Tach reads double actual RPMs....

[RichBaker]

My 90 FJ's tach is reading double what it should, 7k at 35MPH....

Any one else had this problem? How'd you fix it? I'm assuming it works like
most analog tachs do, reading the pulse string as a proportional DC voltage?
It just started doing it, I noticed it on the ride in to work today.

[the fan]

Did you upshift...


...squid.

[Dan Filetti]

I think I remember reading that this is a sign of a bad coil??  Someone check me on that....

Dan

[racerman_27410]

check the coil grounds.... this is where the tach picks up its signal


Kookaloo!

[RickT]

I had that happen to my 93. I took all the electrical connections appart and cleaned them with electrical cleaner. When I put it back together it worked fine. No problems since.

Rick

[racerrad8]

For some reason the tach is reading both coil signal outputs. It should only read one. The might be some sort of diode in the system so that it will one read the coil output.

Is the ignition system still stock, or is it an aftermarket system?

Randy - RPM

[RichBaker]

Dyna box, stock coils(haven't gotten around to installing the DYNA coils yet)....

All electrical connections were cleaned and covered with dielectric grease 2 years ago, I also repaired some wiring issues then. I'm kinda not thinking it's a coil problem because shes running great, no misfiring, starts right away, and idles normally. I'm going to take stuff apart this weekend see what I find.  I was thinking something similar to what Randy suggested, but with the way she's running, and because of an issue I had 2 summers ago, it may be the tach itself.  At the time, I'd start her up and the tach wouldn't read at all.... no needle movement, until I revved her past ~3k, several days later it started acting normal.

And YES, I AM upshifting.....

[rktmanfj]

Dyna box, stock coils(haven't gotten around to installing the DYNA coils yet)....

All electrical connections were cleaned and covered with dielectric grease 2 years ago, I also repaired some wiring issues then. I'm kinda not thinking it's a coil problem because shes running great, no misfiring, starts right away, and idles normally. I'm going to take stuff apart this weekend see what I find.  I was thinking something similar to what Randy suggested, but with the way she's running, and because of an issue I had 2 summers ago, it may be the tach itself.  At the time, I'd start her up and the tach wouldn't read at all.... no needle movement, until I revved her past ~3k, several days later it started acting normal.

And YES, I AM upshifting.....

I could send you a spare tach to try out...

BTW, mine does the 'dead' thing once in a while.  Tapping the lens brings it to life every time.

Randy T
Indy

[mikeholzer]

Yeah, about every third morning, my tach lies down at zero and waits for a little jolt to get it going. Starting to sound like a common problem. I'll wait until I have the 2X issues before I chase after it.

[RichBaker]

Thanks for the offer, Randy. I've got a complete cluster sitting on a shelf, bought it for the cover when mine cracked.... just have to dig it out and do some wrenching.

[RichBaker]

Update:  It appears to be a heat-realated issue.... over ~95F it reads double actual RPMs, since the temps went back down, it's acting normal. I'm waiting for the heat to come back and then I'll swap in the other gauge cludter and see what happens. For now, we're running low 90s for the daily high temp and all is normal.......

[Dan Filetti]

Rich-
This may or may not be related.  On my '85 I was having a hiccupping issue, whereby when I sat at a light, on a hot day, the bike would start to cough and if fed no throttle, it would eventually stall.  I figured this was a carb problem, but once I had gone through them, I still had the issue.  Turned out the issue was a bad coil that, when hot, would intermittently fail and cause the cough...

Maybe the heat is effecting your coil in a slightly different way and somehow doubling you RPM -not sure.  If the swapped out tach does not help, maybe the aforementioned Dyna coils need to be installed to solve the issue?  At least see if you can put a timing light on the coil wires when the 2X RPM behavior is taking place.  If the bike is running well, though, I'm not sure my idea holds any real water.  But think of it this way, it's a good excuse to install those coils anyway.

Stab in the dark, but I figured I'd throw it out there.

Dan

[RichBaker]

Yeah, Dan.... I've gotta get those coils installed eventually... maybe this'll be the incentive. 

[MSG Groves]

Hi, I'm new to the site.  I just bought a 93 and it is doing the same thing.  Took it for ride today and it ran great. Weird but every once in a while the tach would drop back to normal and stay there for awhile and then go back to double..  Have you figured out what was actually causing your problem.

MSG G.

[RichBaker]

Not yet, I'm planning to take the fairing off and go thru things when the temp gets back down to where I'm not sweating off 10 lbs of water when I'm working outside.... prob'ly next month.