Tachometer Calibration/Adjustment ?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

CCCC · 11-06-2009, 05:28 PM

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

**Hart_Rod** · 11-06-2009, 06:21 PM

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

alfeefish · 11-06-2009, 07:51 PM

Quote:

Originally Posted by Hart_Rod

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

I want to know the also ?

JimKshortstep4x4 · 11-06-2009, 09:08 PM

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

CCCC · 11-07-2009, 12:56 AM

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

turp mcspray · 11-06-2009, 09:07 PM

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.

tcb-1 · 11-06-2009, 09:47 PM

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.

unclewatts · 11-07-2009, 12:03 AM

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

**clay68c10** · 11-07-2009, 02:49 AM

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.

CCCC · 11-07-2009, 01:07 PM

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

cwilkie · 05-10-2014, 10:55 AM

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

badpeanut · 05-10-2014, 11:45 PM

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.

**TBONE1964** · 05-11-2014, 12:05 AM

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

69GMCLonghorn · 05-12-2014, 01:50 PM

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.

gonnuts · 05-12-2014, 09:40 PM

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

). Anyone know if this is in fact the calibration adj. ? Name: image.jpg
Views: 1194
Size: 22.8 KB

**TBONE1964** · 05-12-2014, 10:08 PM

Quote:

Originally Posted by gonnuts

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

). Anyone know if this is in fact the calibration adj. ? Attachment 1253202

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.

gonnuts · 05-12-2014, 10:33 PM

Quote:

Originally Posted by TBONE1964

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.

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

11-06-2009, 05:28 PM	#1
CCCC Registered User Join Date: Mar 2007 Location: San Pedro River, SE Arizona Posts: 735	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 __________________ CCCC 67 Sting Ray Coupe 327 4sp; 68 K10 Suburban 327 4sp; 69 K20 Lifted 383 4sp; 69 C80 Fire Truck 478 V6 5sp/2sp; 69 C20 Longhorn 327 4sp; 70 C30 Wrecker 350 4sp; 70 C30 Flat Bed 350 4sp; 71 C20 LWB 350 4sp; 72 K5 Blazer 350 4sp; 72 C30 Flat Bed Gooseneck Hauler with sleeper 350/TH400; 72 C20 Longhorn 402 4sp; 52 Mack LTL Supercharged Cummins 5sp x 3sp; 01 Dodge 5.9 HO Cummins 6spd; 02 Safari Trek 2430 8.1 Vortec on Workhorse Chassis; 03 Silverado 2500 HD 4wd Duramax/Allison Moved but still in the family 70 C20 Custom Camper 350/TH350; 72 K5 Blazer CST Highlander 350/TH350 Recently sent to new homes 72 C30 MoHome 402 4sp; 72 K5 Blazer Conv 350 4sp; 75 GMC 6000 Aerial 350 w Allison; 72 GMC 3500 Utility 350 4sp; 70 GMC 3500, 350 4sp

11-06-2009, 09:08 PM	#4
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	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 __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

11-07-2009, 12:56 AM	#5
CCCC Registered User Join Date: Mar 2007 Location: San Pedro River, SE Arizona Posts: 735	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 __________________ CCCC 67 Sting Ray Coupe 327 4sp; 68 K10 Suburban 327 4sp; 69 K20 Lifted 383 4sp; 69 C80 Fire Truck 478 V6 5sp/2sp; 69 C20 Longhorn 327 4sp; 70 C30 Wrecker 350 4sp; 70 C30 Flat Bed 350 4sp; 71 C20 LWB 350 4sp; 72 K5 Blazer 350 4sp; 72 C30 Flat Bed Gooseneck Hauler with sleeper 350/TH400; 72 C20 Longhorn 402 4sp; 52 Mack LTL Supercharged Cummins 5sp x 3sp; 01 Dodge 5.9 HO Cummins 6spd; 02 Safari Trek 2430 8.1 Vortec on Workhorse Chassis; 03 Silverado 2500 HD 4wd Duramax/Allison Moved but still in the family 70 C20 Custom Camper 350/TH350; 72 K5 Blazer CST Highlander 350/TH350 Recently sent to new homes 72 C30 MoHome 402 4sp; 72 K5 Blazer Conv 350 4sp; 75 GMC 6000 Aerial 350 w Allison; 72 GMC 3500 Utility 350 4sp; 70 GMC 3500, 350 4sp

11-06-2009, 09:07 PM	#6
turp mcspray Registered User Join Date: May 2007 Location: Rapid City South Dakota Posts: 2,359	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. __________________ Turp Mcspray New life for an old 2wd, farm blazer http://67-72chevytrucks.com/vboard/s...d.php?t=505987 My Blazer build http://67-72chevytrucks.com/vboard/s...d.php?t=342299

11-06-2009, 09:47 PM	#7
tcb-1 Nothing to see here..... Join Date: Jun 2007 Location: Shawnee, KS Posts: 4,625	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. __________________ Doug THANK YOU to our American Soldiers & Veterans - POW MIA "You will NEVER be forgotten". The democracy will cease to exist when you take away from those who are willing to work and give to those who would not. Thomas Jefferson

