My mechanic has found a discrepency in RPM readings between several sources.

At idle, all three control units in the box on the passenger side of the engine, the shop's scanner and "fluke" meter read 500 +-20rpm, but the LH Module indicates 700-750.

At 1,500 RPM per the three control units and two external meters, the LH module indicates 2,000rpm.

Any ideas would definitely be appreciated. Could the faulty LH reading lead to hesitation/stalling off the line?

Can LH modules with different part #s all from 500e be swapped?

How does the LH module read RPM (source) and what does it do with it?

Thanks very much in advance!

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RJ
'92 500E
'75 Triumph TR 6

I have two suggestions.

Take out the crankshaft sensor and see if it has any debris on it. If so, clean it off.

Also, I would check and see how much A.C. current (noise) the alternator is letting out. I know that if the LH sees enough noise, it can think it is seeing additional crank reference.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

Thanks Donnie,

We've replaced the Crank Position Sensor.

Someone else recently suggested the Throttle Body might have gone bad (idle position switch), so we'll test throttle position and swap with another car tomorrow to see if that makes a difference.

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RJ
'92 500E
'75 Triumph TR 6

First, let me thank everyone who has helped with trying to diagnose my idle problem both on the forum and offline via phone calls (Partshop Phil, Donnie, Bergwerks, etc.), email (Shanta, Donnie) and, in particular, by providing a donor vehicle to test components (John).

Having tested John's LH module, we find the descrepency to be common and swapping his LH module into my car did not solve the hesitation or low idle problem.

We also swapped in John's Throttle Body without effect.

Anyone have any experience with a VANOS gone bad? That's the next theory as this all started when the exhaust cam broke off the timing gear (non-vanos) and we think might have screwed up the intake vanos (e.g., not advancing or not coming "home" all the way at idle).

Thanks again!

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RJ
'92 500E
'75 Triumph TR 6