Rookie question - tach frozen at 700 rpm in '78 280z

[johnny haywire]

Hey amigos,

So, I've been trying to go through the car (a '78 280z Black Pearl) bit by bit to work out the electrical bugs (got the horn and the hi-beams working) and now I'm moving on to the tach. Since I acquired the car last September, it hasn't worked and has stayed frozen at 700 rpm. I've scoured the forum and found mostly things to do with 240z tachs, compatibility problems with aftermarket coils (mine is stock) and one thread that suggested a frozen tach was not wiring-related, but a failure in the tach itself instead.

I've checked the connections at the back of the tach and they all seem clean and intact. I've also cleaned the contacts for the wires that connect to the coil.

Here's my question (in 2 parts): Is the frozen needle a sign of a burned out board in the tach and thus the whole unit needs to be replaced/rebuilt, or could a wiring issue cause this? I'm hesitant to attempt to pull the tach if I don't need to because I have a dash cap and it's pretty tight up front.

The second part of my question: can anyone tell me which wires to chase/test or where to start? I'm pretty new to Datsuns and haven't done much electrical work on cars. The mechanical stuff, plenty, but I'm kind of a rookie in the electrical department. I do have the FSM for the car, but the electrical section is pretty overwhelming.

Many, many thanks in advance!

- Paul

Edited April 4, 2012 by johnny haywire
spelling errors

[Zed Head]

It could just be mechanical, not electrical. I had one that would not drop below 2,000 when it got warm but would drop to normal when it cooled off.

[thomas461]

I had a similar problem with the tach in my 74 260Z. Like you I was a bit nervous and apprehensive. I downloaded the service manual and got it out of the car. I then shipped it to Bob's Speedometer Shop in Michigan. They did great work and I got my factory tach back looking better than it did when it was new. They can test it for you and tell you if anything was wrong before they start work on it.

I did a review in the review section on Bob's Speedometer.

http://www.classiczcars.com/reviews/showproduct.php?product=34&title=bobs-speedometer-260z-tach&cat=7

[johnny haywire]

Thanks for the tips and info, guys. If it is an internal issue with the tach, I'll definitely go the route of Bob's Speedometer Shop.

Pete, can you tell me what steps you took to determine that it was a problem inside the tach and not wiring-related? Did you have power going to the back of the tach?

[ZCurves]

Paul -

I have a '78 and purchased a Color Wire Diagram from Prosperos Garage, which has been real help when I re-assembled my Z - it is very accurate. You can get one here: http://www.colorwiringdiagrams.com/datsun.htm

I read that you had an FSM - good. Here is link to a PDF version that I have cleaned up : http://www.mediafire.com/file/y2x2e66oh939z9t/1978%20280z%28S30%29.pdf

It would be a great idea for you to also put your hands on the Fuel Injection Bible. You can search this forum for a PDF copy to download.

I wish that I had known about Bob's Speedometer Shop when I was going through my gauges! I believe they work really well, just have to send my clock out for a redo. Good luck, have fun and post pics!

[johnny haywire]

Thanks for the links, ZCurves! Definitely going to get that color wire diagram and thanks also for the FSM!

Slammed with work here for the next few days and I have to change a rear window track on my '86 4Runner before I get to the tach issue, but I will definitely update this thread with my findings and what route I take if the tach needs replacement.

[johnny haywire]

Just wanted to update this post. I picked up a replacement tach last week, taking the gamble on the previous owner's word that it did work when the car was parked (3 years ago), but it wasn't too expensive, so I went for it.

The needle on the new tach did bounce around before I set about installing it, whereas my stock tach was frozen at 700 RPM. I have a full dash cover, though, so getting the old one out required some major surgery. I had to make a cutting scribe out of a wooden block with an X-acto blade screwed to it to trim back the dash cover. Just ran it around the rim to cut it back. Took about 20 minutes, though. This was the easiest route, since my dash cover has some damage near the glovebox already. It didn't come out too badly, though. I'm somewhat pleased with my work...

And so, with a bit of luck, some Model Especial, and some patience, I was able get my old tach out. Then the new one went in...and it works!! But I forgot to change the bulbs beforehand. Talk about a pain in the arse. And there was no way that baby was coming out again. Note to self: always change to new bulbs beforehand.

The problem with changing the gauge bulbs when installed on a '78 (and probably all Zs), as many of you know, is that you need two hands up there, one to hold the bulb socket, and the other to push in the bulb and twist. But there is little room for that, especially with my hands, so I set up a 1/4 screwdriver handled socket driver with a 8" extension and a small socket (maybe a 10mm, I forget) lined with bicycle tire inner tube that allowed it to grip the bulb and twist it while holding the socket with the one hand that could get up there. The Sylvania 53 bulbs are sadly just as dim as the stock bulbs, but they all work now...

So, to sum it up, in my case, the frozen needle was a sign of a dead tach.

And thanks for the tip, Andrew, my color wiring diagram from Prospero's Garage is on its way!

- Paul

[Captain Obvious]

Here is link to a PDF version that I have cleaned up : http://www.mediafire.com/file/y2x2e66oh939z9t/1978%20280z%28S30%29.pdf

I've got the scan from XenonS30, but I'll snatch a copy of yours if it's cleaner. Do you think it's worth the bandwidth if I already have one?