay is known good.

There is no resistor in the aux fan circuit.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

OK, I already did that - swapped the relays that is...no change and A/C compressor still works!

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

That's too bad. Relays are a lot cheaper than fans.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Ha! The story of my car repair life!

Although, I'm not sure the fan is bad - I don't have any voltage to it...

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Actually that's not entirely true if you have read thru my latest post on my window issue. Turned out to be a fuse!

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Is the key in RUN?

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Yes, and then I even tested the wires as they lead to the fan right in front of the car while the car was running with A/C on...But, again, if the parameters had not been yet - refrigerant temperature - or pressure - and engine temps, then the fan wouldn't be activated. My gauges showed temps in the 105C range and the Hi side psi was running around 400...

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

"Parameters" won't matter if you jumper the temperature switch at the receiver/drier. The only parameter that matters anyway is refrigerant temperature. Engine temperature is not a factor, unless the engine compartment gets hot enough to trip the switch on its own, which I have seen happen.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Did you read Roy`s suggestion of running 12v directly to the fan?


I tested the switch on the receiver/drier - jumped the wires and I hear the relay click, but no aux fan. I checked the wires leading to the fan right in front of the condenser and I'm not getting any voltage there. I did get 12v's at the R/D switch. Checked fuse and changed it to be sure...

Answer: You missed a test = jump 12V direct from the battery to the fan = this will verify a good or bad fan. No voltage at the fan could be a damaged receiver dryer switch or wire.


Charlie

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