Anyone else have an inconsistent tach reading?

[lunchkrispy]

First of all, hello and thanks for all the info I've gathered by lurking around on here. So the other day I noticed my tach was reading 3400 RPM at 70 mph on my way to work. Seemed high. Then on my way home it was at 3000 RPM at the same speed. This is on an '84 P-type so manual transmission. Clutch doesn't feel like it's slipping, and at consistent speed on a flat freeway I can't imagine it would.
Anyone else experience this? If it's just a normal issue with the gauge then I'll ignore it but does the ECU depend on that reading for anything?

[84supra84]

Me! I do!! Not quite the same, but When I am accelerating, My (stock) tach jumps from 3500 to 4200 and things like that. Like there are little dead spots. Thats why i have the Autometer Tach that no body here likes. I can't stand seeing the jumpiness, so i look at it. :laugh:

Sorry i cant actually help you:duh:

[ra24man]

Pretty much just old car syndrome guys...it could be a combination of both the tach and the speedo causing erratic displays...or any of the wiring attached to or mechanical connections to either circuit.

[Donn29]

Pretty much just old car syndrome guys...it could be a combination of both the tach and the speedo causing erratic displays...or any of the wiring attached to or mechanical connections to either circuit.

Ya usually stuff like this is a ground issue, or old brittle wiring. Try to find other things that are messed up and see how they relate. IE same grounding location, same fuse... etc.

[84supra84]

Ya usually stuff like this is a ground issue, or old brittle wiring. Try to find other things that are messed up and see how they relate. IE same grounding location, same fuse... etc.

When i go to the shop today I'll look at mitchell and see what surfaces. Thanks! If i can fix it, i have a legitimate reason to ditch the tach.... Maybe

[lunchkrispy]

Chido. So long as it's just a gauge problem I can ignore it.

[sowsley]

Before I replaced my clutch a few weeks ago, when it slipped (always at high rpm), I felt it. It was fine at low speeds and at takeoff. There was no oil on the old disk plate, it was just worn down. It cost a few dollars to replace it, so if you don't feel it slipping, just let it go for a while unless you are in the mood or position to spend money on it.

[woggin]

i have a problem with my speedo, it cant decide how fast im going, it jumps, sumtimes upto 10km/h(6mph) its really annoying, sumtimes i can get it to sit at the speed, but most time it just jumps all over the place???

[ra24man]

I've seen the speedo adapters cause the speedo to run high at times sporadically, I've also seen speedos cause a similar problem. Tachs are, as Donn mentioned, usually ground related. These cars may still run good after a couple hundred thousand miles, but many of the overlooked old parts will need attention from time to time. Grounds are a huge issue with these cars. When I finished my manual swap years ago on my first 85P the starter wouldn't work. I thought that was really odd, because I had triple checked the neutral start switch wiring to make sure it would work. Upon further inspection I found I forgot to hook up the small 12 gauge ground strap at the transmission mount. Once it was connected the starter turned over, no problem. A quick inspection of all the major grounds in and around the engine/transmission resulted in a couple of poor wire conditions (just a few strands left at various locations). I replaced with new grounds and low and behold the blower was faster, the dash lights and headlights were brighter and the starter was noticeably revived. Probably the most commonly overlooked aspect of these cars at their age in my opinion.

[pimptrizkit]

my tack is slow...

i say it's common..

[moonpie33]

wow mine works perfect, lol always consistent.

[Donn29]

my tack is slow...

i say it's common..

Read marks last post!

Grounds are so important! Any problem with electronics come down to a poor connection, almost always, but the poor connection could be inside a sensor or device too(AFM/TPS for example).