KZJ78 Tachometer not functioning

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Jul 23, 2008
The tachometer on my KZJ78 is stuck and reads roughly 1100 rpms under all conditions, vehicle running or vehicle off and key not in ignition.

Any ideas where to start looking?

Maybe more basically, can anybody give me a tutorial on how the tach works? Where does it get it's signal and what kind of gauge is it - electrical or mechanical? What would be a checklist of where to start testing to trace the problem? With any luck, it's just a dirty electrical connection or some other easy fix.
I don't know a lot about how the gauge itself works, that is something I have been trying to figure out also. It gets an electrical signal from the ECU, which gets its signal from a pickup sensor on the injection pump. Your problem strikes me as a physical issue where the needle is hung up on something inside the gauge, rather than an electrical or signal issue. There is some more information here: What runs the LJ-78 tach? - The LJ and KZJ tachometer setups should be pretty much the same.
OK... I need ideas on how most efficiently to test this. Let me know your thoughts on how to figure this out, or if you have an EWD that might provide info for me or others to figure this out, let me know.

I removed the instrument cluster and cover and found no obvious problems - no physical obstructions and while I didn't completely disassemble the cluster, could not see any evidence of leaking capacitors, corrosion, etc that might strongly suggest the gauge itself is bad... though it still might be as with a slight jiggle and tap the tach needle dropped from permanently reading about 1200 rpm to now reading 0. Since I didn't see obvious problems, I put the instrument cluster back in and started the engine to see if the fact the tach now read zero plus any small jiggling etc might have fixed anything. It still doesn't work... reads zero at all times.

I had the dash apart while also installing a new stereo and found from some prior owner a wire tagged on to what I think is the ECU (rectangular aluminum box behind and above the glove box, 2 connectors on the back) that was labeled "speed sensor". The label was not a factory label but appears to have been added by somebody like a stereo installer. It was tagged onto a green wire in one of the connectors on the ECU. That tagged extension from the prior owner is not being used for anything at the moment, but I've left it in place and coiled up for now until I confirm it can be removed (since I'm not quite sure what it was for... probably a nav system input, but not positive since I didn't think the KZJ78 had an electronic output for vehicle speed). There is a green wire of approximately the same color as the one on the ECU connector that got the "speed sensor" splice that is on the instrument cluster connector on the same side as the tachometer, but green wires aren't uncommon so it may not be the same circuit.

Two options come to mind for the source problem: 1) Tachometer is bad... signal it's receiving is good, but the tach itself needs to be replaced. or 2) Tach is good, but it's not getting a signal. Technically, they aren't exclusive of each other.... I could have both a bad tachometer that is simultaneously not getting the right signal. I'm still inclined to think it's a bad tach since the unit was stuck at a fixed reading even when the car was off. That said, especially given I know there's a goofy tag on an ECU wire labeled something to do with speed, I'd like to try and confirm the tach is getting an appropriate signal.

Any suggestions on how and where to check this signal? I'd like to avoid cutting up the wires around the ECU if at all possible. Here are a couple photos that might help with ideas. I don't have an EWD matching the KZJ78 (1995) to help me identify wires:

Connector on back of instrument panel that is closest to the tachometer:

ECU connector with the prior owner's "speed sensor" wire spliced in:
Have any of you taken a tachometer similar to these apart? I took out the tach and took a look. I was hoping to see a leaking capacitor or something obvious I might be able to replace and here's what I found.... rather tricky to get a good photo.

On the bottom half of the photo near the L-R center, you can see two copper spiral springs. One is in line with the camera angle so all you can see is the side and that spring is parallel with the face of the tachometer and the plane the dial rotates in. The spring on the other side of the white plastic piece between the springs is at an angle - that's the one you can more obviously see is a spiral spring as it's angled to the camera pov. Should this look this way? I've never seen the innards of a correctly working tach, but just looking at it I would think those two spiral springs should be in the same plane, and if I'm right it means my tach is dead and likely unfixable. I'd like to confirm that before I order a new tach as they run about $400.

Speaking of which.... I'm finding two part numbers - 83242-6A100 and 83242-6A120. Often I can find a photo of the part, but this time nothing I've tried has an image (one site did at least have an image of the box, but not the part itself). Can anyone confirm the difference between the two? I suspect that 83242-6A100 is for the majority of KZJ78 units while the 83242-6A120 is the one for the units that had a locking diff as it included extra lights / indicators. I haven't been able to confirm which is which. I have confirmed on that both of these part numbers are unique only for the KZJ78 and 71 series... I was really hoping one of them would have been shared with a Corolla or Tacoma to make it easier and cheaper to source. If anyone has another tool to research a compatible part, let me know.

Closing the loop on this one in case anyone else is chasing the same demon and finds this on a search.

I confirmed that the tach was physically defective. There are two coiled copper springs in that photo above on either side of a white disk. One of these springs was bent and rather that coiling around the spindle for the tach needle in a perfectly perpendicular plane, it was at a severe angle. When the needle would move, the spring would scrape on nearby parts. I have no idea how this happened but there was damage on the dash underneath the tachometer area, so maybe there was some kind of physical impact to the dash.

I searched worldwide for just the tachometer but it was not available at any parts supplier at least that I could find. There are two versions that differ in that one also has indicators for a rear differential lock - neither part was available. Instead I was able to purchase and entire instrument cluster off eBay. I think it shipped from the UAE. I stripped the tach out of that cluster, popped it in my current cluster to replace the broken tach, and it worked perfectly. I’m pretty sure the replacement instrument cluster was also from a KZJ78 as it looked identical in every respect to mine. There might be 12v and 24v variants, or at least I was getting those questions from the eBay seller.

So long story short - the problem was mechanical not electrical, new tachometers are not available anywhere, so the solution is to buy an instrument cluster and raid it for the tach.

Users who are viewing this thread

Top Bottom