A/C & Tach problems - PeachParts Mercedes-Benz Forum

dieselmania · #1 08-21-2004, 09:52 PM

I've been having a problem with the A/C and tach operation after driving thru an automatic carwash or driving thru standing water. The engine will begin to falter and the tach will rise and fall for a minute, then the tach will quit altogether and the A/C will quit. Everything will be fine about an hour later, after it has dried out. Turning the engine off and restarting will not help, it must dry out. Immediatley after this happened, I pulled the cover behind the battery to check the relays. Both were dry and tight. I have ruled out belt slippage which I know would open the speed sensor relay.
I know there is a crank sensor, could this be the problem? It seems to happen when the underside gets wet (the carwash has an undercarriage spray).
Or is there another sensor under there which affects tach operation?
I should note that I am running without the lower engine compartment encapsulation in place, however I have run it this way for years and the problem is new in the last few months.
Any help would sure be appreciated.
Thanks, Brian

pberku · #2 08-21-2004, 11:26 PM

The AC speed sensor and the engine speed sensor both feed their signals to the KLIMA relay. The output of the two speed sensors is then compared and it if differs by more then 10% (I think) the KLIMA relay shuts off the AC. The presumption being that the belt is slipping or that the AC compressor is seized.

The engine speed sensor also provides the input for your Tach. In your case it sounds like the engine speed sensor is not providing the proper signal when wet, thus the Tach stops working and the KLIMA relay shuts off the AC as it sees different speeds at the engine and AC relay.

Fix your engine speed sensor and you should be OK

Phil

Jassper · #3 08-22-2004, 02:53 AM

Hello!

There is a small gap between AC compressor and it's clutch when the clutch is not engaged. When water gets in this gap the compressor won't engage. Either wash the engine without wetting the compressor a lot or blow the gap with compressed air. AC will work.

pberku · #4 08-22-2004, 12:45 PM

This does not explain why the Tach quits at the same time. The only probable cause that will cause both the Tach and the AC to quit simultaneously is the engine speed sensor.

Phil

psfred · #5 08-22-2004, 01:21 PM

Get a new or used sound encapsulation panel for the bottom of the engine! You shouldn't get any water up there with the pan intact.

Chances are the alternator is getting shorted by water splashing into the regulator along with soapy water getting into other things like the AC clutch. Dirty water on the serpentine belt will give you fits, to, worse if the belt is fairly old.

I believe the crank position sensor is on the rear of the engine on top of the flywheel (not completely sure!) -- check for a rubbed wire shorting out.

Peter

dieselmania · #6 08-22-2004, 03:00 PM

Thanks Peter, I'll look for that sensor. I still have my lower cover, I left it off partly because one of the nut/clips is missing and partly because it's easier to do my maintenence. I'll take your advice though, and start using it again.

psfred · #7 08-22-2004, 11:36 PM

New nut and screw set is about $1.50 at the dealer, well worth the money. Get four.

I'm surprised you can drive it with the front cover off (unless your rear one is off or missing as well), on mine the rear cover flopped down and scraped on the road if the front one wasn't in place.

Be carefull not to overtighten the screws, they only have to be snug, else they collapse the littte folded sheetmetal "nut clip" and it falls out. They should snap in and stay put.

Leaving the bottom panel off is an invitation to water in the alternator, the voltage regulator is on the bottom of the alternator!

Peter

dieselmania · #8 09-12-2004, 10:55 PM

Here's an update since I finally had time to check the RPM sensor today. The sensor has been working fine except when the car gets wet so I was suspecting a connection problem rather than a bad sensor, and I found it. It was right by the battery where the RPM sensor connects to a two pin plug. The plug had fallen onto the battery tray and the plastic was disintegrated. The metal pins and sockets were completely corroded from battery acid. I scoured the sockets with a dremel and buffed out the pins. Then I reassembled the connectors,using dielectric compound and wrapping all with Scotch rubber splicing tape followed with Scotch 33 vinyl tape. Then I secured the connection up under the cowl, safe from water and away from the battery. Took it thru the carwash and nary a problem, for the first time.
I wanted to post back as this forum has helped me a lot and maybe this will help someone with the same problem.
Anyway, on to the dash pods next!
Brian

kggkgg · #9 04-12-2005, 04:17 AM

Can you guys tell me if my 1995 E300D (W124) has a crank case sensor as described in this post?

If so is that the same as the 'tach amp' ? If not do I have one of those too and if so where do I find that?

I have a dead tacho and a/c . The a/c works if I bridge 5 and 7 at Klima so I am suspecting the tach signal is the problem. Just not sure what components there are and where to find them.

Any input gratefully received.

Keith