W124 A/C Diagnostic
 

After not having A/C since I bought the car, I tried to diagnose why the A/C was not working on my '87 300TD. I went through the test procedure section A of "83-605 Checking compressor cutoff and remedial measures". I pulled the Klima relay of and this is what I found:
Step 1: The measured voltage was nominal between terminal 5 and 1, 13V, OK
Step 2: The measured voltage was nominal between terminal 5 and 10, 13V, OK
Step 3: I bridged terminals 5 and 7 and briefly started the engine. A/C compressor clutch engaged and I had cold air blowing inside. OK
Step 4: Terminals 5 and 7 remain bridged and I measure the Volts (AC) at terminals 9 and 11. The nominal value is at least 0.3 VAC but I only measured 0.15 at idle and this went up to .25 when I revved the engine to 1500RPM. Since it did not pass the test, It says with engine off, to measure the Ohms between terminals 9 and 11. The nominal values should be between 530 and 650 Ohms. I was at 620, right in the range. Is this sensor good or bad???
Step 5: Since I have a diesel, I measured the Volts (AC) from the engine speed sensor between terminals 1 and 2. The nominal value is > 4 VAC and I got 5.68 VAC at idle. It also mentions to test the Ohms of this sensor and the nominal range should be between 1.7 and 2.1 kOhms. I measured 8.6 kOhms. Is this sensor good or bad??
Step 6: The measured voltage was .02 DC well in the < 1 V DC nominal range. OK

How do I interpret these results? By failing Step 4, I would think the Ohms would also fail, but they were OK. Does it mean the compressor is slipping? The belt is tight. By failing the second part of Step 6, I would think this sensor is bad. However, this is the same sensor that drives the tachometer and it seems to be working fine. If all these tests were completed OK, that would mean the Klima Relay is bad.

Do I buy:
a) Klima Relay $120
b) Engine Speed Sensor $50
c) A/C Compressor Speed Sensor $120
d) Rebuilt A/C Compressor $200 +

Any input of experience in this diagnosis would be appreciated!

1987 300D 186Km
1987 300TD 198Km
2004 CLK 500 Convertible 2Km

Buy a 1980 123 with the manual control AC..... seriously, electricity on a TOO complicated car is a real bummer.... This kind of stuff can drive you crazy...
Someone will post who has already been crazy and recovered.... but may not be soon on a Holiday Weekend post...

Steve mentions that he has seen an OVP failure cause A/C control problems. Don't know if your car has an OVP.

http://www.peachparts.com/Wikka/AcDiags

The compressor protection circut are needed since a locked up compressor would pretty much strand you. Its also nice to have since a Nippondenso compressor is not cheap.

Don't spend your money on a compressor without gauging the suction and discharge pressures. If the compressor pumps pressure and isn't rattling like it is about to fly apart, you do not need a compressor.

I have never worked on the electrics on a car like yours and will leave comment on that to someone who has. I am interested in this and I hope you will post what you find. Generally a relay is just a switch that is switched by an electromagnet. If you have correct power in and correct power out, you can't fault a relay.

Car repair shops have generally bad reputations and A/C shops are at the bottom of the heap for trustworthiness. You really need someone qualified to ensure that the refrigerant and lubrication in your system are up to spec. The fact that you got cold air out of it is a good sign. Try to find someone you can trust.

Electrical problems can be very frustrating, sometimes it is useful to walk away and make another attempt later.

I ordered a new Klima Relay. I will keep everyone posted!

1987 300D 186Km
1987 300TD 198Km
2004 CLK 500 Convertible 2Km

I bought the new Klima Relay and it fixed the problem. I had cold A/C for about a month.
After that, a new problem developed and the A/C only worked when the engine was cold. It stopped working when the engine reached normal operating conditions. When the engine is restarted, A/C works again briefly and then stops.
I brought the car to a local shop who measured the amount of refrigerant and determined it to be OK, ie. none leaked out. I told them to replace the low/high pressure switch since it was something I could not do on my own with my limited tools. They said the A/C works fine now.
I am skeptical and will watch what happens. Has anyone seen anything similar?


1987 300 TD 200Km
1987 300D 186Km
2004 CLK 500 Convertible 4500m

I just witnessed this in my 87 300d turbo. only mine would work during the first start of the day. one difference is that mine was converted to 134A by the PO. Very puzzling indeed, I picked up a kit that had a pressure gauge with it, and my 134 level was in spec, but still no compressor activation except on the first start of the day.

djankov

I know the link I supplied from Steve B in my earlier post is long and detailed but if you take some time and read it I think you will find that it describes your exact symptoms you are experiencing.

The link also explains how the system operates and how to fix it.

Thanks, I printed it out to take a look, I'll have to read it over after work.

Sounds like you might have a slipping belt.

Good luck,

Today I picked up my car (300TD) and the A/C worked for 10 minutes until I got on the freeway. Then it stopped. It never went back on, even after turning off the engine and turning it back on. I went back to the basics and jumped the 5 and 7 terminals on the relay plug to trigger the clutch on the compressor. It did not work, the compressor did not turn. It always worked before. Does anyone know of a reliable place that sells compressors? I see places all over that have it for about $180-200, but I know if it is not properly rebuilt, there will be problems again.

TimFreeh, I read the link you supplied, Thanks!

LarryBible, I replaced the belt, idler pulley, and spring about 6 months ago, Thanks


1987 300 TD 200Km
1987 300D 186Km
2004 CLK 500 Convertible 4500m

Not sure what you meant by compressor did not turn...
Are you saying that it is frozen up ? Will it turn by hand?
If it is not frozen then it might be the clutch is not operating properly. Even with your " hot wiring".... did you check to see if electricity is actually getting to the compressor clutch.... the Brotherton site makes it look like several things can control this... maybe something lower on the tree has cut electricity to the wires you thought would ' hot wire' it.....

Turn on the defrost and see if that engages the compressor...

Today I tested the A/C system again. With the engine cold, I measured the Ohms between the 7 terminal and ground ( had to removed the Kilma Relay) as mentioned by stevebfl in another thread
http://www.peachparts.com/shopforum/showthread.php?threadid=100449.
The resistance was 4.5 Ohms. I started the engine and turned on the A/C to max. The compressor ran fine for about 10-15 minutes. At that point, the clutch dissengaged and I could not get the compressor to turn on again. Then I measured the resistance and found it to be 82.5 Ohms. I think I have a clutch that overheats and will not engage. I don't think it is worth to get only the clutch rebuilt, so I guess I am forced to get a new compressor. Does everyone agree?

Thanks,

1987 300 TD 200Km
1987 300D 186Km
2004 CLK 500 Convertible 4500m

I really really hate to sound like PEH...
but AC systems are in the category of " if it is working" then don't mess with it....
I consider it working if something not in contact with the refrigerant can be unbolted and fixed and put back on....
and I think this is the case ... if you have diagnosed this correctly... I would pull it , repair it, put it back on.... and keep on trucking.....

I agree with Leathermang - the Nipponsenso A/C compressors are fairly reliable, it seems like its not unusual to get 10-20 years of service out of one depending on how much you use the A/C in the summer. I'd expect less service life in Texas and alot more in New England.

I also agree with your diagnosis - it seems the solenoid coil is a bit flakey.

I took a quick look at my W124 service CD and it appears its not too difficult to change the clutch and it also appears that you might be able to change the "coil pack" associated with the electro-magnetic clutch without changing the entire clutch assembly. It's not clear if you have to discharge the system to change the clutch but the manual does advise removing the compressor if you have a six cylinder engine.