82 xj750 at my wits end with tachometer

I might have even posted about this in the past, im not sure, but it sure is irritating.

Tach is very slow to react. if you hold an RPM it will eventuall get there. When you first start it up its stuck at the bottom until a bit of riding it will loosen up. it does seem to be temp related to, warmer days it works better but never right.

I have taken this thing apart too many times to count, tried many types of lubes, cleaners, even no lube. too many to list, it has been over a dozen attempts with differnt things.

everytime it seems to work. all is normal for a while but eventually goes back to suck.

thinking it is probably time time swap w ith a good used unit, unless anyone here has a bright idea?

2nd issue, used units have not nice faces for the most part. my faces are pretty as always beeing behind a fairing. i havent tried, but can i just pop the needle off and swap the guts? tach is its own unit true so thats easy, but i dont want to be pulling on needles and breaking them if unsure. in my past expierence some are glued some are crimped and some just slip on.

lastly anyone have a parts set of gauges and want to part with just the tach for reasonable?

 

have you read this link
Speedo and Tach dissection with pics

is this a mechanical tach or electric tach
mechanical tach you have lubed the cable? cable not bent or kinked? have you inspected the internal cable?

electric tach, clean all connections, resolder the coil connections for the tach coil

 

His tach is electrical. It may be an electrical problem rather than a mechanical one. Try reflowing the solder joints on the circuit board.

 

i have read that thread, thanks for the link though. cleaned all types, if you think it helpful I can list them all.

I have NOT tried to reflow any solder joints, but the inside of this thing is very clean for what that's worth.

today it was pretty cold, in the 40s and the tach worked pretty good. slower than it should but no hang ups. where as before today it was not working.

electrical huh? what about the tach signal from the motor? can anyone shed some light on that?

 

The tach gets its signal from the #2/3 ignition coil via the grey wire.

 

anyone know a good way to test it, besides another tach?

I went through some old pics. I have pics of the tach taken apart. and the needle did come off, so swaping the guts seems viable. however is it worth the $50 gamble? which is the cheapest I have seen for a functioning, though looking very tuff gauge cluster.

 

Oscilloscope.

 

that's fair,

naturally I don't have one.

so I think for me at this time. a know good used gauge will be the most useful to figuring this out.

 

Actually it's not entirely fair. You can rule out an intermittent break in the wire by running a jumper from the primary side of the coil to the tach input. Honestly, given your symptoms, I have no confidence that will get you anywhere.

 

the last part was a little vague to me. what wont get me anywhere? the jumper test you suggested, or swapping the gauge?

 

The jumper wire. If the tach works just fine when the temperature is warm, and never low in the rev range, then the problem is not likely to be a broken wire.
Thoroughness dictates that all troubleshooting options be mentioned though.

 

i found one through the forums. just the tach itself. plugged it in, worked instantly. swapped it into my cluster and all is well now. my previous one must have been a dud.

 

Shoot me a PM if you're interested in getting rid of the old one. I've worked on and assembled a few XJ gauges for other members and can always use some parts.