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  #1  
Old 03-27-2008, 09:56 AM
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Why no tach amp in 85 300d?

while retrofitting a tach to my 82 240d, I noticed that my 85 300d does not have a tach amp under the hood. What drives the tach? Is the amplifier in the tach in the 85?

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Old 03-27-2008, 09:59 AM
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The EGR computer drives the tachometer which picks up its signal from the flywheel sensor.
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Old 03-27-2008, 10:17 AM
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good info, forced- any ideas on failure causes in this year?? I shoved ciggarette butts somewhere- but it didnt work......
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Old 03-27-2008, 10:38 AM
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Quote:
Originally Posted by ForcedInduction View Post
The EGR computer drives the tachometer which picks up its signal from the flywheel sensor.
I don't think it can be the EGR computer because the '84 doesn't have a tach amp and it doesn't have an EGR computer.
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Old 03-27-2008, 10:45 AM
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Originally Posted by Brian Carlton View Post
I don't think it can be the EGR computer because the '84 doesn't have a tach amp and it doesn't have an EGR computer.
I believe it does, especially since a tach can be added to the later 240D using the EGR computer.
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Old 03-27-2008, 11:04 AM
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Originally Posted by ForcedInduction View Post
I believe it does, especially since a tach can be added to the later 240D using the EGR computer.
Where is the EGR computer on the '84??
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  #7  
Old 03-27-2008, 11:09 AM
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Originally Posted by Brian Carlton View Post
Where is the EGR computer on the '84??
Should be in the passenger kick panel.
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Old 03-27-2008, 11:13 AM
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Originally Posted by ForcedInduction View Post
Should be in the passenger kick panel.
Does it have a different name or function? It doesn't have anything to do with the EGR on the '84.
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Old 03-27-2008, 11:36 AM
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Thanks, that helps my curiosity, especially since i had a hard time even finding the tach wiring in the mb wiring prints when I was putting the tach in my 240. I ended up just using all the components from a 82 300D and tracing the wires the hard way.
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Old 03-27-2008, 11:41 AM
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Quote:
Originally Posted by mobetta View Post
good info, forced- any ideas on failure causes in this year?? I shoved ciggarette butts somewhere- but it didnt work......
A very well hidden fuse located behind the glove box is a very common cause for in-op tach.
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Old 03-27-2008, 11:47 AM
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Here is a pic of the EGR computer with the passenger kick panel removed. {courtesy of TheDon}



It does effect tach function.....
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Old 03-27-2008, 11:58 AM
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Check out this Thread.

My 84 FEDERAL model TD does not have a tach amp nor does it have an EGR computer like a California model would have. I don't know about 1985 models, but you may find you don't have an EGR controller either.

Below is a detail on how the engine speed sensor connects to the tachometer - note that there is no tach amp, and that the federal and California models are different!

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Old 03-27-2008, 12:34 PM
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now I am confused, does only the california version have the egr computer?
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Old 03-27-2008, 12:50 PM
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its all magic
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Old 03-27-2008, 02:12 PM
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Quote:
Originally Posted by kingdoc1 View Post
now I am confused, does only the california version have the egr computer?
I don't know. My only experience with a Benz of any sort is my 300TD. Look for the EGR controller - if it's not there, you simply don't have one and the tach amp has been folded into the tachometer itself.

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