Klima Relay 2 - Electric Boogaloo - PeachParts Mercedes-Benz Forum

Bengoshi2000 · #1 08-13-2021, 09:14 AM

Put a new Klima relay in my 91 300D on Monday. AC fixed. I enjoyed cool air until halfway to work this morning. Driving along and suddenly, no cool air. Before I went into work I was able to verify the compressor clutch was not engaging (which was the symptom prior to replacing the klima).

I plan on checking the pressure switch this weekend, but my primary concern is that something has shorted and blown the new klima. I'm going to do an autopsy on the old relay.

So, is there anything in the circuitry of the ac system that could short out and blow a klima relay?

Bengoshi2000 · #2 08-14-2021, 10:47 AM

I bypassed the pressure switch with no effect. Pulled my new relay, reinstalled, no effect.

I cut open the old relay and found a couple burnt traces on the board. The burned traces are on circuits fed by pin 11 (AC RPM Sensor) and pin 1 (ground). I'm going by this diagram: Compressor speed sensor.

I also found two cold solder joints for the internal relay itself (both fed by pin 5).

I guess my primary question is whereabouts is the AC RPM Sensor (and is it capable of shorting out the klima relay)?

123boy · #3 08-14-2021, 07:43 PM

Did you try bypassing the relay?

Bengoshi2000 · #4 08-14-2021, 08:25 PM

To do that I jump pins 5 and 7, correct?

123boy · #5 08-15-2021, 07:16 PM

Not sure as I don't have the relay schematic in front of me. I'll try and get that info for you. Of course, I'm sure you realize, you're jumping the relay ONLY to verify everything is working when you bypass the relay solenoid. Disclaimer: Not jumping to drive around, to work, etc.

Very short duration. Test purposes only.
Good info here: Compressor speed sensor.
1 Ground
2 Engine RPM
3 Not used
4 Accelerator microswitch (normally open/grounds at full throttle)
5 12V+ from fuse box, hot in run
6 Kickdown switch/solenoid
7 AC compressor
8 Not used
9 AC RPM Sensor (red)
10 Pressure switch (normally grounded)
11 AC RPM Sensor (white)
12 High temperature switch (normally open/grounded when too hot)

INSIDIOUS · #6 08-15-2021, 10:10 PM

Suppose you wanted to energize the compressor, switch in the cabin. Any problem with 12v?

Bengoshi2000 · #7 08-16-2021, 06:47 AM

I ran across that compressor speed sensor link yesterday and I'm highly inclined to build the dual-relay workaround that mxfrank designed. I just need to find the appropriate relays.

But before I go there, I'm really curious what is in the compressor speed sensor circuit that would have caused the pin 11 traces on my old KLIMA relay to burn out. Would some sort of short at the speed sensor cause that?

Within the next year or two I plan on doing a teardown/rebuild of the ac system... I just don't have the time, patience or $$ to do it right now. I'd LOVE to convert to a manual ac setup but I'm sure that would be uber expensive.

Quote:

Originally Posted by 123boy

Not sure as I don't have the relay schematic in front of me. I'll try and get that info for you. Of course, I'm sure you realize, you're jumping the relay ONLY to verify everything is working when you bypass the relay solenoid. Disclaimer: Not jumping to drive around, to work, etc.

Very short duration. Test purposes only.
Good info here: Compressor speed sensor.
1 Ground
2 Engine RPM
3 Not used
4 Accelerator microswitch (normally open/grounds at full throttle)
5 12V+ from fuse box, hot in run
6 Kickdown switch/solenoid
7 AC compressor
8 Not used
9 AC RPM Sensor (red)
10 Pressure switch (normally grounded)
11 AC RPM Sensor (white)
12 High temperature switch (normally open/grounded when too hot)

Bengoshi2000 · #8 08-18-2021, 09:39 AM

I'm going to give a bypass relay a shot using MXFrank's diagram:

Compressor speed sensor.

and the relay he referenced in this post:

Compressor speed sensor.

Two questions:

1) do both relays need to be the lower 180 ohm "ice cube type" or just Relay 1 (compressor relay) in the diagram?

2) what value for the diode between pins 12 and 4?

Cyclotronguy · #9 08-18-2021, 06:28 PM

Diode

Clearly not a zener....

I would guess any general purpose diode (AKA rectifier) Personally I'd want at lest 200V piv, just in case we have back emf from the clutch coils raising havoc.

You should be able to look at the old diode and sub in something equiv in terms of ampacity. Should be pretty cheap from Marlin P Jones or Jamco. By cheap meaning less than the cost of a cup of designer coffee

Kent

Bengoshi2000 · #10 08-20-2021, 07:51 AM

I have some 1N4007 diodes left over from a radio restoration project. Do these specs look to be robust enough for this task?:

1N4007 Characteristics:
Maximum Recurrent Peak Reverse Voltage 1000V.
Maximum RMS Voltage 700V.
Maximum DC Blocking Voltage 1000V.
Average Forward Current: 1.0A.
Peak Forward Surge Current: 30A.
Maximum Instantaneous Forward Voltage: 1.0V.
Maximum DC Reverse Current At Rated DC Blocking Voltage: 5.0µA @ 25°C.

BillGrissom · #11 08-25-2021, 02:04 AM

I don't know the 1991, but in my 1985 300D, the same rpm signal feeds the dash tachometer and the Klima relay. If the tach needles shows 0 rpm, the AC clutch will not engage. Those route thru the EGR Box which is the R kick panel of mine. The problem there is often than the OVP relay above it fails (or its integral fuse). I used instructions here to remove the EGR Box and jumper across its pins, then my tach and AC worked again.

Another time when my AC failed, the culprit was a failed anti-ice switch in the cabin evaporator. A fairly generic part. I used one for a 1970's Chevy which fit other than 1 bolt hole. I recall the maker is Ranco or such, but just look at yours. It should conduct between its 2 terminals when above 32 F.

I can't think any way the rpm signal could kill the Klima. It is just a low-voltage sinusoid with little energy capability, produced by the magnet pickup on the transmission bell-housing (termed "variable reluctance").

My 1984 300D actuates the AC clutch via a standard Bosch relay (but round pins) on the L inner fender. I don't see a diode, but you could google the 1984 wiring to see if they used one somewhere in the circuit. You can buy Bosch-style relays with those protective devices, though probably not needed since people don't even use them in switching radiator fans which likely give more back emf when turned off than an AC clutch. The 1N4007 diode should suffice, as I doubt the AC clutch draws more than 1 A continuously.

Bengoshi2000 · #12 08-27-2021, 12:14 PM

Two steps forward... one step back.

I built the bypass relay setup:

Round One - failure. I used a 4 pin relay instead of a 5 pin (for the cutout relay).
Round Two - failure. This time the relays were correct. I triple checked. Nothing.
Round Three - failure. I used the Panasonic relay simply to see if I could get the compressor clutch to engage. (pin 10 to 85, pin 1 to 86, pin 5 to 87, pin 7 to 30). No dice.

Evidently, I'm not getting any voltage on pin 5 (or I'm not getting the signal voltage from the PBU on pin 10). At least there's the possibility I didn't fry the new Klima relay and the problem resides elsewhere.

Back to the multimeter...