Tach Only Works Out Of Bucket!

[scott0757]

OK. I've tried everything I've read in the many posts for wacky tachs. I've disassembled everything several times. Had the tach out and apart to look for corroded internal solder joints. Checked all voltages and all check well. Checked all connections from all the related threads mentioned. Spread out the wire bundle in the tach/light assy and looked for shorts/pinched wires and found none.

When the tach is in my hand, it is rock steady throughout the full range of RPMs.

When I set it into the bucket to remount it and it is sticking out about 1/4 inch from it's seat, it continues to work fine. I bolt it down with the OEM insulators and it is bouncy and unstable (rapid full spectrum needle swings) when running at anywhere from 2500-6500 RPMs.

Everything tells me this is a pinched wire but I have yet to find any and cannot create the problem when squeezing the wires by hand when I have the tach out in my hands.

It's beginning to look like the can of the tach must be both electrically as well as physically isolated from the metal bucket/ground. If tried rubber washers on the mounts, added rubber bands about the tach body and no success. Might there be a magnetic field affecting the tach when it's trapped in the bucket that is not present when it is out of it?

Any ideas as I've about given up. Riding with the tach in my hand is kind of inconvenient...

[xs11jack]

Cracked Circuit board?

Is there a way to softly flex the tach? It maybe a bad circuit board and when the tach is tightened down the board is flexed and only intermittent contact is being made. The only other thing I can think of that will help is a 12 in long stick duct taped to the top of the helmet and the tach taped to the stick(remember to tape it with the face facing you!!!). This leads to the fact that you will always know what the RPM is, especially went ogling the girl on the sidewalk in a bikini and are about to hit a parked car. You will remember exactly what RPMs you were at when you dived into the cars trunk!!!

[TopCatGr58]

Hey Scott,

Here's a photo of the back of the tach:


Note the smaller bolt ends in the middle of the back plate.

And one of the innards:


You can see the circuit board, and there are two prominent phillips head screws just beside the center shaft area sticking into plastic pieces. I don't know if these are the ends of the ones sticking out the back of the housing?? The housing is fairly robust, so I would wonder about how much flexing could occur when tightening the housing bolts down? The chrome housings are metallic, so I don't think there is any magnetic fields causing problems.

Wonder if the wires/bundle directly behind the inner tach housing is getting compressed and pushing IN on the bottom center housing causing flexing of the case/board??

Here's how mine is routed:


Don't know what else to suggest?? T.C.

[scott0757]

The two nuts on the bottom of the tach shell (one has the wire cage) are screwed to double-ended studs. The other end goes through holes in the tach circuit card and into the solid metal casting /machined housing that holds the guts of the tach/magnet/return spring. It's the opposite side of the exposed tach innards you show.

The studs that hold the tach shell into the outside/chromed housing are welded studs on the tach shell. Tightening these will not flex the circuit card. Not much flexing going on there I'm afraid.

Also when I press the tach into position without tightening the mounting studs, I get the same condition so something apparently being crushed/compressed in the wiring but I'll be damned if I can fine it. I guess at least I know it's not some place more difficult to get at. Just tired of having a useless, annoying , dancing tach.

I'll keep you posted. Thanks.

If not, I'll try the tape and stick method also suggested....

[bikerphil]

tack

Sounds like this is being caused by a vibration when you mount it on the bike. Try checking the solder joints on the pc board. Mabye one has cracked and requires re-heating. If you do find a bad solder joint, use a low wattage soldering iron.

[scott0757]

They all looked nice and shiney so I left them alone. I was thinking about a vibe issue, but saw little that would be moving about inside the tach that wasn't supposed to be.

You may be right. Wonder how many shots I get at peeling back the tach bezel ring before it cracks off... Not a fun process....

I may have to go there again...Ugh. WIll take pictures this time.

Thanks.

[scott0757]

Here are other images of the open tach and printed circuit board. Note two double-ended studs hold PCB to tach mechanism and hold the tach assy in it's can. They are not the same studs that mount the tach to the exterior/chrome can.

Bottom as removed from can


Side view of tach assy with mounting studs


Bottom with PCB removed


Interior of tach assy can


Tach Assy with routed wiring


I reheated all solder joints and retraced wires as best I could to avoid pinching (not much room in there). Gave up. May be slight improvement but still seems to oscillate, but maybe not as wildly. We'll see. Have to get to rebuilding the carbs, valve shims...

[scott0757]

Sorry. Only posted thumbnails.

[scott0757]

Sorry about the image issues..