Two cars, two problems.

1. 82 300D that cools when car first starts, then after engine warms it blows warm/hot air even though the compressor is running and trying to cool. So far I have:
Disconnected the circulating pump - reduces the heating but does not eliminate it.
Next tried substituting parts from a part-out 1982 on which air worked prior to parting. Substituted push button unit, control box, dash and plenum sensors, installed new mono valve insert, exchanged the electro-magnet around mono valve. Have checked all five electric/vac valves. After all this the problem persists. I am starting to run out of ideas - for some reason something is telling the sytem to send heat.


The second problem is a car with a Nippon-Denso compressor with centrifugal sensor. I suspect the relay, can anyone tell me which pins on the relay that can be jumped to by-pass the sensor. I have done it in the past but loaned out my A/C manual with the schematic and my notes. Using a jumper from the battery, the compressor runs continually - I believe the relay can be jumped to by-pass the switch in the relay and in doing so the system will cycle normally but the protection is lost.

Thanks for your help.
Bob

On the 300D remove the glove box & try substituting the aluminum control box & the black plastic control box from the donor car. (they are located behind glovebox)

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Jimmy Jenkins

On the 300D you should squeeze the heater line to restrict it and be sure that the water isn't flowing. Otherwise it sounds like a refrigeration problem and I am not so good at the parts changing strategy of auto diagnosis. It really don't work well with the refrigeration.

On the later car there are enough info at the compressor relay to wire a dumb relay to the compressor on signal which will allow cycling. There is no reasonable way to just bypass the compressor speed signal.


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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Thanks Jimmy - I have replaced both the control box (the under dash aluminum box) and the black one next to it which is the blower control. Neither made a difference. As you might see from my original message, I have replaced virtually the whole system of controls with absolutely no change in system performance. I am really puzzled by this one.

Again, thanks for trying.

Bob

Steve - The refrigeration is working, compressor runs and the return line is cold. If the heater hot water supply line is closed, the system will cool. Normally on the climate control systems I find substitution (based on dagnosis) is the most reliable procedure. I have found that Mercedes test equipment for climate control and ASD systems is not totally reliable.

Bob

Well once you have determined that water flow is your problem then WHY is the water flowing? DO you have monovalve electric control? If not then why not. If so then why doesn't the valve close?

If your control isn't appropriate then I would evaluate the temp sensors and temp wheel values at the controller. We have a MB climate control tester for those old guys but we always diagnose by symptom and function evaluation; mostly from the wiring diagram. BTW be sure the aspirator hose is fully attached to the in car sensor. No air flow will mess with control.

[This message has been edited by stevebfl (edited 06-29-2000).]

Steve - My planned approach for Friday is to do exactly what you said. Will test the mono-valve circuitry. In looking at the schematic this evening I noted that it gets its positive directly from the fuse - would be nice if the problem was as simple as the fuse being blown or corroded. Often these complex & frustrating problems end up being so simple as a fuse - that should have been the starting place on the task.

Do you know the pin numbers that can be jumped on the A/C relay on the other vehicle I mentioned in the query?
Thanks, Bob

The relay would have to be wired according the the particular vehicle. Basically you would need the power that is to be switched (probably term 30), power that is ignition switched (probably some term 15), the compressor on signal (varies whether a ground or hot by different models) and finally the terminal going to the compressor. A diode protected relay should be used to avoid spiking the pushbutton controller.

If the car is a 300E, there was a great hack by "German Automotive" on this site that did exactly that. If you can't find it, let me know - I have it in a file.

Chuck

maybe your mono valve is not working properly.
or opening at he wrong time
thats how your defrost works.the ac is on but the heat is also on