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Old 05-27-2005, 10:24 PM
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Question stevebfl..surprise! a/c problems

This will be a long post but bear with me. I have read dozens of threads, but none have the figures I'm looking for. I have an 86 190d 2.5 non-turbo that I bought in September. The previous owner had rigged the a/c compressor to operate from a switch in the ice cube just a switch for on and off. It was a rather sloppy install, and I didn't like the fact that it could freeze up the evaporator so I wanted to try restoring it to it's original wiring...crazy yes.

It appeared that the klima relay was new, and the rpm sensor on the bellhousing was new...the old one was left dangling from the bottom of the car. Following the procedure for compressor cut-out after verifying the pbc was ok, I checked all the inputs to the klima relay. The compressor rpm input checks ok, the temperature input checks ok, but the dreaded engine rpm signal does not match what the manual states it should be. This signal, according to the schematic for this specific car, comes first from the ring gear speed sensor into the elr idle speed control unit, then out another pin of the elr to the klima relay and td diagnostic connection, daisy chaining off the same klima pin to the egr unit, and finally daisy chaining off the same egr pin to the tach. The original signal from the ring gear speed sensor is perfect: 4v AC and increases with engine rpm as stated in the procedure. Once it exits the elr the signal is different as it enters pin 2 (GM+) of the klima connector. According to the procedure it should be the same 4v AC, increasing with rpm, but instead it is 3.2v AC and drops with increasing rpm. If it fails this test one is to measure the resistance at sockets 1 and 2 (pins 31, GM+) of the klima connector to be 1.9k +- 0.2k. The resistance I measure is in the mega ohm range and it increases as long as I keep the probes on these sockets. I have replaced the elr unit, as this seems to be the only unit the signal passes "through". I still get the same signal. The curious part is that the tachometer works fine, and this is at the end of the rpm signal chain.

Finally, my questions are: if it is supposed to be the same signal going to the klima relay as that which comes originally from the sensor, why does it pass through the elr, and can it be spliced from the sensor to the klima instead? The signal, although it does not match the manual specs, makes everything work fine including the tach and the elr unit. Is it the correct signal? Is it a mistake in the manual? I have read that it is supposed to be converted to a DC pulse signal, but why would it be measured at the klima relay as AC per manual after passing through the elr which is the only unit that the signal goes through (ie only unit that could convert the signal to DC pulse)?

If all else fails I'll just bypass it, but I wish I could keep the pressure cutout and temp cutout. If anyone with working A/C and a similar diesel system could measure the signal that would be very helpful. It's the green/black wire running from relay to relay behind the battery, and at the rpm diagnostic plug in the same area. It's getting hot down here in Florida isn't it Steve? I go to UF and live in Jax so the commute is a sweat.
Thanks for making it through this post
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Old 05-28-2005, 10:02 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
If you look at the article in DIY that I wrote about this AC you will see the scope patterns for the three signals you refer. The compressor speed signal is an AC wave with about 1.5-2v amplitute. The engine speed signal is a huge signal by comparison, probably over 20v amplitude AC wave.

This AC engine speed signal is converted into a DC square wave pulse by the idle controller or EGR controller, I don't remember which. All the other devices looking for engine speed will be looking for this square wave DC signal, although I suspect that an AC signal would be interpreted similarly as the controller is only looking for an up down signal when looking at square waves.

Yes, it is getting hot here in N Florida.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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