View Full Version : crazy tach

09-22-2009, 10:37 AM
I've recently gotten my tr3a running and among the many things that need to be done my tachometer is erratic and reads very high all the time. Has anyone encountered this before and what type of shop would work on it?

09-22-2009, 10:50 AM
Can I ask if the needle returns to rest on the wire post when the engine is turned off? Does the needle bounce when your running the engine or does it maintain a certain RPM? What is the reading when you are at idle? What is your reading at 30mph in 4th gear? You may be able to just detach the needle and reattach it where it belongs.

09-22-2009, 11:06 AM
First thing to try is loosening the cable a bit from the tach and see what effect that has. It seems that many new cables are being supplied too long and put pressure on the workings of the tach (or speedo). The cure may be as simple as putting a flat washer in the joint.

There are any number of places around that rebuild such instruments, but the work is not cheap. Mo-Ma is probably the best well-known, as they do many Pebble Beach cars and such:
1321 2nd St NW
Albuquerque, NM 87102
(505) 766-6661

But the lesser known shops do well too, such as

North Hollywood Speedometer & Clock Co.
6111 Lankershim Blvd.
North Hollywood, CA 91606
Phone 818-761-5136
Fax 818-761-4857


West Valley Auto Electronics (was WV Instrument Specialists)
19314 Vanowen St.
Reseda, CA 91335

Likely there are some closer to you, I just happen to know the ones near me.

09-22-2009, 11:22 AM
The cable is original as far as I know. The needle does bounce with the engine running and appears to rest on the post when off. It takes a few seconds but rapidly jumps to near 5000 rpm and bounces around 4 to 5000 at idle and when revved up its still bounces around the same area.

09-22-2009, 12:05 PM
You could take the Tachometer out and apart. That is what I did and sprayed it with WD-40 and my tach now works great. There was a whole lot of grime that came off the internals. The cable also needs to be lubricated. I believe most suggest graphite but I simply removed the cable and sprayed WD-40 through it with good results. Plenty of grime came out of the cable too.

09-22-2009, 12:07 PM
Eric - My own tach was acting just like yours when I first got the car 3 years ago. So bad I actually completely disconnected the cable from the back of the gauge.

Later on I tried Randall's suggestion (slightly loosen the fastening nut) ... and it's been working perfectly ever since.

Thank you Randall!


Don Elliott
09-22-2009, 01:21 PM
I rarely ask tech. questions but while we're on the topic, here's my situation. The tachometer has been smooth all the way up to 5500 RPM ever since I put in a new cable and sheath in my 1958 TR3A in 2002.

But this summer, it has started to act up. It is fine at any RPM up to about 3500 RPM. Then it jumps up to 5000 to 6000 RPM and swings back and forth rapidly. I know the engine is powerful, but the engine may really be doing about 3500 to 4500 RPM when this happens. As I continue shifting through the gears to 4th overdrive and the RPM of the engine drops, the tach will read correctly at any RPM from 700 to 3200 RPM.

I took the cable and sheath out completely and checked it all over. The length looks okay (it ran fine like this for 7 years) and I wiped out and dried out all the oil and grease off the inner cable and tried to clean out the inside of the outer sheath. Then I dusted the inner cable with fluffy dry graphite. But it still continues to do it.

What do you suggest ?

09-22-2009, 01:31 PM
Don, the cable drives a short shaft inside the instrument, that just rides in a plain brass bushing. The other end of the shaft has the magnets that spin inside a cup to move the needle against a spring.

It sounds to me as though the brass bushing has become worn, and at certain rpm the shaft starts to wobble. Cleaning and some light lubrication may solve the problem, but a rebuild or another tach may be the only real solution.

Check out Tony's article at https://home.comcast.net/~rhodes/speedo.pdf

Although written for speedometers, the tachs work just the same (without the trip meter and odometer).

09-22-2009, 01:55 PM
I think that I'll remove it and take a look. Phil are you able to protect the face from damage , I've read that the print is water color and fragile. While I'm at it I will check out the spedo It isn't working and the cable is disconnected at the trany. I'm hoping just a cable issue. Man so many things to get her back in working order.

