Tach just bounces between 0-2000 RPM, is it bad or is it wiring?

[JeremyL]

As the title says my tach just kind of bounces up and down between 0 and 1800-2000 RPM. It doesn't act normally up to 2000rpm either, it just starts bouncing all over (up to 2000) as long as the bike is running. I read about how these tachs have little bearings inside that can cause "tach wobble" but I'm not sure my mine is really tach wobble since most of those guys seem to at least have a tach that pretty much works, just bounces a little. I also have the Clymers manual and am looking at the wiring diagrams to test the tach and check the wiring but the diagrams only show the "Tachometer Lights" and not the tach itself.

I would love to just hook the tach directly up to a power source to see if the needle will even go above 2000rpm so I can decide if it's a short/poor connection or a mechanical problem with the tach itself. My bike is kind of odd with it's wiring so I'm afraid someone's just rigged up half of it. The "Headlight" indicator comes on when you put the headlight on HIGH beam (and the Highbeam light stays on as long as the bike is on). I have an 80 motor in a 79 frame so I was afraid its all chopped and spliced up.

[3Phase]

Jeremy, hopefully it's just a bad connection in the headlight shell or down at the alternator harness connector. The alternator connector is behind the fuse block under the right side cover.

If you want to test the tachometer needle drive motor with a battery you have to take the tachometer out of its housing, then remove the front bezel and the glass.

The White wire at the tachometer goes to a circuit board inside the tachometer that counts the AC pulses so it can't be driven by a battery. After you remove the bezel you can see the Red wire and the Black wire that actually drives the DC needle motor. Disconnect them from the circuit board and connect them BRIEFLY to a 9 volt battery.

The needle should immediately move to full scale and stay there.

If the needle doesn't move or only moves intermittently and jumps around then one of the wires is bad/broken.

It could also be that one of the two small coil springs (they look like the coil spring in a watch that makes the watch go, "Tick, tick, tick, tick ...") is loose or broken.

The two coil springs are attached to opposite ends of the motor armature. They help hold the motor armature in place and they make the tachometer needle return to zero. The springs are also used as conductors to carry the current from the Red and Black wires to the motor armature.


XS1100 tachometer needle drive motor and its upper armature return spring.






XS1100 tachometer needle drive motor and its upper armature spring.

The upper spring broke off of the armature and the tachometer did not work.

[DAVINCI]

Not to split hairs, but it's not a 'motor', it's a galvanometer. Analog voltmeters and VU meters work on the same principle.

[3Phase]

Not to split hairs, but it's not a 'motor', it's a galvanometer. Analog voltmeters and VU meters work on the same principle.

Thanks, Randy!

Jeremy, it's too late to edit my post so you'll just have to think, "galvanometer," where I wrote, "motor."

[BA80]

A couple more things.............Once you get the bezel off, which is a real PITA because it was never intended to come off after original assemby, then you have to get that 30+ year old piece of EXTREMELY BRITTLE plastic that is the needle off. It was never intended to be removed 30 years after assembly and is very near impossible to remove without breaking.

The moral of this story is, PM andreas and get one from him that works. I tried to fix my tach when it started doing that too. I have a lot of spare parts for one now. Minus the needle and a couple of tiny screws.

I got one from andreas a couple of years ago and it's still working great. I think he only charged me $40. I can't remember for sure now.


JAT

[bikerphil]

When repairing my tach, I found corrosion on the PC board inside. I cleaned it off with a toothbrush and contact cleaner and reheated a few solder joints that looked bad and no more bounce. A miracle I must say.

[BA80]

And here all this time I thought there were no angels in Florida.


Truly a miracle.

[bikerphil]

Yep, Angels were here last week too playin' baseball. Funny thing, I lived with that tach bouncing for 15 years, one day I was bored and was determined to figure it out. Never had to go as far as the photos above though. Electrical connections sure can be a factor.

[BA80]

I'm sure that's all it was in mine too but I had a real tough time getting the bezel off without destroying it and then when it came to the needle it just disintegrated when I touched it.

Obviously mine was further gone than yours or I'm just a bit too heavy handed. Possibly a bit of both.

[Shacknasty]

I used a dinner fork to gently lever off my tach needle. it worked and if you don't believe me, well, fork you too!

[BA80]

I used a dinner fork to gently lever off my tach needle. it worked and if you don't believe me, well, fork you too!

.......Now that's forking funny, I don't care who you are.

[je2wheeler]

.......Now that's forking funny, I don't care who you are.

What do call a brick layer who uses a fork? - - - A mortar forker

[JeremyL]

I'm about scared to crack open the tachometer. I take things apart all the time so that's not really the issue but mine has been out in the sun for years, apparently, so I'm sure it's brittle inside.

I took my headlight out and was looking at the wiring and for the most part it looks right but there's some stuff rigged up like one of the wires for the headlight is snipped and is running to the brown line that goes to the tach (I think -- it was on the 3 wire connector going to the tach housing). Anyways, I started reading the tech articles here about the headlight not working because of the RLU so I started working on that. Now I've got too many things going on at once lol. I am going to clean the stator wires and harnesses behind the fuse box and see if that helps my tach. I looked at it again and it's really bouncing around between 0-1000RPM but it's not really showing a reading at all, just bouncing around wildly the entire time. I'm thinking it sounds like a dirty connection somewhere (I hope).

If it was the alternator causing the problem, wouldn't the battery run dead all the time? I charged it up the other day and took off on and hour ride and the next day it started right back up again.

[3Phase]

If it was the alternator causing the problem, wouldn't the battery run dead all the time? I charged it up the other day and took off on and hour ride and the next day it started right back up again.

Maybe.

I f the re is an intermitte nt connec tion to th e alt ernator the n the b attery wil l st ill c harge, ju st n ot as qu ickly an d effic iently as it shoul d.


When a coil spring in the galvanometer breaks away from the armature it can still make contact when the needle/armature is at rest but it loses that contact almost as soon as the armature starts to move. Engine and road vibration will also make the tachometer shake and bounce in its housing and the spring will touch the broken armature connection.

The fast make-and-break causes the needle to appear to waver and hover at a low RPM reading, similar to the way the old thermal regulators for automotive gauges worked to limit the voltage and current going to the fuel level sending unit in the gas tank.

Sometimes the tachometer needle will jump to a high(er) RPM reading because only one of the coil springs is restraining the armature movement instead of both springs. The broken end of the coil spring can 'catch' the solder blob on the armature and tachometer will work, briefly, but it shows roughly double the actual RPM.

[JeremyL]

Well I started tracing down tach wiring then I saw that my headlight was jumpered over to the tan tach wire in the 3-wire connector (I guess the tach backlight wiring?). So I started trying to figure out if my headlight wasn't working because of an alternator problem because I guess if I figured that out then I'd not even mess with the tach until the alt was fixed. But I bypass my headlight relay and it worked just fine today so at least I know what the problem with the headlight was.

But I'm back to my tach now. I checked the resistance of the white wires going to the stator/alt and got .5 or .6ohm (usually .6). I saw that it should be .4 -+10%, which would be .36-.44 so is .5/.6 close enough or is my alt bad? My multimeter only shows to the tenths place so it just bounced between .5 and .6ohm between any two white wires.