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Mercedes speed sensor

wrooper on Sat May 22, 2010 11:38 AM User is offline

Year: 1991
Make: mercedes
Model: 300te
Engine Size: 3l
Refrigerant Type: r12
Ambient Temp: 80
Pressure Low: 35
Pressure High: 240
Country of Origin: United States

Hi all,
My car will cool well when parked and testing but when driving the compressor cuts out and will only reengage by restarting the vehicle.

I have narrowed down the problem to a compressor speed sensor based on the restart fix. Apparently the AC computer compares engine speed to compressor speed and cutsout compressor if speeds vary to protect the system from seized compressor parts.

My belt is new and belt tension is good

Is there a way to bypass the speed sensor for confirmation of the diagnosis? My car does not have the Klima relay which is apparently easy to fool. Mine uses an combined relay for fuel inj.AC etc called a MAS.

Shouldn't I be able to jumper the MAS pins that receive the engine and compressor signals to "fool" the MAS?

Anyone know which pins to jumper?

Anyone have an opinion of my diagnosis? I believe I have an original Nippondenso compressor with 232k miles

Anyone know of another sensor that will only reset with engine restart?

Thanks in advance

TRB on Sun May 23, 2010 10:48 AM User is offlineView users profile

Sorry personally do not do much Mercedes work. Maybe someone else will chime in on this thread.


When considering your next auto A/C purchase, please consider the site that supports you:

NickD on Mon May 24, 2010 6:10 AM User is offline

One option is to replace the speed sensor to keep the car stock, not cheap at 214 bucks, but that is Mercedes.

Or perhaps since it is working most of the time, connections require cleaning.

In making modifications like this, one needs both the electrical service manual and the vehicle, sorry, I don't have neither. Also a lot depends where the tach pulse originates from, if directly off the ignition system, more distributor loading can affect your spark output resulting in engine misfiring. I had some of these coils apart, that's right, just a coil of very fine magnet wire with a permanent magnet at the core with a slotted wheel that varies the flux density as the compressor rotates.

Idiots don't even know how to make a coil correctly, entire coil was impregnated in a thermal expansion epoxy that would break connections when the coil heated up and they want a fortune for this piece of crap. Correct way of making a coil is to seal it with tap, a this coating of silastic that acts as a thermal buffer, then encase it with epoxy to make it water proof.

I classify this so called belt slippage circuits with air bags that have killed just as many kids as saved lives of adults that refuse to wearing their safety belts and anti-thief electronics that keep the right owner with the correct key from starting their vehicles.

Oh, and if your compressor pulley bearing seizes, for more likely to happen as those are also crap today, that speed sensor is worthless. Your belt will break and leave you stranded.

Ran into a guy yesterday at the boat launch with onstar, backed up his boat, got out and closed his doors with the engine running and his doors locked him out. Borrowed a cell, called GM, he didn't have that door unlocking feature in his vehicle so called his son that he happened to have with an extra set of keys. Not only inconvenienced him, but everybody else waiting to launch. All this new electronic stuff is crap in my opinion, don't need it.

Edited: Mon May 24, 2010 at 6:18 AM by NickD

MBDiagMan on Mon May 24, 2010 10:21 AM User is offline

If the belt and clutch are in good shape and not slipping, it is likely that the compressor is internally worn allowing thrust of the internals which trips the sensor circuit.

Have a great day,
ASE Master

Edited: Mon May 24, 2010 at 10:26 AM by MBDiagMan

NickD on Mon May 24, 2010 10:50 AM User is offline

I try not to speculate on problem sources, can be anywhere. Thing here is to either find the problem and fix it, or to modify the circuit to get rid of it. Either can be a problem.

MBDiagMan on Mon May 24, 2010 11:04 AM User is offline

Good point! Can I change the word "likely" to "possibly?"

I have seen this in the 124 series MB's a number of times. The circuit was changed in the later 124 cars, but I'm not sure when. I think 92 was the circuit change. The early cars have a Klima relay in the compartment behind the battery. That is where the rotational sensor can be by passed. It is a relay except it recieves an input indicating that the rotational sensor matches engine speed. A regular ice cube relay can be wired in it's place to eliminate the rotational sensor input. A Factory Service Manual will show you the coil and contact wires that you need to attach to the new relay. If one is not comfortable with such wiring issues, I would highly recommend finding someone with adequate electrical knowledge.

Hope this helps.

Have a great day,
ASE Master

wrooper on Mon May 24, 2010 5:06 PM User is offline

wrooper on Mon May 24, 2010 5:18 PM User is offline

In my first post I mention that my car does not have a Klima relay. I have been aware that compressor wear may be the issue but with 232k I am willing to be stranded when compressor finally gives up.

On a positive note I may have found my issue. I think it may be the evaporator temp sensor or the aux coolant pump. Either will route it's signal [or signal interference in the case of a worn aux pump] through the panel control unit in the dash. I have unplugged the aux pump under the hood and replaced the control panel and now have normal operation for the past 30 miles. Will require more shakedown to be sure.

I was on my way to the Radio Shack to buy a resistor [10-15k ohm] to put inline in place of the evap temp sensor to fool the system when I discovered my AC working normally but only in max cooling setting. When I got home I replaced the panel control unit and unplugged the aux pump. Apparently the aux pump begins to draw excess amps as it wears and wreaks havoc with the panel control unit

Thanks to all and I will update when more miles are under my belt

MBDiagMan on Tue May 25, 2010 7:45 AM User is offline

You REALLY need to check the current draw of your aux pump. It should not draw over 1.2 Amps. If it does it will very easily blow the CCU.

BTW, losing the compressor while on the road will incur an inconvenience FAR beyond being uncomfortable while driving home. This kind of failure would cause LOTS of debris to be strewn throughout the entire system requiring complete dismantling, flushing et al. This is what the rotational sensor circuit is there to prevent. As Nick said, these sensor circuits have sense been found to be less than perfect, but if working correctly it will prevent the system damage I describe above. The compressor will be disabled before it makes a total mess of the innards. At least that's the plan.

Good luck

Have a great day,
ASE Master

wrooper on Tue May 25, 2010 10:40 AM User is offline

I have seen several posters on other sites who have disconected their aux pump for years with no ill effects. It apparently disables some "feature" of the climate control system that improves comfort during heating season. At 232k I am not inclined to replace it for this add'l comfort.

Thanks for your replies,at day two my fix is holding [it's 92 and humid here today]

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