Tach calibration - The 1947 - Present Chevrolet & GMC Truck Message Board Network

greenhorn male · 04-30-2005, 01:25 PM

I tested 2 tachs (used, in a cluster) using jumper wires and alligator clips. With light guage wires I could barely get them to work. So I switched to heavy guaged jumpers. One tach (marked 8 cyl) registered 1K, the other one (no visible markings) registerd 800, the sears tach/dwell meter registerd 750. These were taken at idle with one vacuum port open (to air cleaner). The strange thing is both tachs seem to top out about 2200 rpm when I opened the throttle some. I was going to swap one of these clusters in because my temp guage doesn't work in my truck.
Question: Is that a calibration screw in the back? Why does it seem to top out at 2200-2300 RPM? When I install my HEI, what would it do to the readings?

OhioDan · 04-30-2005, 04:48 PM

I'd like to know also. My tach in the dash reads about 500 at idle, and my tach I use at work says it's about 650-675. Maybe someone has an answer?

**VetteVet** · 04-30-2005, 11:02 PM

Just a guess but were the truck engines originally six cylinder inlines. The tachs might be calibrated different for sixes.
VetteVet

68w/sbc406 · 05-01-2005, 04:30 AM

did tach dashes ever come in a 6 cylider??? id say cause they are old and inaccurate. i dont know how to calibrate them so TTT for ya

byrd · 05-01-2005, 07:32 AM

Rod on here can tell you how to zero the tach out. I didn't think the factory tachs are compatable with hei?

Randy

72gord · 05-01-2005, 08:33 AM

My son and I put a factory tach in our truck that came out of a C-60 with a 366 in it, ours is a 350 with HEI and the tach works fine

OhioDan · 05-02-2005, 03:56 AM

Quote:

Originally Posted by byrd

Rod on here can tell you how to zero the tach out. I didn't think the factory tachs are compatable with hei?

Randy

I hope you're wrong about these tachs being compatible with HEI. There sure seems to be a lot of members using them. Rod who?

byrd · 05-02-2005, 07:05 AM

Rod the moderator. I 've been told that about the tach from vendors, so I'm not really sure it they wanted me to buy something or telling the truth

any body else want to help me out on this? According to classic industries their aftermarket tach is not compatable with hei, so am I wrong to assume the factory tach won't either? Randy

Huck · 05-02-2005, 08:28 AM

The original factory tach's are designed to work with points dist's. HEI sends a signal of diferent wave length. There is a potentiometer adjustment in the rear of the tach (plastic screw) that can be adjusted but wouldn't help a tach that tops out at 2200 rpm. There are different part numbers for 6cyl and 8cyl tachs but have heard that you can adjust accordingly.
In my experience, about 1/2 the old original tach's you buy don't work even closely to actual rpm. KInd of like the AM/Fm radios--they might work but they suck when it comes to performance. You can buy a new replacement tach from the board venders that will work with HEI. It looks almost like an exact replacement except for the placement of the dial screws. About $100.

68w/sbc406 · 05-02-2005, 09:04 AM

Quote:

Originally Posted by byrd

Rod the moderator. I 've been told that about the tach from vendors, so I'm not really sure it they wanted me to buy something or telling the truth

any body else want to help me out on this? According to classic industries their aftermarket tach is not compatable with hei, so am I wrong to assume the factory tach won't either? Randy

and also a guy on here burned up his camaro tach cause he didnt get the HEI filter also.

Rod · 05-02-2005, 09:15 AM

Yes tachs came in 6 or 8 cyl.
Yes you can calibrate an original tach back to zero.
Yes I have seen original tachs running on HEI equipped trucks with no filters.
Yes the originals are not as accurate as you may think.
Yes I have had tachs that had limited rpm, pretty much they get stuck ...fixed them all.
Yes I did list some for sale I had, waited a bit then put them on ebay.

Yes I am an Administator of the board. Step above a Moderator.

I've fixed tachs before..but been so long ago I'd have to take one apart just to figure it out again. Someone send me a bad one that's stuck or sticking dosen't rev high enough and I'll fix it when time permits. I dont have a bad one to play with.

chevyfarmer · 05-02-2005, 01:51 PM

Great topic. I have a tach in my 68 with a HEI that appears to be working fine (Although I have never tested the accuracy). I do not know what filter some of you are referring to. What is it, where would i get one, how does it install??? etc. Am i going to burn up or damage the tach without it?? I have ran it like this for about 50K without any problems, but dont want to kill the tach either. Thanks

**pjmoreland** · 05-02-2005, 04:08 PM

If you know your speedometer is accurate (mine is), then you can do a pretty simple test of your tachometer as follows.

My factory tachometer reads about 2000 RPM at 65 MPH. I have a 700R4, so with the torque converter clutch locked, the transmission has a 0.70 overdrive. My rear end is 3.73. I have 255-70R15 tires, which have a diameter of approximately 29 inches (15 inches plus two times the sidewall height, which is 70/100 x 255 /25.4). So...

2000RPM x 1/0.70 x 1/3.73 x 29in x 3.14159265 x 1ft/12in x 1mile/5280ft x 60min/1hr = 66.08MPH

That's pretty close in my opinion.

