HOWTO - Tachometer Repair
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Lead Driver
Joined: Dec 2002
Posts: 5,491
Likes: 16
From: Kuna ID
HOWTO - Tachometer Repair
I've been fighting an intermittent problem with my tach for awhile now. It started last year. It just stops working sometimes. Sometimes it wont work at all for a week or 2 and when it does work it rarely lasts all day. It will usually jump at random while driving. I have replaced the tach and the circuit paper, cleaned all the contacts on the dash and plug, and jiggled every wire I could find but nothing seems to trigger it. I dont know what else to do. 
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
Sounds like the problem I had on my tach. What I found was inside on the circut pc board of the Tach was some old yellow colored glue that turned brown and started conducting current. I cleaned off the browning yellow glue and resoldered some cold joints and that took care of that problem. Since then I fixed about 5 tachs for the same thing. Funny thing is Mitsubishi TV's of the late 80's/ early 90's had the same yellow glue that browns up and causes all types of problems...
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
Once that yellow glue turns brown, that stuff will easly scrape off. Just scrape off the glue from the componet's legs, to keep from conducting current across. If there is some glue not turning brown, it will be harder to pick it out. Get some pc board cleaner to rinse off the pc board, let it dry and that should be it. My tach has not acted up in 8 or so years now. That yellow glue was some ones idea to keep things from virbrating and causing bad connections. But after years in heat they never though it would turn condutive.(although I wonder about that....). I did the same thing you did. Replaced tach, wires and ect... then hooked up a aftermarket tach to the back connectors in the cluster. That showed the problem was inside the tach unit and the aftermarket never acted up but the orginal did.
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Lead Driver
Joined: Dec 2002
Posts: 5,491
Likes: 16
From: Kuna ID
Eddie you ROCK!! 
I went out and took apart my spare tach and it was full of yellow glue. I spent about 20 minutes scraping it off and hosed it with cleaner. I checked the tach in the truck before I swapped them and still it wasn't working. I swapped in the one I had just cleaned and it worked. I then took the other tach and opened it up. The glue was brown in this one and a lot easier to clean, only took about 5 minutes and there wasn't as much either. I threw it back in and it started working too. Finally, I hate having dead gauges in my dash.
I went out and took apart my spare tach and it was full of yellow glue. I spent about 20 minutes scraping it off and hosed it with cleaner. I checked the tach in the truck before I swapped them and still it wasn't working. I swapped in the one I had just cleaned and it worked. I then took the other tach and opened it up. The glue was brown in this one and a lot easier to clean, only took about 5 minutes and there wasn't as much either. I threw it back in and it started working too. Finally, I hate having dead gauges in my dash.
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i just posted a thread in 87-96 forum with my diesel. Mine is a 91 but i'm going to go check that. cant start it right now, i'm waiting on an injector line to come.. threads wrecked, and the injector threads wrecked. all this to fix fuel seap.
Cargo Master
Joined: Dec 2004
Posts: 3,362
Likes: 22
From: Florence, SC
You've probably already done it, but look where the tach wire fastens to the coil terminal.
The thing is, my tach IS working; the problem is the tach reads too high once it goes into overdrive. (302 carbureted V8 w/ AOD) How do I fix this problem??
Cargo Master
Joined: Mar 2007
Posts: 2,935
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From: Arcadia, Fla
Sounds like some mouse had fun. Does the tach show idle speeds or change when you rev the engine? What color is the wire that was chewed up? Not fun when a rodent eats up things. Just got rid of them from my truck. This time they ate up the brake booster vacuum line..... going to recrute a rat snake or black snake again..
Cargo Master
Joined: Dec 2004
Posts: 3,362
Likes: 22
From: Florence, SC
Yes, the tach works perfectly fine otherwise. Seems to be reading too high when it goes into overdrive. It doesn't make sense to me. Aside from reading too high, the tach seems to work normal, yet there is a chewed up wire in the engine compartment. The wire is yellow and actually says "Tachometer" on it. Any ideas?
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Lead Driver
Joined: Dec 2002
Posts: 5,491
Likes: 16
From: Kuna ID
Are you sure the tach is off, maybe your OD band is worn and your tranny is slipping. Just a thought. If the wire has tachometer printed on it, it's most likely not a factory wire. A PO probably had an aftermarket tach in there at some time.
Cargo Master
Joined: Dec 2004
Posts: 3,362
Likes: 22
From: Florence, SC
AOD transmission was just rebuilt, so there are no problems on that end.
The truck was my father's, he bought it new, and it came with a factory tach, the one that is still in there.
I changed from EEC-IV/TFI to Duraspark II ignition, using stock Ford parts and a Duraspark wiring harness from Painless. When it was first hooked up, everything, including the tach, worked fine. Now, the wire is off and the tach is reading screwy and I don't know where the wire went to.
The truck was my father's, he bought it new, and it came with a factory tach, the one that is still in there.
I changed from EEC-IV/TFI to Duraspark II ignition, using stock Ford parts and a Duraspark wiring harness from Painless. When it was first hooked up, everything, including the tach, worked fine. Now, the wire is off and the tach is reading screwy and I don't know where the wire went to.
