I really need some help here.

86 Comanche 2.8

I have already soldered the resistor in and opened the trace as outlined in this writeup (for fieros, but it's the same tach and same engine)
http://www.fieros.de/en/articles/tach.html

I tried another used tach from a junkyard and it still pegs high. I guess I should have checked for 12v and gnd, as well as coil pulse first, so I'll go do that. Otherwise, I'm not too sure what else to do. These 84-86 xj/mj really don't have a whole lot of info floating around.


Edit: just checked the +12v, gnd, and coil pulse and they're all there. The problem is definitely in the tach. Now to figure out what specifically is bad on this thing, because they just don't exist on parts sites.

 

Is it pegged with the key off or only with the key on ? because if it is only with the key on It sounds like to me that you are having a ground problem ,

 

Sooooo it's not actually "pegged". There is no peg. And both tachs will spin freely. I have a good 12v and a clean ground at the tach stabs in the cluster, and the logic probe pulses consistently with rpm when it's running.

Both tachometers, when installed and the engine runs, will point the needle straight down. They initially both point to zero when the key is on and engine is off.

I might try a handheld tach for troubleshooting, but I'm almost certain I have 2 bad tachs.

 

For future searches, a 1993 tachometer definitely doesn't fit in the spot.

You also cannot just connect 12v, gnd, and coil (from 86 side) to 12v, gnd, and ign with jumpers. It doesn't work. My guess is that the ign signal to the tach is a different wave pattern from the ECU on the 93, and the 86 just taps off the coil.

The search continues

 

For those unfortunate ones that have an 84-86 with this issue:

http://www.howtoalmanac.com/kevin/pr...chfixtruck.htm

There's a diagram on here that actually shows the internal schematic of that black and white resistor chip. The one from pin 4 to 10 is what goes bad. Break the connection somewhere on the board and solder on a variable resistor across that break.

The fiero page didn't do a great job of explaining any of that.

Works like a charm now.