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  #1  
Old 07-13-2013, 04:06 PM
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85 300D - Tach reading too high

I just finished up an OM617 engine swap from a W126 to a W123 chassis. Long story short, I used the tach wire from the W126 engine that was attached to the engine, and connected it to the W123 car. I'm getting high reading on the tach. Should I use a tach amp off a W126 to correct this?

Here's what I'm seeing for the speed and tach. The trans is shifting to 4th, and feels fine. And I can tell the engine isn't over-revving, so it has to have something to do with the swap between the two chassis.

Any ideas?

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85 300D - Tach reading too high-img_20130713_135518_517.jpg  
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #2  
Old 07-13-2013, 04:28 PM
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What's the tach reading at idle? Should be 650 to 750 tpm.
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Old 07-13-2013, 09:16 PM
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Originally Posted by funola View Post
What's the tach reading at idle? Should be 650 to 750 tpm.
Hmm, I'll have to check...
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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Old 07-13-2013, 09:25 PM
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Anyway, I had some various pieces of days available over the last couple weeks as I transitioned to another job, so I tried to wrap up the swap project on the car. The wife will be getting this car for her DD. I will be selling her 87 W124 300D when that happens. The 87 is her current car. She doesn't like the size of the W126's, and she hates the sluggishness of the 240D automatic. She loves the W124, but I prefer as the guy who works on them the W123 chassis, which she's fine with if it will get out of it's own way. She's OK with the 85. So, I had a few hours today to go over and pick up the car from where I did the swap. Ran great. Needs to be cleaned up, but no rust anywhere, and the interior is in extremely good shape. Here's a couple pics I quickly snapped now that it's with most of the other cars back at the house.
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85 300D - Tach reading too high-img_20130713_200959_121.jpg   85 300D - Tach reading too high-img_20130713_201050_728.jpg  
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #5  
Old 07-21-2013, 09:06 PM
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Quote:
Originally Posted by jusme View Post
Hmm, I'll have to check...
It's reading around 1000-1100RPM at idle. Definitely too high. Should I try replacing the tach amp as a first step?
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #6  
Old 07-21-2013, 09:57 PM
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Quote:
Originally Posted by jusme View Post
It's reading around 1000-1100RPM at idle. Definitely too high. Should I try replacing the tach amp as a first step?
The 85 does not use a tach amp.

Here is a link with diagrams for you to check out and may help with your problem.

1985 300 D Tach Problem



.
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Old 07-21-2013, 10:11 PM
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Quote:
Originally Posted by jusme View Post
It's reading around 1000-1100RPM at idle. Definitely too high. Should I try replacing the tach amp as a first step?
Does it sound like it is idling at 1100? Do you have another 617 to compare idle sound to? There is an adjustment on the tach circuit board but you'd need a photo tach i order to calibrate it. Otherwise adj it by ear.
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Old 07-21-2013, 10:15 PM
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Originally Posted by funola View Post
Does it sound like it is idling at 1100? Do you have another 617 to compare idle sound to? There is an adjustment on the tach circuit board but you'd need a photo tach i order to calibrate it. Otherwise adj it by ear.
No, it's definitely idling at the right speed. So, there's an adjustment screw on the board? I didn't notice it when I had the cluster out, but then again I wasn't looking for it.
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #9  
Old 07-21-2013, 10:20 PM
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Originally Posted by DeliveryValve View Post
The 85 does not use a tach amp.

Here is a link with diagrams for you to check out and may help with your problem.

1985 300 D Tach Problem



.
Well, there was one on top of the inner fender on the driver's side... I wonder if the PO moved it over when he was going to go with the W126 engine. hmmmm...
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #10  
Old 07-21-2013, 10:24 PM
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I'm not familiar with thr W126 "tach wire". Check that it has only one tooth on the harmonic balancer that the mag pickup is sensing. If it has 2 tooth, that could explain the high reading.
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Old 07-27-2013, 10:52 PM
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Quote:
Originally Posted by funola View Post
I'm not familiar with thr W126 "tach wire". Check that it has only one tooth on the harmonic balancer that the mag pickup is sensing. If it has 2 tooth, that could explain the high reading.
The wire I'm talking about is the feed coming off the sensor on the balancer on the front of the engine.
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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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  #12  
Old 07-28-2013, 01:54 AM
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85 will need the pickup on the engine adapter plate. The sensor at the balancer is the wrong reading for the 85 tach. Well also cause the moms relay to shutdown the ac compressor due to engine and compressor perceived speed mismatch...
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  #13  
Old 07-28-2013, 01:19 PM
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Not sure what the op has. In his first post he said he swapped a 617 from a w126 into his 85. What year is the w126 and what's different re the tach?
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  #14  
Old 07-28-2013, 03:33 PM
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What year was the 126 engine out of? The engine doesn`t make any difference actually. It is where the tach Pick is located.

The 84 - 85 picks up the signal off the FW from the adaptor plate. there is a flat spot in the casting for the sensor to bolt to. this sensor wire then sends a signal to the Black Box behind the pass side kick panel. the box is only to operate the EGR and to operate the Tach.

If you are using an engine from a 81 - 83 and it`s adaptor plate, then there is no place to connect the sensor wire to.

The 81 - 83 uses a pick up off the crank in the front of the engine and sends it`s signal to the Tach Amp on the L/F inner fender, then to the Tach.

The 84 - 85 only has the female plug on the inner fender with a small cap. this is probably a place for the dealer to hook up to for diagnostic checks.

Now if you add a Tach Amp to this diagnostic port, and then are trying to use the Tach the 84 - 85 uses, then it probably won`t work. you will have to swap in a Tach from a 83 - 84 to get it to work properly.

The other option is to ( A lot more work) swap the Adaptor plate for the rear pick up.


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  #15  
Old 08-02-2013, 03:43 PM
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I would guess it's an 81-83 engine. The pickup is at the front of the engine. What's odd is the tach works smoothly, very fluid. But, it's the speed that's registering too fast. Now, someone mentioned an adjustment screw on the tach circuit board. Is this the case on an 85 W123? I'd love to address it from that angle if it's that simple. I can get it close by ear. Well, closer.

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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D
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