1971 300SEL 3.5: AC - no 12V at compressor

[werminghausen]

I want to renovate the AC. It never worked since I own the car.
I unplugged the plug on the compressor (2 leads) and wanted to test 12V.
I started engine...turned temp switch down and turned on the AC fan.....but no 12V between the 2 leads?
There is only a temperature switch for the AC aux fan which shouldn't affect the signal for the compressor...no low pressure switch. So I assume I should have 12V at the compressor plug....but NO.
Does anyone know how to tackle this problem?
Martin

[Pooka]

You need to first test for 12V at your high/low pressure cut off switch. Current flows to this switch first and if it senses pressure in the correct range in your A/C system allows 12V to flow to your compressor's clutch.

This high/low switch will be located on your filter/dryer. It looks just like any other plug with two leads. One will be 12V in and the other will be 12V out.

So check for current at this switch. Just drag out your volt meter and ground the negative to any metal part of the car body and then probe either slot for 12V. If there is power to one side then just jumper the plug so current will flow from there to the compressor.

Then test for 12V at the compressor. If you have 12V there then you have either:

Low pressure in your system.

High pressure in your system.

A bad high/low switch.

And you will have to go from there.

But don't think you can run your A/C system long with no pressure. Not only will you have no cold air but you can also damage the inside of the compressor. A few seconds is all you will need for a proper test because all you are testing for is if the A/C clutch is pulling in or not.

This can also be done with the engine off. Just watch for the clutch to pull in as they normally make quite a 'SNAP' sound when they do.

[JPMOSE]

Pooka...there is no pressure switch on this model.

Martin...you need to check the A/C relay...it may just need to be cleaned (most likely).

I am very busy, but will download the shop manual pages for the A/C system on your car. It may be a couple of days but I won't forget.

JP

[werminghausen]

Thanks JP and Pooka

I should have told that there is no freon in the system. I stripped the old hoses (I ordered new aeroquip hoses) and have the condenser out for cleaning inside with carb cleaner.

It is correct that there is NO low/high switch....Only thing there is...a temp switch which turns the AC aux fan on/off at 52 C. This temp switch sits on the drier but has no connection to the freon circuit. I believe the temp switch just turns the aux fan on and off and nothing else?

AC relay...got it. I need to find this thing. Where is the AC relay located and what input comes to the relay. I guess the input is from both knobs (AC fan and temperature) but not sure.
I need to tackle this AC system because it is sooo hot here.
Martin

[JPMOSE]

Martin,

I will have to e-mail you the pages from the shop manual. I am sending the entire section on A/C, Heating Ventilation that was added in the later shop manual. Hence, it mainly applies to new models as of 1968 which includes late W109 and W111. Additionally, I will send you the wiring diagram specific for the W109 3.5 (if you enlarge it on your computer...it is quite clear).


JP


Here are a couple files for starters

[JPMOSE]

Here's the Electrical Circuit section for anyone else viewing this posting in the future....

[Pooka]

Ooopppsssss!

I guess I have worked on so many old A/C units they just all ran together.

So..... Time for another bold technical statement that may be wrong.

The A/C on these cars was more or less an afterthought since the car was designed in an era when A/C was not common in autos. By the time the cars were introduced to the market A/C was available on all American luxury cars so Mercedes had to come up with one, too.

One thing I seem to recall is that the relay for this could be in one of several places and that there was nothing special about it. It just looked like some of the other relays.

But it seems there is usually a relay board mounted high on the inner fender on the drivers side of US cars that contains this relay. Pulling the relays and cleaning the plugs and the inside of the sockets can often solve the problem since the problem is not the relay but built up electrical resistance.

Let us know what you find if you would.

[werminghausen]

Thanks Pooka,
can someone send a picture of the AC relay in the engine bay or the 3.5?
Martin

[werminghausen]

Thanks Pooka,

I was lucky to find a picture of the backside ECU unit (controlling D-jet) on the right side front beside the radiator. AC relay is NOT located there. All sorts of relays are there (relay aux fan, relay cold start valve, relay voltage supply, relay fuel pump...) I just looked at the backside of the ECU...the two round relays are there (aux fan, cold start valve...meaning cold start injector) plus below the boxy fuel pump relay and the bigger box voltage regulator...... but no voltage supply relay is there! At least I think this one is somewhere else.
I was looking for the 'relay board mounted high on the inner fender on the drivers side' as you said ...potentially finding the AC relay....but nothing can be found on the driver side....there is one separate fuse mounted but no single relay.

I found a couple relays on the passenger side firewall. Can someone identify these 4 relays ( 3 round ones and 1 boxy). I'll attach a picture.

Martin

[werminghausen]

update:
I looked at each of the 4 relays right side firewall but none of them seems to fit the AC relay (which is called 'change over relay' #42 in the diagram I have) because the cable colors are not matching at all. So the AC relay must be somewhere else.

1) First test was the signal from the AC temperature switch in the dash (remember the AC fan is working but I have no proof that the temperature switch is working). First test was continuity at the two leads of the switch with engine off.
I pulled the switch, disconnected the 2 cables (blue and blue-black) and measured continuity/resistance. There was no continuity meaning infinite resistance. So I thought the switch is bad.

2) Second test : testing signal at compressor leads (with engine running)with the 2 temperature switch bridged (I connected the two cables)...No signal at compressor at the 2 compressor leads.

3) Third test: then I pulled tested +12V lead at the compressor plug separately and ....YES there was +12V (I was taking ground directly from battery-).
This means that ground cable of the compressor plug is not working!
There is a second, separate ground cable directly connected to the compressor body and this ground connection is not working either!
This means the ground cable is disconnected for some reason.

4) Fourth test: I went back inside the car and connected the temperature switch again with the 2 cables AND....this time it worked. I had still +12V at the compressor lead.

The temperature switch is confusing me a bit, I don't understand how it can have no continuity measured separately and have continuity when plugged in.
Results:
a) The AC relay (where ever it is) must work
b) Temperature switch seems to work fine
c) Ground of the compressor clutch lead (brown cable) is dead.

Question to you: How is Ground cable repaired best?
Martin

[werminghausen]

Can someone tell me how to repair the Ground for the compressor ?