W108 - 1972 280SE 4.5 - A/C Not Cooling

[Baxterdown]

Hi Guys,

When I bought my car last year, the A/C never cooled properly. As the months went by it cooled less and less until it stopped cooling altogether. Now that it's starting to warm up again here in FL it's time to get the A/C fixed!

For starters, the fan works fine and I checked the freon a few weeks ago and it's full. So, I would venture to say the problem is not a leak.

Someone told me the problem is most likely the evaporator. According to him that's usually what goes bad with these cars. But, he didn't elaborate.

I looked a this thread and it looks like removing the evaporator is a HUGE job.

Do you guys have any suggestions on other places to start troubleshooting before I go trying to remove the evaporator?

Jose

[Idle]

The first place to start: Is power getting to the black wire running to the compressor clutch? If yes, then....

Is the clutch engaging when 12V is supplied to the black wire? If not then replace the clutch as it has failed.

If the clutch is engaging and turning the compressor.....

Look at the sight glass in the top of the filter/dryer. Is it clear? If you see bubbles it is low on Freon. If it is white and milky you are low on Freon. When it is running clear it is full.

This is, of course, just a rule of thumb way of doing it. The best way is to have a set of gauges so you don't overpressure the system when adding Freon.

If power is not getting to the black wire.....

See if power is getting to the low pressure switch which is located on or near your filter/dryer. If the sensor sees low pressure it will not allow current to pass to your compressor clutch. You can jump this for test reasons and see if your compressor clutch pulls in.

If this is the case then you have one of two things going on: Low pressure or a bad low pressure switch. A quick test to see if the low pressure switch is bad is to jump it, turn on the A/C and look at the sight glass. If it bubbles and then clears up and you car is blowing cold air you have a bad low pressure switch.

BUT.. From the sound of it you have a leak in your system. It could be anywhere but they are usually at a connection where an O-ring has dried out and is no longer holding pressure.

To test for this there are various methods but the best is to get on to Ebay and pick up an electronic leak detector. You can then install a can of Freon or R-134 or whatever else you are running in the system and use the detector to look for leaks.

And all of this is just a start. It could be all of these or none of these but all of these tests are simple and quick. They should be done and if no problem is found then it is time to start looking at things like the switches and sensors.

[Baxterdown]

Idle,

Thanks for the detailed info. I looked online and on EPC and I think I've figured out where the compressor clutch is located (this is all new to me). I'm gonna look for the black wire and test it for power.

I'm assuming the filter/dryer is located near the compressor clutch. I found images online so I'll look for it as well.

Thanks again,

Jose

[68_280SE]

The A/C systems on the w108 cars are fairly simple, as there are no high and low pressure switches in the system. The compressor on the 4.5 cars is the old GM A6 type and is located on the lower drivers side of the engine. The dryer is a long cylindrical type located on the passenger side of the condenser. You should see it when you open the hood. This sight glass is located on the bottom of the dryer on liquid line leaving the dryer. The expansion valve is located inside the evaporator box under the dash. If the expansion valve has to be replaced, then evaporator box has to be removed to gain access to it. The amount of work involved in pulling the evaporator box out it pretty high. I've never seen an evaporator go bad in a w108, but i've seen plenty of clogged/stuck expansion valves.

As long as the compressor clutch is coming on when you turn on the A/C, then the most probably cause of poor cooling are low charge and/or a clogged/stuck expansion valve. The easiest way of determining this is to connect a set of A/C gauges and see where the high and low pressures are. Also a worn out engine fan clutch will cause poor a/c cooling and overheating at idle. If the compressor clutch isn't coming on, then check the fuses in the auxiliary fuse boxes along the firewall.

Low high and low low pressures are a good indication of low charge

High high side and low low side is a good sign of a clogged or stuck closed expansion valve.

One more suggestion. I don't recommend converting W108/109 cars to 134a. These cars have a fairly small stock cooling capacity with R12 and when switched over to 134a, you loose cooling capacity. Since you live in south florida you will definitely notice the difference between R12 and 134a.

John

[Idle]

I agree that sticking with the R-12 is best. But if someone has already converted the car to R-134 you would just have to deal with that.

But the use of the gauges is the best way to go and they are not tough to use or to learn. I would look for a set on Ebay as the R-12 sets are pretty cheap now and most of the better ones still have a lot of life left in them.

The compressor clutch is the round thing on the front of the compressor. It pulls in with a loud 'snap' when you are testing it with the engine off. With the engine running you can see it just sitting there with the drive belt running around it and doing nothing. Then you turn the a/c on and suddenly it starts to rapidly turn.

The clutch used on the old York compressors only has one wire going to it. The GM units, which are round, sometimes have two. The York is grounded through the body of the car but some of the GM cars have both a ground and a positive wire. You will just have to determine which one your car has.

Photos would be nice if possible. Just a shot of the compressor, and not even a full picture but whatever you could get, would tell us a lot on how to proceed.

[Tomguy]

Jose,
How did you check the freon level? If the compressor is running you cannot get an accurate pressure reading from a gauge. Having pressure in the system isn't enough to know. You really do need gauges to do anything A/C related (trust me here! Others will say the same thing. Even the cheapo set I got from Harbor Freight for $40 has proven essential in setting pressures right on both my Jeep I converted to R134a and my 300M that I have replaced the compressor & evaporator on).

Does the compressor kick on at all? If not, you can't really check the pressures anyway. Sure, you can see what the static pressure is, but it won't give an accurate representation of the proper high/low side values, unless someone else with a w108/109/111/112 with the same A/C system would give you their static reading to compare to. While the system may seem complex, it's essential to know the basics and once you do, it's easy to troubleshoot:

Compressor compresses freon to the high side (condenser).
Expansion valve/tube to bottleneck pressure (so high side stays high and low stays low).
Evaporator (low pressure).
Accumulator/Dryer (removes moisture from system).
Back to compressor.

If the expansion valve is clogged, the high side will be too high and low side too low, so the freon won't flow between the two sides. As a result, the evaporator can't remove heat to transfer it to the condenser.

If there is not enough freon, the high side won't get high enough and the heat won't be able to be dumped from the system at the condenser. Obviously, any leaks can cause this.

If there are no leaks, your evaporator is likely fine. It's unlikely to become clogged unless you have another issue, like a dying compressor spitting metal into it, and even then the expansion valve is more likely to become plugged/stuck before the evaporator goes bad.

The condenser is the high-side part. If your high side pressure is too high, it may blow out. A bump, or piece of road debris (like a rock) can also pop the condenser. It can also become clogged (especially during love-bug season in FL) and lose efficiency.

Without pressure readings though, everything else is just a guess. If you do need to put freon in, I'd suggest using the kind with UV dye, so you can find your leaks.

[Baxterdown]

Hi guys,

Thanks for all of your replies! Sorry I didn't update you sooner, but I woke up Saturday morning to this!



And to illustrate how bad of a leak it was, here's a quick video:

Radiator Leak - YouTube

Needless to say, it was off to the mechanic IMMEDIATELY. It turned out to be the water pump, which we're having to replace. So, since the car is there already I decided to have the mechanic do the A/C work as well (I don't have the tools or knowledge anyway).

He hasn't diagnosed the A/C yet; he's working on getting the radiator fluid leak first. But should start on it tomorrow. I'll keep you guys posted.

Jose