Ok... My tach is sticking--maybe that's not a good word--it is erronious. The tach will often read about 1000-2000 RPMs too high. Then when i bang the dash around (gently with my fist) the tach will settle back down to what the reading should be... I was thinking of just popping off the tach and oiling the back--good idea, bad idea? Then again, is it simple to get to the tach? I see about 4 screws that I need to take off to remove the plastic cover to the instrument panel. Is that all?

Anyway--any tips from you guys is much appreciated =P

 

Bad idea. The problem is probably electrical. There's a giant old style circuit board (very crude looking by todays standards) behind the cluster.
You could have a problem with your ignitor, instead, but you mentioned that it corrects itself with a good whack.
You will have to remove the cluster to access the back...even the you'll have to take the whole thing apart. I would look at every other possibility first. Working on the cluster is like surgery. Everything needs to be VERY CLEAN...if you care about no lint/dust on the gauge faces. The more you care about not accidently getting a $%$#$^& scratch on your gauge face, the more likely you'll end up with one!
Also, this would be the first time I've ever read about that being the reason for your problem. It is usually related to the trailing ignitor.
(Someone elses department.)

BTW, Oiling the tach or speedo does'nt work. There is actually a very small threaded brass pivoter that can be backed off a little to allow the needle to be a bit more responsive, but it does'nt seem to make a whole lot of difference.
I've done it, and when I'm in the driveway and blip the throttle, ithe tach only spikes to 3K. But I know when I steadily increase the throttle to match the tone of the peak during the blip, I'm at 5000!
Makes me worry 'bout my 7.5 K shifts!

 

EEK! So basically the tach is a not that responsive? Hmmm... The reason why I want to fix that problem is cuz i'm kinda a perfectionist--i hate my car dirty, and small problems tick me off =)

As for the trailing ignitor, could there be an electrical problem there? I just though the tach would stick temporarily because something back in the cluster is preventing it from completely freely. Any clue how I can determine the source of my problem?

Thanx =)

 

Yeah...Like I say, not really my area. I only know from taking the tach apart and doing some rev-buzzer fun.
But I know the older firsts use the tach as a battery indicator when you first turn on the accessories part of the starter. Maybe (if you have one that's like this) you have the same problem as someone else that posted just less than a week ago.
Forget the names or the cure.
Where the hell is REVHED and peejay?!

 

Nah... I don't think mine is a battery indicator one--it's an 85... Hehe, they better read this post soon =P

 

It's most likely an electrical fault - scrap the tach.

All a tach is is a voltmeter, with electronics that convert the pulse signal coming from the ignition system into a voltage. I've taken one apart and it's pretty neat to play with. If it randomly goes up too high, you've got a problem somewhere in the circuitry, like a resistor (maybe the adjustment pot?) intermittently failing or maybe a random short/break elsewhere.

My gut feeling is that the adjustment pot is failing. Maybe. I dunno - electronics diagnostics, especially intermittent fault diagnostics, is not my bag. Basically, if the RPM drops down at random, it's probably an ignition problem, if the RPM jumps up at random, it's probably a tach problem. Much easier and less frustrating to just swap out the tach, rather than screw around trying to fix it. (Unless of course you don't mind constantly tearing apart and reassembling your dash, and want the experience, in which case go for it! But you're on your own)

BTW it's only '79 and '80 that have the combo voltmeter and tach. In '81 Mazda introduced stunning technology: a separate voltmeter.

 

hrm, My tack is sticking on decel sometimes at about 1,300rpm, and then won't drop back down. I can hear that the engine is idleing normally, and then after a while the tack will dart down to 800rpm, normal idle. If I get on the gas before it settles the tack goes up from that point and gives an incorrect reading. I don't wanna have an aftermarket tach sitting out there, any shots at replacin the stock unit? - ala - who's installed something like an autometer tach into the stock position..