Dead Tach... Can it Be Fixed?

[JWMUSTANGMAN]

I've got an 80 bronco. Pretty much a base model, but was ordered with the sliding windows and gauge package. 6 cyl 4 speed. I recently had a parts truck that was also an 80. It was a ranger XLT with a V8 and 4 speed, but had a tach. So 80 tach cluster with gauges should be a direct swap into my 80 bronco with gauges, right? I installed the tach cluster in the bronco. All gauges and speedo work (after replacing the broken speedo cable). I even have all the lights. For whatever reason tho, the tach has no signs of life. It didn't work when it was in the other truck either, but I assumed maybe someone had messed with the coil wiring on the other engine or something. I've done some research and I think everything is good to the tach. With the cluster plugged in, the 4 terminals on the back of the tach all are getting what they need. B+ has good voltage, - has good ground, The signal is coming in the form of a rapidly flashing ground, and the 8 terminal is NOT grounded since mine is a 6 cyl. Is it safe to assume the tach itself is dead? Is there any trouble shooting it? or just get another one? Like I said, all the circuit board connections are good and working, and I can test at the 4 pins on the back of the tach and see what should be there. Anybody got a way to troubleshoot the tach itself? Or a source for a replacement? If I just replace the tach only, 80-86 will work, right? Thanks in advance.

[Gary Lewis]

Hey, you got the picture bit figured out on this post.  

On the tach, you've done the troubleshooting I would do and it pretty well proves the tach is the issue.  But if you are replacing it you do have a few options other than the Bullnose trucks, namely in the medium duty trucks.  Go read in our page on tachs at Documentation/Electrical/Tachometers.

But somewhere we have a writeup on cleaning the circuit board of resin on the tachs, and that seems to have gotten some of them working again.  I'll look for it, but maybe someone else knows where it is?  I think you use a Q-tip and cleaner, maybe isopropyl or contact cleaner, and clean the board of the brown stuff that may be on it.

[Gary Lewis]

Haven't found what I was looking for, but did find this post where a guy fixed his tach.

[JWMUSTANGMAN]

Haha, yep! Pic loaded!

Thanks. Glad to have you confirm my thought process. I’ll comb over the tach write up again and see what I can find about cleaning the resin. 👍🏻

[mat in tn]

well, there is one point that has worked for me in the past. there are four points where the tach mounts to the printed circuit. inside the terminals are soldered to the tach these soldered joints have been known to fail occasionally. re heating them with a soldering iron or gun to re flow the solder may actually be a solution. but testing both sides of each joint with an ohm meter will tell which one if this is the case.

[JWMUSTANGMAN]

Great idea, Mat. I'll do that next chance I get to pull the tach. I got it all back together for now and I'm driving it today. haha

[JWMUSTANGMAN]

Update: the dang thing just decided to start working yesterday. It's still working today. I'm gunna ride it out and see how long it lasts. I haven't touched a thing. I was just driving it with the tach not working. It decided to work for a while, so we will see how long!

[Gary Lewis]

I had one that worked when it was warm.  Never did figure out what its problem was.

[mat in tn]

there is a bad connection somewhere.  but roll with it. it is a symptom at least. the tach does work. now to find the bad connection if it fails again.