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  #1  
Old 10-01-2006, 11:31 PM
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Intermittent problem: Tach dies and idle drops, gets rough.

This has happened three times in the past week: my tach dies, the idle drops and gets rough. Above idle it is fine, but tach does not function. Shutting down and starting up cleared up the problem, until the next time.

Any ideas?
Thanks, John
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  #2  
Old 10-02-2006, 09:17 AM
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May now be a constant problem, did it all the way home, idle stinks, but runs fine above idle, and tach does not work.
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Grand Wagoneer 4BT project in progress!
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  #3  
Old 10-02-2006, 09:39 AM
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My guess is that you lost the speed reference for both the tach and the computer idle control. Without the speed reference, the idle falls to a low value and the engine runs rougher.

It's either the sending unit or the computer itself (less likely).
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  #4  
Old 10-02-2006, 04:04 PM
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Quote:
Originally Posted by Brian Carlton View Post
My guess is that you lost the speed reference for both the tach and the computer idle control. Without the speed reference, the idle falls to a low value and the engine runs rougher.

It's either the sending unit or the computer itself (less likely).
Any idea where the sending unit is located?
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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
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Grand Wagoneer 4BT project in progress!
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  #5  
Old 10-02-2006, 04:48 PM
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Quote:
Originally Posted by DieselCJ View Post
Any idea where the sending unit is located?
I don't have specific knowledge of that vehicle.
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  #6  
Old 10-02-2006, 05:39 PM
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Yes, it sounds like the tach sensor is going out - I had to change that on my car too. It's located on the left rear of the engine, under the oil filter housing, around the starter area. A single hex-head bolt holds it in place. It reads the rotation of the flex plate for the torque converter.

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  #7  
Old 10-02-2006, 05:47 PM
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Sorry for hijacking your thread but I have almost the same problem. Some times, when I turn on the car, the abs goes on, tach doesn't work and the elr I think doesn't work either as the rpms are too low. After about a min or so everything starts working, abs light goes off. What might be?
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Old 10-02-2006, 05:48 PM
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If you are also getting an ABS light on, that doesn't sound like the tach sensor... that sounds more like the OVP (over voltage protection) relay. Do a search on OVP and see if the symptoms match yours.

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  #9  
Old 10-03-2006, 12:07 AM
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Thanks for the information, is this the part?
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1WU0020511WU0077NO&year=1986&make=MB&model=190-D-002&category=All&part=RPM+Sensor
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Grand Wagoneer 4BT project in progress!
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  #10  
Old 10-03-2006, 12:15 PM
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Yep, that's the sensor for the tach. It's a minor nuisance to replace because you have to trace the wire back to where it plugs in to the harness, and it's not as easy as it sounds. Took me an hour or two total on a W124... not sure how long on a W201.

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1997 E420 - 155kmi (Bugeyes)
1994 E420 - 145kmi (Blondie)
1993 500E - 193kmi (Lollipop)
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  #11  
Old 10-17-2006, 10:47 PM
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UPDATE:
I replaced the RPM sender (gsxr, yes, it took longer than expected to follow the factory wiring route), things returned back to normal for the first two stops, at the third the tach died and the idle got rough. Any more ideas?

Thanks, John
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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
Jeep CJ-7 with Cummins 4BT/NV4500/AtlastII 4.3.
Grand Wagoneer 4BT project in progress!
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  #12  
Old 10-18-2006, 01:14 AM
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Shoot. I vaguely recall the sender goes to a black box of some sort before going to the instrument cluster... but I'm not sure about that. If so, that would be my next stop. I'd have to dig out the wiring diagrams though. Anyone else know offhand? Maybe verify that the fuse for that circuit is clean, and the OVP relay is good...

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  #13  
Old 10-18-2006, 06:15 PM
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OVP Relay $96 at Stealership.
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  #14  
Old 10-18-2006, 06:58 PM
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Quote:
Originally Posted by jfikentscher View Post
OVP Relay $96 at Stealership.
Howdy,
where is this located in the vehicle? How likely would you estimate this is the cause? Is there anyway I can test it? What does OVP stand for?

Thanks,
john
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1986 190D, 2.5L, 5-speed swap, 180,000 Miles (60K by me).
Jeep CJ-7 with Cummins 4BT/NV4500/AtlastII 4.3.
Grand Wagoneer 4BT project in progress!
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  #15  
Old 10-18-2006, 08:46 PM
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If it is an over voltage protection relay you might be able to monitor it with a meter once you have a look at a schematic. And understand how it is integrated into the circuit and it's apparent function. If you have a friend with one you may be able to substitute it as well for a test. The last is not too likely unfortunatly. You could buy a new one for the best price you can get and ebay it if it is not the problem. Everyone has heard so much about them you might even make a little money. Stranger things have happened. If there is a fuse iside or outside of it clean the connections and check or substitute the fuse.
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