It appears that my a/c system has a bad compressor speed sensor. I'd really rather not replace it, as I'd have to empty the system at the very least. And I may not be able to get access without removing the compressor. So rather than replace the sensor, I'm wondering if there's some workaround for the Klima. Admittedly, a bad belt would be a major issue without the Klima function, but that's a risk I may be willing to take.

In the Passenger compartment.

Suicidal for the Mercedes BUT At Least before it overheats (Or Worse)
the Captain will be Cool.

__________________
'84 300SD sold
124.128

x2 ice cube relay in place of Klima relay. You also lose compressor cutoff from the 105/125 switch or whatever version you have but who drives a 601/2/3 without one eye on the temp gauge?

Sixto
83 300SD can be yours
98 E320s sedan and wagon

I suspect a seized compressor would shred the belt before it shredded the engine.

A simple switch doesn't cut it. I'd have to patch it in through the ACC system so that the compressor comes on and off with system demands. I'm going to try to scope it tomorrow. If there's still a low signal, maybe I can add a simple amplifier.

BTDT -

Klima Relay Deleted om603

Klima Bypass Question

Sixto
83 300SD can be yours
98 E320s sedan and wagon

I can modify a Kilma which only monitors the freon. It will by-pass everything. You will have the full function of the ACC buttons. PM me if you are interested.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Ah...no. I can do the same thing, but there are functions performed by the Klima that I think are important, like:

1) controlling the kickdown solenoid
2) Turning off the compressor when coolant temp is over 105
3) Turning off the compressor at full throttle.

I can probably make all of this happen with two or three relays. Or maybe I could come up with a circuit mod for the Klima board. Or if the sensor has a bit of output, boost it with a single stage amplifier. Or just throw in the towel and replace the sensor.

How about faking the compressor signal?

Sixto
83 300SD can be yours
98 E320s sedan and wagon

This is about the simplest possible wiring plan if I wanted to replace the Klima with dumb relays and not screw something else up. Explanation:

- Pins 4 and 12 are the full throttle and high temp cutout signals. They present ground when either condition is satisfied. These pins get wired to a standard DIN relay, which is set up as NC. Pin 5 on the Klima is the power source for the whole system. Unless a problem is signaled, this relay will present 12v+ on 87a. This will be used to power the coil of the other relay.

- The diode is required to prevent the fan from coming on high speed when the throttle is floored.

- Pins 5&6 are kickdown control, they are simply bridged.

- Pin 10 presents ground when the CC head unit wants the compressor on. This controls the coil on the compressor relay.

- Pin 7 is the compressor clutch.

To sum up: the cutout relay will shut down the compressor relay if the accelerator is floored or if the high temp switch closes. Otherwise, the compressor relay is controlled by the pressure switch and the CC head unit.

Disadvantages:

- not sure if there are situations where the Klima shuts off the kickdown solenoid. The jumper bypasses control entirely.

- No 'belt protection' from the compressor sensor, but that was the goal so it's good/bad.

- Looking at it, I'm pretty bummed by how non-obvious it is. Anyone stumbling into it would really have a head scratcher. It took me a while to put it together, and it would take longer for someone reverse engineer. But I must admit, having spent twenty or thirty years reverse engineering Bosch cr_p, it's almost sweet revenge.


Just for the permanent record, the pinouts on the Klima are as follows:

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)

Attached Thumbnails

 

There is one other function that the Klima relay performs that is not taken care of by the solution above: There is a delay engaging the compressor clutch until the engine is running at an RPM above crank speed.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

That's right, it's another disadvantage: no chip, so no speed sensitive functionality. I have to think through whether it's worthwhile going through this effort and still end up missing some key functionality. On the one hand, no other car I know of is this fussy over compressor control. OTH, replacing the sensor means R&R the compressor. Not only expensive, it's the type of repair that leads to ten more unexpected repairs.

Most other makes don't leave the HVAC system (or anything else) energized when the engine is cranking either. As a result, it isn't something that has to be dealt with since power is cut when the engine is cranked. On these cars since the HVAC is switched "on" when the key is in "run" or "start", the compressor will be pulled in when the engine tries to crank if there isn't an RPM sensor to control clutch engagement. Quirky...

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

CORRECTED DIAGRAMS:

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 12V+ from control unit through pressure switch
11 AC RPM Sensor (white)
12 High temperature switch (normally open/grounded when too hot)

Attached Thumbnails

 

Awesome work, thanks!

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Just take it one step at a time, I still haven't made this work. Pin 10 seems to put out .75V, I can't figure out why.