Tachometer Calibration/Adjustment ??
 

Even with all these old Chev and GMCs hangin' around here, never had one with a factory tach except the mechanically driven 67 Corvette. So - is the 67-72 truck tach the same - mechanical - or is it or electronic, and how does a guy make it accurate?

This one feels to be running (guesstimate) about 50% to 70% higher than actual revs. Is this a reasonable/logical fix (gear, cable, etc.?) or a pain?

Thanks for any info and help.

CCCC

Re: Tachometer Calibration/Adjustment ??
 

It's electric, but I'm not sure how to calibrate it....

Re: Tachometer Calibration/Adjustment ??
 

I want to know the also ?

Re: Tachometer Calibration/Adjustment ??
 

You will have to send it to someone who rebuilds instruments and have them calibrate it.
I just had some gauges for a Road Runner restored. I used a place called Dash Worx. There are lots of places out there. Just do an internet search.

Re: Tachometer Calibration/Adjustment ??
 

I use a Snap-On Tach as a reference and calibrate the tach using the adjustment on the back of the tach, (covered with a piece of tape).

Jim

Re: Tachometer Calibration/Adjustment ??
 

like Jim said, you will need an accurate tach to measure by.... and there is a means to adjust it.

however, if your REALLY concerned about accurate revs, you may add an aftermarket unit. granted this old 40 year old tach is somewhat accurate, but I would guess within 80-90% accurate within any given time..... or that seems to be my experience.

Re: Tachometer Calibration/Adjustment ??
 

I have read many threads in the past about this and our factory tachs are notoriously inaccurate.

Re: Tachometer Calibration/Adjustment ??
 

Thanks to all for the comments and suggestions (maybe I will have to send it somewhere), and especially to JimKshortstep4x4 in that I would prefer to learn to do the adjustment myself, if possible. If I understand correctly, there is place on the back side of the tach where one can change the setting for calibration. Can you be a bit more specific - is it a small opening in which I would use a small screwdriver, or hex wrench, etc.??

Thanks for any additional guidance.
CCCC

Re: Tachometer Calibration/Adjustment ??
 

There's a flat head screw, usually covered with a round piece of tape, on the back of the tach. That's the adjustment. It is pretty sensitive, so you really need to watch the tach when you tweak it. It helps to have a second person hold the throttle steady while you adjust.
I adjusted mine at 2500 RPM and that seemed to work best.

Re: Tachometer Calibration/Adjustment ??
 

Thanks, clay68c10. I will give it a go. My old pal (wife) can hold the rpms and read the second meter. You fellows are great.
CCCC

Re: Tachometer Calibration/Adjustment ??
 

How big of a screw driver do I need and how do I hook up a second tach as a reference?

Re: Tachometer Calibration/Adjustment ??
 

I used a small FLAT screwdriver. I'm able to reach with the cluster installed. I didn't do it while the truck was running. The adjustment is extremely sensitive.

I mounted an aftermarket tach under the dash for calibration purposes by tee'ing off the tach wire and hooking up power and ground.

Re: Tachometer Calibration/Adjustment ??
 

For anyone interested, I have a calibrator made for electronic tachs. It will read 8, 6 and 4 cylinder tachs at three RPM ranges.

Also, depending on the year, the tach will either have a flat head adjustment or a #2 phillips head adjustment. Be careful while adjusting, the adjustment only has a short range of motion. If you have a timing light that reads RPM's that is the best reference to use as a comparison.

Thank you,
Tom

Re: Tachometer Calibration/Adjustment ??
 

My tach in my other truck was reading like 3800 RPM at 60 MPH. So (in my ignorance), ordered another one from LMC. That one would read accurate until I was on the freeway for over 10 minutes. Then it would read high again. I had them send me 3 tach's in total and they were all POS's!! I should have either adjusted the first one, or had it rebuilt if it needed it.

Re: Tachometer Calibration/Adjustment ??
 

My tach is an LMC(I think) and is different than the pic above. It had a hole covered by a QC sticker, I tried a flat & Phillips screw driver neither seemed to fit. I looked up there with a mirror & it looked like a blue plastic ??? But I couldn't see a screw head. I tried taking a picture but couldn't get it to focus(I have the same problem sometimes:lol:). Anyone know if this is in fact the calibration adj. ? Attachment 1253202

Re: Tachometer Calibration/Adjustment ??
 

I am not where I can see the ones I have in my inventory (aftermarket). Some of these may be non adjustable. You may also have a plug in there that prevents adjustment. If you need to work on it, it is best to remove the cluster from the dash to get a good look at it. You can plug the tach in out of the cluster and run it as long as you have the the tin grounded or use the center lug which is also for a ground.

Re: Tachometer Calibration/Adjustment ??
 

Ya... I was just looking for the easy way out:rolleyes: