Diesel Tach Inop.
 

You should try to support the diesel forum here!!

77 ohms says the mag pickup is ok. You bought a new tach amp so assume that is OK. I'd try cleaning all contacts at the tach amp and also crimp the female contacts so they fit tighter on the pins. Also try a known good tach.

If you connect a DVM set on AC volts on the lowest (milli volt) scale across tach amp pins 2 (gnd) and pin 3 (output of tach amp to the tach) and run the engine, you should read a small voltage there that is proportional to engine (rises with) RPM. If you get that, the tach amp and pickup is working and the problem is wiring to the tach or the tach in the cluster itself. Check fuses for the cluster and cluster grounds before trying another cluster tach.

Checked pins 2 and 3 with engine running: idle 44 mVAC. Voltage actually dropped with throttle advance. High RPM gave me about 24 mVAC.
Reinstalled Tach Amp and checked voltage cluster two pin connector. Here read 13 .4 VDC from fuse box. Also signal wire from Amp read 275 mVAC and increased with throttle advance as it should.

Although not positive i am almost convinced it must be the Tach itself. Just don't like swapping components without being more certain.


Update: scrounged up a cluster schematic for Tach. BK/RD wire is hot from fuse box and the other wire GN/BK is signal wire from Tach Amp. The signal wire is a straight run from Tach Amp pin 3.

Funola, i misinterpreted your above post and read pin 3 voltage at amp connector. I went back today and read at the cluster as i should have the first time. If i understand the pickup correctly then having this mVAC at cluster signal wire along with increase with throttle tells me the pickup and circuit through amp is acting correctly.

Looks like the tach body serves as ground to the cluster body then through the large multi pin cluster connector. So i either have a ground issue or dead Tach. Does anyone know what the actual Tach resistance should be reading at the back of the cluster(two pins)? Mine is reading .76 meg ohm. I would think a functioning tach would read higher than this.

Comments appreciated.
Reading

Answer
 

I suggest you inspect the circuit board, and install the additional ground.
http://www.peachparts.com/shopforum/general-information/296474-w123-instrument-cluster-printed-circuit-board-repair.html


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Thanks WHunter. I will try and check it out this afternoon and post the findings of the circuit board.

Could you tell me where the tach gets its ground from? out through which pin on the multi pin connector?

Answer
 

As I recall, it is a shared ground.
You can easily spend days locating this poor ground.
Many times I have been forced to remove the harness from a car, open every millimeter + breadboard + pin out. Just to find and repair a ground.
I can assure you; It is seldom worth the effort.

I suggest you ignore the factory ground.
Giving the cluster a fresh reliable ground is the critical factor.


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Thanks for the post WHunter. I disassembled the Cluster and inspected board. Board looks great as all lights annunciators have been working correctly. I did notice that the solder joint from the speedometer metal housing to the square piece of metal(back of fuel gauge circuit board) did have a broken solder joint. I did attach another ground as you suggested in your repair procedures. I know for certain that i have a good ground, 13 VDC, and Sensor signal from Tach Amp to the Tach Amp itself. Deducing from this that i have a bad Tach? Something tells me that it's something else. Next logical thing to do would be replace the Tach so i guess i will start my search for one and give that a shot.

not getting increasing voltage with rpm from the tach amp is not normal. before swapping in another tach, I'd suggest disconnecting the tach from tach amp (so it is not loading the tach amp), and check that you are getting increasing voltage with rpm.

Funola, i misinterpreted your above post and read pin 3 voltage at amp connector. I went back today and read at the cluster as i should have the first time. If i understand the pickup correctly then having this mVAC at cluster signal wire along with increase with throttle tells me the pickup and circuit through amp is acting correctly. Signal wire from Amp read 275 mVAC.