Hello everyone. I have used this site on many occasions to help me fix my old truck, and now the time has come for me to contribute with a rather strange problem. The truck in question is an 1980 f100 custom flareside 302 ci c6 trans factory a/c Cali emissions.

I am going through hell trying to get my factory tach to work. The tach will work sometimes out of the blue and will just go dead just as quick. It may not work for days and then work for a day and then cut off again. It is driving me bonkers. I have attempted to trace the green wire all the way from the coil to the instrument panel. I replaced the distributor pickup, cleaned and used di electric grease on every single connection from the coil to the instrument panel, took the panel out and removed the tach to put a thin coat of di electric grease on the connections, and on the wire harness for the instrument panel. I've checked all fuses and all good. I have removed the coil and sanded down the connection between the coil and intake manifold and added di electric grease before install.

I even went so far as to see if I un did the connector with the green and black on the drivers side would work if I put some wires in the female end and grounded the black and put the other wire on the terminal on the coil and still nothing.

If y'all have any idea how to get this darn thing to work again I would be forever thankful.

And no I do not just want to install a sunpro or similar tach, I want to keep the truck looking factory as much as possible. Thank you so much for your time as it is much appreciated.

 

I had the exact same situation with my tach. It would suddenly cut out, and then it wouldn't work for maybe a minute, or maybe a week, and then suddenly snap back to life.

I remember reading about issues with the flexible printed circuit and/or the tach circuit board and some kind of brown conductive stuff that can develop over time.

Are you sure your tach is an '80 tach? I wondered if my wonky tach in my '83 was actually from an '80. I think the '80's were different.

I never found the problem, I ended up getting a new cluster.

 

Hi! In the stickies tech and tricks there are a thread dedicated for the tach instalation. Do you have read it? Another things the 80-81 printed circuits from the cluster thereare not the same of the mild 81 to 86 printed circuit clusters, check the color of you printed circuit and be sure thats your tach match with these years clusters

 

Thank you all for the replies. In response to everyone's questions, I am 100% positive that the printed circuit board is for a 1980. My father was the original owner and told me that he did not replace anything in the instrument cluster.

I have read the sticky about tach installation and I used that information to locate the necessary wires under the hood that I then checked all wires for nicks or burns, crimps or bends, and proceeded to clean all the connections and re install with di electric grease to improve the connections.

On the other hand, I remember reading about the brown glue (as I think it was described) that was used in these 80-82 I believe, and took the tach out of the cluster and checked visually the connections on the back and the connections at the printed circuit board and I did not find anything out of the norm. If there is any more information on where this "brown glue" is located precisely so I can dig deeper into the tach if necessary that would be much appreciated.

Also, to cover all the bases the wiring under the hood in not aftermarket in any form, no MSD.

Thank you for your time guys I really appreciate it and hope to find the solution to this issue and post on here to help others out like me down the road, cause this is driving me bonkers.

**may I have to buy a NOS tach from eBay? Finding a 1980 factory tach at the pick and pulls out here is close to impossible.

Thanks again.

 

Start here in that thread. Luke lays it out very precisely.

 

Thank you gary_lewis for the thread in the right direction. I am going to give that a shot in a couple days using the steps set out by luke and I will let everyone know if it worked out for me.

Again, thank you so much for the information.

 

So I went ahead and followed the steps laid out by luke on the other thread and this is the result.




You can see all the gunk and what I would call "earwax" came off the printed circuit board.

My problem seems to be resolved as the tach works now, starts right up and keeps working, but its too early to give a accurate answer. Ill reply to this thread in a week or so with an update.

Thank you again so much for the info!!!

 

Congrat's! That's a good feeling - no cost fix.

 

I purchased a replacement cluster that had this very same problem with the tach. I posted about it, but noone provided the fix above. I simply replaced the cluster with another, and it fixed the problem. Wish I would have known about this.

 

The stickies provide a lot of information if you go looking into them....

 

Tach worked this morning when I started the truck and headed to work, then stopped working when I had started the truck back up later to go to my doctors appointment. I was upset to say the least because it had not given me any problems after I had taken it apart and cleaned the circuit board. I get out of my appointment and start heading to work, tach is still dead. I start the truck up on my lunch break and to my amazement the tach works again. Turned off and on the truck about a total of 4 times since and tach still works. Why would it cut off like that?? I am at a total loss. Maybe its about to reach the end of its life and I may just need to get another one?

 

Perhaps the intermittent problem is with the wiring?

 

I agree with David - you have some connection that is intermittent. I say that because you said the tach either works or doesn't when you start the truck. A bad component probably wouldn't act that way and the tach would probably come on initially and then fail during the drive.

I would pull it back out and inspect it very closely with a magnifier. Look for more of the brown stuff you missed and for solder joints that might be cracked or "cold" - a joint that moved while the solder was molten. A cold joint has a different look to it, usually rough and a slightly white. Re-solder any joint that is suspect.

 

First thing, I should recheck all the connections that lead to the TACH correct? What connections would those be? To my knowledge the connections that I am aware of are the back of the PCB on the Intrument panel, the green and black wire plug near the ignition, and the green wire leading to the coil. If there are anymore connections that should be brought to my attention let me know.

I will take the tach out again this weekend if that doesnt work and I will inspect for more gunk and re-solder if nessesary. When you say it would not be a componet failing you are refering to the componets of the PCB on the tach correct? Such as the capacitors and resistors?

Thank you again for the help. I appreciate it alot.

 

Just wanted to mention that the tach will work or not work when I start the truck, but it has twice already when it was dead, start working all the sudden while I am driving down the road. But it never dies when I am driving, only doesnt work when I start the truck.

I dont know if that information would help.