W124 AC Troubleshooting Confusion - PeachParts Mercedes-Benz Forum

Bengoshi2000 · #1 05-07-2022, 11:18 AM

At the end of last summer, the AC quit on my 91 300D. Research pointed to the Klima Relay so I replaced it and enjoyed a couple weeks of AC before it went out again. Now I'm getting serious about getting the AC operational. I'm working my way through the 50 step guide someone posted years back (that I printed, but can't find the link to).

What I've done so far:

jumper pins 5 and 7 on the Klima relay socket, the compressor clutch engages and I have the nice cool air in the cabin that I'm used to.
between pin 5 and the Neg battery terminal I get 13.6v (at idle).
between pin 10 and the Pos battery terminal I get 13.25v (at idle with the PBU on a setting calling for AC). Same reads .46v when PBU is off. So the PBU is sending the proper voltage signal.

With this information I got a Panasonic relay (again suggested in another thread, can't find it in this morning's mind fog) and wired it like this:

Pin 10 to relay 86
Relay 85 to pin 1 (ground)
Pin 7 to Relay 87
Pin 5 to Relay 30

This should throw the switch between pins 5 and 7 when pin 10 is hot (ie the pbu calls for ac). But alas, no dice.

I know that this configuration bypasses all of the safety/kickdown protections the Klima provides... but I'm starting with basics. If I can't get this simple configuration to work the other features are a moot point it seems.

I have pulled apart the old Klima relay and there are several burned spots on the trace leading to pin 1, which is the ground IIRC.

FWIW, Pin 1 to the neg battery terminal reads 0v and about 1 ohm of resistance.

Do I have a bad ground at the Klima relay socket?

Not sure how to troubleshoot from here. Any suggestions?

jay_bob · #2 05-08-2022, 10:24 AM

Use an 'automotive test light' the thing that looks like a screwdriver with a 12 V *incandescent* bulb in it (not an LED) and has a clip lead coming out the back.

Put the clip lead on the +12 V terminal on the battery and probe pin 1. The light should light up brightly. If not you have a bad ground.

When chasing bad grounds, use the test light to provide the source and load and let the ground path you're trying to trace complete the circuit.

You can also use a surplus bulb socket with a reverse light bulb (1157 or similar) and do the same thing. You need one that brings both sides of the bulb out to 2 wires, not a single wire plus the shell kind. Put the bulb between battery positive and pin 1 on the relay socket. That will actually load up the circuit enough that a bad ground will become apparent.

Also on 124s don't overlook the OVP relay, the fuse holder clips can go bad on the top and that wipes out pretty much all the engine electronics.

87tdwagen · #3 05-13-2022, 11:12 AM

Jaybob provided a good overview.

The only thing I'll add is if you do confirm a bad ground to the relay, you may want to consider also adding a new secondary chassis ground lead to that socket to ensure a good ground. As these cars age, the resistance and corrosion builds up in the factory wiring underneath the sheathing and can drive you batty trying to chase down.

Bengoshi2000 · #4 05-15-2022, 02:35 PM

Everything else on the car works, so I'm hopeful that means my OVP relay is fine. No other electrical issues whatsoever.

That being said...

I checked the voltage between pin 1 and the pos side of the battery and got a rock steady 12.12 volts. This is with the car off.

I made a jumper that I connected to pin 1. It also read 12.12 volts against the pos terminal. Started the car and selected an ac setting, then touched the ground jumper to the grounding point on the side of the strut tower... Nothing. Neg terminal of battery... Nothing.

I confirmed I had a good relay using a 9 volt battery. It clicks just like it should.

What next?

Zulfiqar · #5 05-17-2022, 08:29 PM

Is the clutch magnet good?

Zulfiqar · #6 05-17-2022, 08:30 PM

You can also get the klima unit to spit out the error codes from the diag panel beside the battery.

Sometimes the rpm sensor on the compressor gets iffy.

Bengoshi2000 · #7 05-19-2022, 06:23 PM

So I used a different diagram/schematic to replace the Klima Relay and I struck gold. AC is now working.

But what I have noticed is that now the serpentine belt sounds a little noisy. And the damper shock seems to need replacement. Since the diagram below eliminates the speed sensor cut out, I'm wondering if the entire problem wasn't the belt tension the entire time...

AC Klima Relay Replacement.pdf

Bengoshi2000 · #8 06-08-2022, 08:30 AM

UPDATE

A couple of weeks have gone by and the AC is working. I did get an ac tech to check the pressures and it turns out it was overcharged. Now that the pressures are correct I'm getting 39*F at the center vent (80*F ambient with 30%-ish humidity). Cooling now holds better at idle and the fans cycle as they should.

Question: Would the KLIMA relay shut off the compressor if pressures were too high?

Lesson Learned: Don't guess around with ac pressures. Get a set of gauges, do it right and experience less headaches.