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  #1  
Old 11-18-2017, 07:59 PM
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1985 300d tach works like vacume gauge ?

1985 ,CA emmisions third owner , when I got it from famly member
The tach worked , so did clock
Now clock doesent and the tach reads zero at idle , and rises as rpm goes up
But when floored ( and who dosent just to get going) it drops and acts exactly like a manifold vacume gauge,

Thoughts ? Iv seel lots of posts about non working but havent found these symptoms.

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  #2  
Old 11-18-2017, 09:32 PM
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85 has the tach signal go through the EGR computer behind passenger right leg panel. A known issue with it is water wicking down the harness into the connector shorting things out. Pull the panel and the connector, if water spills out, you've found the problem. I had similar symptoms in mine, cleaned and dried out the connector, sealed the harness inlets in the engine compartment (under battery) with Silicone sealant to prevent water ingress and it's been fine since. It took me a few tries of sealing with the Silicone before I got it totally dry.
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Old 11-18-2017, 11:03 PM
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Quote:
Originally Posted by funola View Post
85 has the tach signal go through the EGR computer behind passenger right leg panel. A known issue with it is water wicking down the harness into the connector shorting things out. Pull the panel and the connector, if water spills out, you've found the problem. I had similar symptoms in mine, cleaned and dried out the connector, sealed the harness inlets in the engine compartment (under battery) with Silicone sealant to prevent water ingress and it's been fine since. It took me a few tries of sealing with the Silicone before I got it totally dry.
Thanks ill look in there tomorrow .
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Old 11-19-2017, 09:34 AM
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The symptom that you have tach problems as well as a non-functioning clock point to the possibility of a bad ground between the left half of the cluster and the right. The ground is provided to the case of the clock and tach through the speedometer case to a tab underneath the bottom speedo fastening screw. The solder connection of that tab to the circuit board cracks. Re-solder it and lay a fine piece of wire across to give a better stronger connection. Also clean the oxide off of the tab.
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Old 11-19-2017, 09:49 AM
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In addition to the grounding issues mentioned above, the tach and clock also have electrolytic capacitors that dry out and fail. Replacing them typically restores functionality.
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Old 11-19-2017, 12:53 PM
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The clock in my 85 also stopped working around the same time as when my tach went wonky. The clock was fixed by replacing the electrolytic caps. The grounds in your cluster could be an issue related to your tach problems but it was not the issue with mine.
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Old 11-19-2017, 01:10 PM
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The symptom that you have tach problems as well as a non-functioning clock point to the possibility of a bad ground between the left half of the cluster and the right. The ground is provided to the case of the clock and tach through the speedometer case to a tab underneath the bottom speedo fastening screw. The solder connection of that tab to the circuit board cracks. Re-solder it and lay a fine piece of wire across to give a better stronger connection. Also clean the oxide off of the tab.
I'm sorry should have been more clear , the clock stoped a year ago , the Tach only started woorking odd about a month ago , like I said it works but very odly
Does not read at idle , at highway speed reads 2,000 +- but when accelerating it drops just like a manifold vacume gauage , so going from 55 to 80 it starts at around 2k and drops preportinatly equal to rpm rise .

Thanks for replying .
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Old 11-19-2017, 01:27 PM
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In an 85, without a good tach signal, the AC will usually not work also. It's a much more complex system compared to 83 and prior W123's
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83 240D 4 spd manual- parted out then junked
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Old 11-19-2017, 08:37 PM
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In an 85, without a good tach signal, the AC will usually not work also. It's a much more complex system compared to 83 and prior W123's
Today i took the EGR computer out , as mentioned it was corroded , i used electronic cleaner and nylon brush to work on it .

Havent used dielectric grease yet as im not sure its clean enough

Whay can I use to clean the contacts beside what i did?

Put it back together and now the tach does not act like a vacume gauage but it
Is clearly linked to the throttle , once I take foot. Off throttle the tach drops to Zero. Addt pedel and it goes back to relitive RPM

Is there a TPS on these?
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Old 11-19-2017, 10:20 PM
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Try brake cleaner and a tooth brush. My EGR computer, even though it drank water and had some corrosion, was not fried, and still works. If you do not want working EGR but want a working tach, you can bypass the EGR computer. There are threads with instructions on how to do it. What's a TPS?
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  #11  
Old 11-19-2017, 10:41 PM
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tps=throttle position sensor
a modern car thing
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Old 11-19-2017, 11:18 PM
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tach, etc

Pardon my ignorance, but can someone tell me what the EGR computer is and where it is? I have some similar tach symptoms in my 85 wagon - reads accurately when you push the accelerator pedal and falls when you let off. I would love to clean the EGR computer but I don't know where it is or what it looks like. And what is the best way to clean it?

Thank you all in advance,
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  #13  
Old 11-20-2017, 12:03 AM
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Originally Posted by will_t View Post
Pardon my ignorance, but can someone tell me what the EGR computer is and where it is? I have some similar tach symptoms in my 85 wagon - reads accurately when you push the accelerator pedal and falls when you let off. I would love to clean the EGR computer but I don't know where it is or what it looks like. And what is the best way to clean it?

Thank you all in advance,
Obviously im not an expert but the computer is located on right kick pannel
There is a squair door ( 6x6 ish) the computer is inside to get it out slide it back
A bit it will come off its mount .
Not knowing this i took the pannel off

And mine now does the exact same thing . Runs up fine but take foot off and the rpm drops to zero.
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  #14  
Old 11-20-2017, 03:13 PM
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Sorry if this is a repost :
Cleaned egr computer contacts
Still no joy

I'm totally unconvinced that there is not a mechanical or vacuume connection in here somewhere

Driving at any speed - let's say 40
Rpm show 2000+- if I lift foot off throttle the gauge drops to zero way faster than the engine could lose even 100 rpm .
Put foot back on throttle and rpm come back .

It's as if there is a tps or a mechanical connection
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  #15  
Old 11-20-2017, 03:42 PM
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RPM readout on the 85+ is handled by the EGR computer in the 617's and EDS computer in the 603's. The tach output is an amplified signal from a magnetic pickup on the crank (flywheel end or front balancer depending on design). There's no vacuum or mechanical connection whatsoever to the Tach signal on these cars. It's all electronic.

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