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Old 07-03-2003, 01:05 PM
Registered User
Join Date: Sep 2002
Posts: 134
Question 124 Acc

This is my first post, I would like to thank all people in this site for their help and advice, I learned alot of information
My car is 124 300DT 1987, running good with @175K miles...

I have a problem with the ACC [AC], I started trouble shooting, there are 4 main concerns:
[since this is a repeated problem I appreciate more explanations, I did search(s) in this issue, there is alot of info., but not organised in a good sequence for diagnosing the problems...I appreciate more detailed explanations so that this post could be a source for many users]

1. getting the compresser to engage:

1.1. It starts and engages for "some time" and disengage, usually less than 1 minute. The time depends on whether the AC [clutch] was working before; if I jumped the terminals [klima input 5, and 7], the clutch engages. Letting the compressor works for sometime, then install the klima, system works for longer time, 5 minutes or so... Also, up and down RPM would cut the compressor [with klima]. [I do not know why this behavior]

1.2. I checked the output signal of the compressor RPM sensor, at the klima is less than 0.1v at idle, and can increase to 0.2 or 0.3 v...which does not match the minimum required [0.3v at idle]. SO I need advice on how to replace the compressor sensor [ do I have to discharge the system, is the sensor in touch with refrigerant, does the pressure affecting the sensor output - may explain the previous behavior, could be the compressor bad, and replacing sensor will not help, is there a way to clean/verify the sensor is working....please advice]

1.3. The signal of engine speed (RPM), it comes to the Klima at about 10 - 11 V, in the diagnostics, it suppose to be (more than 4 v) I am not sure if this is acceptable!!!???. Also, this signal stays within this limit even with higher RPM, I think they have this as steady and the klima compares the compressor speed to this signal..the klima cuts the compressor if its speed is out of ceratin range for few seconds or so (usually between 5 to 30%, as I read in other place not mercedes)

2. Another technical issue, the signal comming from the ACC bush-buttons unit, that tells the Klima [little brain activates the compressor if everything is OK!!] to activate the compressor, this signal is available at the pressure switch [red]..It suppose to be negative (ground)...when I tested it, it was (+), possitive battery voltage...I accepted that there could be different systems with different signals...BUT the same signal comes to the Klima at terminal 10, and the diagnostic says that this is a negative...I tested there and it was a negative ...I am totally confused, how a possitive signal can be converted to negative!!!??? please help me on that...

3. System pressure seems to be OK, no leak, always engages [no need to jump the pressure switch terminals]. however, it happened that the clutch was slipping when hot, could this be because of high pressure building up [the aux. fan is not working]. Or the clutch needs cleaning [oil inside], or bad clutch...
please advice me on how to take the clutch out and clean it, without discharging the system.

Again, thank you for your help and the time you spend in reading and advicing...

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Old 07-03-2003, 03:31 PM
gsxr's Avatar
Join Date: Aug 2001
Location: USA
Posts: 8,031
Really not sure if I can help much here, but I do know there is a tachometer on the compressor. It compares compressor speed to engine RPM. If they are more than "X" amount different, the computer will kill the signal to the clutch engagement to save a slipping clutch, or prevent the serpentine belt from damage. Might just be your clutch problem is causing all the other problems...?

Dave M.
Boise, ID

1997 E420 - 155kmi (Bugeyes)
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Old 07-04-2003, 07:07 PM
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Join Date: Sep 2002
Posts: 134
Thanks gsxr, yes there could be a problem with the clutch, but the system does not work all the time, which means that the electronics NOT OK. From the test results, the AC compressor rpm does not generate the required voltage [should be > 0.3v at idle, it gives only 0.1 or less]

SO, I need help on how to take out the AC comp. sensor [installed at the back of the comp.]. DO I need to discharge the system. Is this sensor in contact with the refrigerant inside the comp., any one did that before!!!??? please help

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