11-06-2009, 06:21 PM	#2
Hart_Rod ************* Join Date: Apr 2005 Location: Pensacola, FL Posts: 17,862	Re: Tachometer Calibration/Adjustment ?? It's electric, but I'm not sure how to calibrate it.... __________________ Rob - https://www.instagram.com/hart_rod_c10 As Iron Sharpens Iron, So One Man Sharpens Another. Proverbs 27:17 FOR SALE: DBW pedal bracket - http://67-72chevytrucks.com/vboard/s...d.php?t=651123 FOR SALE: Hood Brackets http://67-72chevytrucks.com/vboard/s...d.php?t=679945 1987 Silverado SWB - 34.5K original miles http://67-72chevytrucks.com/vboard/s...d.php?t=801834 1969 SuperBurb - http://67-72chevytrucks.com/vboard/s...d.php?t=200387 1968 Farm truck - http://67-72chevytrucks.com/vboard/s...d.php?t=358692 1968 SWB - http://67-72chevytrucks.com/vboard/s...d.php?t=551258 1948 Chevy - http://67-72chevytrucks.com/vboard/s...=122164&page=3

11-07-2009, 12:03 AM	#8
unclewatts 70 Chevrolet=Obsession Join Date: Jun 2009 Location: nice ca Posts: 1,067	Re: Tachometer Calibration/Adjustment ?? I have read many threads in the past about this and our factory tachs are notoriously inaccurate. __________________ I'm Just like my truck. Old, Ugly and Grouchy. Except my truck starts easily in the morning and doesn't smoke! IT'S NOT EASY BEING GREEN! -Kermit the frog 1970 Chevrolet C-10 Longbed Stepside : 350, Muncie M20, 3.08 GM corp. positraction Forest green exterior/light green interior(Where all my money goes) 1996 Toyota Corolla (Parts runner)

11-07-2009, 02:49 AM	#9
clay68c10 Senior Member Join Date: Aug 2004 Location: Centennial, CO Posts: 2,996	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. __________________ ClusterTruck: 68 C10 Bought in 89, wrecked in 03. Slow low $ rebuild started '17. 6.0 & TKO http://67-72chevytrucks.com/vboard/s...d.php?t=788602 93 K1500 Short Step, 350, NV4500, EBL flash ECU. Vortec heads & roller cam someday... 05 Jeep Wrangler Unlimited. 3” lift & 6-speed 02 E320 the insurance total “free car” 13 Tahoe LTZ

11-07-2009, 01:07 PM	#10
CCCC Registered User Join Date: Mar 2007 Location: San Pedro River, SE Arizona Posts: 735	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 __________________ CCCC 67 Sting Ray Coupe 327 4sp; 68 K10 Suburban 327 4sp; 69 K20 Lifted 383 4sp; 69 C80 Fire Truck 478 V6 5sp/2sp; 69 C20 Longhorn 327 4sp; 70 C30 Wrecker 350 4sp; 70 C30 Flat Bed 350 4sp; 71 C20 LWB 350 4sp; 72 K5 Blazer 350 4sp; 72 C30 Flat Bed Gooseneck Hauler with sleeper 350/TH400; 72 C20 Longhorn 402 4sp; 52 Mack LTL Supercharged Cummins 5sp x 3sp; 01 Dodge 5.9 HO Cummins 6spd; 02 Safari Trek 2430 8.1 Vortec on Workhorse Chassis; 03 Silverado 2500 HD 4wd Duramax/Allison Moved but still in the family 70 C20 Custom Camper 350/TH350; 72 K5 Blazer CST Highlander 350/TH350 Recently sent to new homes 72 C30 MoHome 402 4sp; 72 K5 Blazer Conv 350 4sp; 75 GMC 6000 Aerial 350 w Allison; 72 GMC 3500 Utility 350 4sp; 70 GMC 3500, 350 4sp

05-10-2014, 10:55 AM	#11
cwilkie Chance Join Date: Sep 2001 Location: Belcourt ,ND, USA Posts: 750	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? __________________ 1968 Chevrolet Stepside BB 396 w/TH400 1972 Chevrolet 3/4 Ton 4x4 BB 496 w/TH400

05-10-2014, 11:45 PM	#12
badpeanut Not a bad nut after all.. Join Date: Dec 2000 Location: ND Posts: 1,759	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. Attached Images __________________ '05 Chevy 2500HD 4x4 '72 GMC LWB 4x2 driver '72 Chevy LWB 4x4 plow truck '71 Blazer 4x4 driver '70 GMC LWB trailer

05-11-2014, 12:05 AM	#13
TBONE1964 Cluster King Join Date: Nov 2012 Location: Junction City, OR Posts: 5,263	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 __________________ Contact me on all of your gauge cluster needs. I specialize in restoration, repair and parts sales for 67-72 Chevy and GMC trucks. email me at tbonegarris@yahoo.com I am also a dealer for Counterpart for gauge cluster parts only. Also see my facebook page, CG&C

05-12-2014, 01:50 PM	#14
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	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.

05-12-2014, 09:40 PM	#15
gonnuts Registered User Join Date: Sep 2010 Location: Langdon, Ab Posts: 348	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). Anyone know if this is in fact the calibration adj. ? __________________ 67 Chev SWB, 400 SB .040 over, vortec heads, comp XE268 cam, performer EPS intake, edel. 650 AVS carb, 700R4



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.