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  #1  
Old 03-19-2009, 01:08 PM
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How interchangeable are the tachs???

I have an '86 560SL - two piece clock/tach gauge of course, and I'm wondering if these tach are interchangeable with other models? I have found several clock/tach gauges on W124's and W126's at my local pull-a-part...

I know the instrument clusters are different, but can I swap out just the tach mechanism?

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE
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Old 03-19-2009, 06:42 PM
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OK, I replaced just the tach "guts" from a pull-a-part W126, or maybe a 190E (I pulled 2 and don't know for sure which is which now). They were both 7000 rpm and were stamped with 004, which is the same as mine from my 560SL. The circuit board was shaped a little differently, but all the components looked the same. I removed just the tach mechanism and installed it on my 560SL plate with my original clock. Fired up the car and the needle shows the car idling right, but when I punch the gas, the needle only goes to about 1300 rpm, or so.

So, it works at idle but won't work on acceleration - any ideas? I was very careful with the needle this time and didn't push it on too hard - either you can't interchange these because some signal, or the tach itself is different; OR whenever you pull the needle off, it is hard to get it back without maybe bending something inside the mechanism??? Anyone had this problem before?

I don't know, although the needle moved smoothly and returned to 0 before I put everything back together.

ryan
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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Last edited by rgnprof; 03-19-2009 at 06:47 PM.
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  #3  
Old 03-19-2009, 07:45 PM
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What was wrong with the old tach?
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Old 03-19-2009, 07:59 PM
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It's a rather long "I screwed it up" story... It had quit working and I tried to fix it and I unscrewed 2 screws that I shouldn't have and the whole thing came apart on me...
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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE
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  #5  
Old 03-19-2009, 10:08 PM
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That sucks. I know because I've attempted to fixed many things, only to make it worse. Still, the experience counts down the road.

I did some searching, but couldn't determine whether tachs are interchangeable. It seems to me that they should be, but then again I wouldn't be surprised if there are signalling differences between different-cylindered engines. Were the donors cars both 8-bangers?

Also, the evolution of the ignition systems over time suggests to me that the window of interchangeability will be limited to those models sharing the same components. To clarify, our 1986 tachometers get their signal from the EZL ignition control unit. Do you know if the donor vehicles were so equipped?



I'm sorry that I can only offer more questions. I hope that a guru will respond soon.
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  #6  
Old 03-19-2009, 11:00 PM
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Thanks for the reply! I think you're right as far as the number of cylinders go - both of the donor tachs came off of 6 cyls - I think! I know for sure one was a 190E (W210), but I'm not sure about the second.

I have no idea regarding the ignition unit and the signal - or if it would even make a difference. I'm not sure about all of the differences in the ignition systems in the 107's over the years - let alone the other Mercedes models!

ryan
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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE
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Old 03-20-2009, 06:31 AM
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I know the signal for my '73 comes straight from the ignition points so you basically get a pulse per cylinder which is how I think they all are. Are you sure the unit worked ok before you fitted it.
Mine has been bad since I got the car and it does exactly what you describe, it is ok at idle but only shows less that 1/2 the true rpm at higher revs.
There must be an off the shelf electronic module that can replace these with a stepper motor to drive the needle or similar....I've yet to see one though.
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  #8  
Old 03-20-2009, 08:22 PM
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No, I'm not sure the salvage yard tach I picked up works at all - you takes your chances, I guess. But I did talk with a local guru today and he was pretty confident that the only tach that would work would be from another 1986-89 107 560SL. He thought it possible one would work from another 560, but probably not.

ryan
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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE
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  #9  
Old 03-20-2009, 08:40 PM
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Well, even though it's not fixed yet, thanks for sharing your experience to date. I learned something...

...that these cars can be a PITA!

Sorry, I'm a bit down after a full day of fighting a reluctant Oxygen Sensor, and then finding out that one of my new halogen bulbs isn't working...and then discovering that the old bulb wasn't working either, so it's another wiring puzzle (yes, I tried new fuses). Arrgh.

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