04-30-2005, 01:25 PM	#1
greenhorn male Senior Member Join Date: Jan 2003 Location: Bay Area, Northern California Posts: 1,344	Tach calibration I tested 2 tachs (used, in a cluster) using jumper wires and alligator clips. With light guage wires I could barely get them to work. So I switched to heavy guaged jumpers. One tach (marked 8 cyl) registered 1K, the other one (no visible markings) registerd 800, the sears tach/dwell meter registerd 750. These were taken at idle with one vacuum port open (to air cleaner). The strange thing is both tachs seem to top out about 2200 rpm when I opened the throttle some. I was going to swap one of these clusters in because my temp guage doesn't work in my truck. Question: Is that a calibration screw in the back? Why does it seem to top out at 2200-2300 RPM? When I install my HEI, what would it do to the readings? __________________ 03 Tahoe 72 GMC Super Custom 2500 Custom Camper, my reliable workhorse. Dual gauged tanks, dual battery trays.

04-30-2005, 04:48 PM	#2
OhioDan Registered User Join Date: Apr 2002 Location: New Carlisle, Ohio,USA Posts: 1,404	I'd like to know also. My tach in the dash reads about 500 at idle, and my tach I use at work says it's about 650-675. Maybe someone has an answer? __________________ LIFE AIN'T EASY WHEN YER FAT AND GREASY! GMC= Got More Class! 1970 GMC LWB, Fleetside, 350/4spd, Flowmaster "40's", 2002 GMC 16" Rallies, cowl hood,and added a bunch of options that weren't there when I started.

04-30-2005, 11:02 PM	#3
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,709	tachs Just a guess but were the truck engines originally six cylinder inlines. The tachs might be calibrated different for sixes. VetteVet __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

05-01-2005, 04:30 AM	#4
68w/sbc406 Got Light Emitting Diode? Join Date: Apr 2004 Location: Newberg, OR Posts: 2,485	did tach dashes ever come in a 6 cylider??? id say cause they are old and inaccurate. i dont know how to calibrate them so TTT for ya __________________ 1968 1/2 ton 2wd lwb 6 lug disc and bags up front. Next c notch and rear bags '02 Chevy Silverado LT ext cab short bed 2WD 5/7 drop on 22's(the family car) 1993 Cherokee work ride/weekend wheeler

05-02-2005, 09:15 AM	#11
Rod Join Date: Dec 1999 Location: Eastern - Manitoba Canada, Winnipeg Posts: 4,369	Yes tachs came in 6 or 8 cyl. Yes you can calibrate an original tach back to zero. Yes I have seen original tachs running on HEI equipped trucks with no filters. Yes the originals are not as accurate as you may think. Yes I have had tachs that had limited rpm, pretty much they get stuck ...fixed them all. Yes I did list some for sale I had, waited a bit then put them on ebay. Yes I am an Administator of the board. Step above a Moderator. I've fixed tachs before..but been so long ago I'd have to take one apart just to figure it out again. Someone send me a bad one that's stuck or sticking dosen't rev high enough and I'll fix it when time permits. I dont have a bad one to play with. __________________ Senior active founding member. 70 3/4 ton GMC Suburban 4x4 250/4 spd. 71 1/2 ton Suburban Chevy 4x4, 350/350 72 Chev 1 ton tow truck 402BB w/Holmes 440 wrecker on propane 2005 FLSTSCI Harley Springer 85 FXSB Harley Lowrider 72 Triumph 650 Bonneville 5spd "Poor people don't have hobbies" Quote from wife.

05-01-2005, 08:33 AM	#6
72gord Never enough time!! Join Date: Mar 2004 Location: sw Wisconsin Posts: 602	My son and I put a factory tach in our truck that came out of a C-60 with a 366 in it, ours is a 350 with HEI and the tach works fine

05-02-2005, 08:28 AM	#9
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	The original factory tach's are designed to work with points dist's. HEI sends a signal of diferent wave length. There is a potentiometer adjustment in the rear of the tach (plastic screw) that can be adjusted but wouldn't help a tach that tops out at 2200 rpm. There are different part numbers for 6cyl and 8cyl tachs but have heard that you can adjust accordingly. In my experience, about 1/2 the old original tach's you buy don't work even closely to actual rpm. KInd of like the AM/Fm radios--they might work but they suck when it comes to performance. You can buy a new replacement tach from the board venders that will work with HEI. It looks almost like an exact replacement except for the placement of the dial screws. About $100.

05-02-2005, 01:51 PM	#12
chevyfarmer Registered User Join Date: Feb 2005 Location: Owosso, Mi Posts: 242	Great topic. I have a tach in my 68 with a HEI that appears to be working fine (Although I have never tested the accuracy). I do not know what filter some of you are referring to. What is it, where would i get one, how does it install??? etc. Am i going to burn up or damage the tach without it?? I have ran it like this for about 50K without any problems, but dont want to kill the tach either. Thanks __________________ 67 short wide 355ci 3 on tree 69 short wide 250ci 3 on tree 68 long wide 283 3 on tree 72 2wd Blazer project 454 5 speed

05-02-2005, 04:08 PM	#13
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	If you know your speedometer is accurate (mine is), then you can do a pretty simple test of your tachometer as follows. My factory tachometer reads about 2000 RPM at 65 MPH. I have a 700R4, so with the torque converter clutch locked, the transmission has a 0.70 overdrive. My rear end is 3.73. I have 255-70R15 tires, which have a diameter of approximately 29 inches (15 inches plus two times the sidewall height, which is 70/100 x 255 /25.4). So... 2000RPM x 1/0.70 x 1/3.73 x 29in x 3.14159265 x 1ft/12in x 1mile/5280ft x 60min/1hr = 66.08MPH That's pretty close in my opinion. Last edited by pjmoreland; 05-02-2005 at 04:08 PM.



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