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Ctubutis linked this old thread to a new post I started, and I would like to update what the initial problem I had with my tachometer reading too high.

If the tach is reading too high, and you have a V8 engine with the Duraspark II ignition, the problem is in the tach ground wire. For models with an inline six engine, there isn't a ground wire. The V8 models used a ground wire, and if that ground wire is not making good contact, the tach will give a reading like it was an inline six engine. The V8 engine needs the tach ground wire to read correctly.

 

Yeppers. I solved Gary Lewis' tach reading too high in just a couple minutes time. His was a DS3 - DS2 conversion.
Near the DS2 box is a connector with 2 wires, 1 green, 1 black. The green wire goes to the coil, and the black wire goes to ground, but ONLY on a V8 truck. A factory 6 cyl truck will not have the black wire on the engine side of that plug.

As for the crud inside the tach causing erratic behaviour, these trucks are not the only ones that suffer from the issue. I need to disassemble the tach in my RX7 and clean it up. It works fine, till the temp drops to under 40F. I rarely remember about, since I rarely drive the car in temps cool enough to cause the tach to "wig out".......

 

I know I'm late to this thread, but followed the link from the pinned thread... my '86 6.9 Diesel tach is not registering at all... Seeing as how the sending unit doesn't have any circuitry in it.. (I'm assuming) Are we talking about taking apart the gauge in the dash? Or something in-between the two? That would be a wonderfully simple/cheep solution!

 

If you find out that the problem is the tachometer itself and not the sending unit, a friend of mine has a spare diesel tachometer.

 

I would take a look at the tach's pc board, as it could very well have the crud shorting it out. Can't hurt before writing the tach off as bad. As for the diesel sending unit, I never messed with them so others could tell you more about them.

Yes you will have to take the cluster out of the dash so you can take the tach out of the cluster. The PC board is attached to the back side of the tach face.

 

Hey guys, my tach was caddywhompus as well, actually it just quit working completely. I didn't have high hopes for the cleaning procedure but figured it wouldn't hurt to try. What do you know, it worked!

I took some photos along the way. This is just a repeat of the information already discussed here but maybe these will be useful to someone.


1) Remove instrument cluster from dash, then remove the tach from the instrument cluster. I’ll let you figure out how to do all this on your own, but it’s not too hard.

2) You do not need to remove the needle from the face! So don't try, you will probably just break it. Instead, start unscrewing the back cover from the tach. There are two black screws holding the back cover to the face, and then four nuts on the four terminals:

 

3) Once the back cover is off, you will see the top side of the PCB. You actually need to get to the bottom of this PCB. To remove it, carefully untwist the two small wires that run from the PCB to the needle mechanism. One of my wires was wrapped around a capacitor. Get them unwrapped and untwisted and then the PCB will just lift off the mechanical portion. The wires are just long enough you can flip the PCB over and see the bottom.

While you are working on this, be gentle. The needle on the face of the instrument is exposed and as you can see in my photos, face down on the bench. You don't want to put a lot of pressure on the instrument face.





Here you can already see the yellow/brownish gunk that is on the contacts. This is what we need to clean off.

 

4) Now to clean off the bottom of the PCB. Once you have the PCB separated from the dial mechanism, you can press down on it ok since it will be supported by the four sturdy posts. Just don't yank the flimsy wires too hard that attach to the dial mechanism.

For cleaning electrical circuits I like to use 99% rubbing alcohol. The very nifty pointy Q-tips can be found in the "lady section" of your local well appointed super store. You may be amazed at the sorts of convenient tools they have hidden away in the lady area, if you go look.

 

5) Before photos: All the brown crud is what to remove. If you look very closely in the photos, you will see not only the chunks around some contacts, but also very thin "pools" of it that have spread across the surface of the PCB. These are probably what are actually shorting various connection points.





6) After photo: all the junk is gone. This took me a dozen or more Q-tips and about 30 minutes.

 

7) One other thing I did was re-melt each connection on the back of the board, in case any joints had gone cold over the years. This was probably not necessary as all the joints looked OK to the naked eye, but I figured it wouldn't hurt.

8) Re-install and cross your fingers. Although my expectations were at zero, what do you know, it worked! The tach spins up smooth as silk.

In Conclusion: If your tach is dead, maybe give that dirty sucker a cleaning! You might just fix it to good as new basically for free.

Thanks so much to Eddiec1564 for the original idea of this fix!

 

Hey, thanks for all the pictures! I know my tach is dead, I was just gonna suffer with it and not care... but, I think I'll give this a try soon! Thanks again!

 

If you do, post back here to let us know if it worked!

 

I'll try, but won't make any promises :x Got a lot going on these days, so fixing the odometer is going to have to wait a bit...

 

Also in the 1st photo you see that brownish glue that is on the components(resistors,diodes ect) in the center of the PC board. Got to remove that too. That crud was my problem with my tach.

GOOD WORK on the photos too!! I didn't have a camera when I did my tach so many years ago.

 

Great pics, they really show the problem well. Going to have to do this on my truck, but I'm going to take it a bit further. I will also be redoing the face, to read 1/2 of what the current values are. No need for a tach that can read 6K, with an engine that is mechanically limited to just under 3K........