09-22-2009, 02:20 PM
I am not 100% sure about your 3A tach but I cleaned the face of my TR4 tach with Windex and it came out looking very nice. I'm sure mine is not water color. For new seals between the tach face and the housing, some have used the rubber o-rings for the oil cannister if you have one of those laying around. It is highly likely that the original seal is completely disintegrated. Simply slice the o-ring and lay it around the bezel. No big deal if you have a space without the o-ring.


09-22-2009, 02:24 PM
Thanks I'll let you guys know how it comes out.

09-22-2009, 04:12 PM
I can confirm that the paint on the original TR3A tach & speedo (at least the earlier ones) is very fragile, and will come off with almost any contact; even just dry brushing with a camel hair artist's brush. Dampening the brush with Endust and only touching the black part of the face seemed to produce the best results for me.

09-22-2009, 04:28 PM
Can some one walk me through taking off the tach and or spedo can't see it well and didn't find any thing in my literature that would help.

09-22-2009, 04:46 PM
You mean, "removing from the dash?"

1) Unscrew cable (at center).
2) Remove two illumination bulbs (one each side, a little below center) and high beam indicator bulb (speedo only, center bottom)
3) Remove two round securing nuts. (behind the securing bracket, a little above center, one each side)
4) Pull gauge forward, out of dash, and retrieve securing bracket from the floor, where it has fallen.

Since you are fiddling around near things that can arc and sputter, it's wise to disconnect the battery first.

09-22-2009, 04:53 PM
Unscrew the knurled collar that holds the cable to it, and pull out the two lampholders.

For the speedo, also remove the holder for the high beam indicator (which also just pulls out) and the trip meter knob if your car happens to be one of the few that still has it. Originally there was a tiny cotter pin located between the speedo case and the edge of the dash, which held the knob and it's shaft to the shaft protruding from the speedo. But it's frequently broken off, and no telling what some DPO has used in it's place (my car has a sleeve that is a tight push fit over the broken speedo shaft).

If it has been on there for a long time, it would probably be a good idea to wire brush the exposed threads of the studs as best you can, followed by a few drops of oil or WD-40 on each one. Then undo the two knurled round nuts and collect the lockwashers that should be under them, followed by the funky bracket that they bear against.

Then just push gently against the back of the case, and catch the gauge as it falls out the front.

09-22-2009, 05:53 PM
Ok, so I am typically a instruction reader (hereditary), but I got impatient and just went for it. And voila I got it out no problems. So now Phil suggested cleaning and I'm going to guess that I remove the two screws on the back of the tach and the whole thing springs apart like a jack in the box.


09-22-2009, 06:03 PM
Eric - you're an instruction reader. That's half the battle won already!

Now, as Randall suggested, here's the other half:


Take it one step at a time. You don't want any surprises - like tiny parts flying all over the room.


09-22-2009, 06:14 PM
Ya I just loaded that, not working on the spedo yet kind of hope that is a cable issue. wanting to get tach working first. I tried the advice of loosening the cable which was already fairly loose. It worked perfectly for a moment idle speed 1500 and when I revved the engine it jumped to 5500 or so. I disconnected the cable from the engine and it seemed to spin easily. I could remove and clean it but it's the only descent grommet of other rotted 50 year old one hate to tear it and don;t have a replacement yet.

09-22-2009, 06:21 PM
and I'm going to guess that I remove the two screws on the back of the tach and the whole thing springs apart like a jack in the box. Gotta get the bezel and lens off first (but you knew that).

09-22-2009, 09:22 PM
Well I cleaned up the tach mechanism, really not that bad. A little grease that was not as solid that I would expect for 50 year old grease. Wonder now if I should have used a more solvent type liquid in stead of wd-40. The back of the gauge was very clean so just cleaned the lens and carefully put it back together. When running the tach idles well but when revved it jumps to red line almost immediately. It then slowly comes back to1500 or so, which seems a little high. The engine idles well and not terribly high. I would have guessed 1100.