Factory tachometer reading high - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Kcorey · 03-31-2020, 06:42 AM

Hey guys , I’ve got a 72 k20 with factory tachometer . I rebuild the engine last winter after buying truck , and ever since I installed it my factory tachometer reads roughly 3-400 rpm high at idle . And when I rev the truck up or accelerate , it seems to respond rather quickly ( meaning rpms show they are rising but definitely not matching the actual engine rpms). Just looking for some thoughts , it’s a 350 with a MSD hei distributor . Thanks

**HO455** · 03-31-2020, 03:11 PM

Mine is the same way. I believe the 50 plus year electronics have degraded and are causing the erroneous readings. I know that member Tbone1964 can fix the problem.

**Sheepdip** · 03-31-2020, 03:40 PM

Quote:

Originally Posted by HO455

Mine is the same way. I believe the 50 plus year electronics have degraded and are causing the erroneous readings. I know that member Tbone1964 can fix the problem.

I'll second TBone, he does good work.
And while your at it and got the dash out, have him fix you up with a volt meter to replace the worthless amp meter.

custom10nut · 03-31-2020, 10:03 PM

Two possibilities here.
As mentioned, the 50 year old electronics are probably worn out, but they will usually read way too low (mine read a consistent 225 when the actual rpm was 750).
The fix is a new tach or a rebuild, which will cost you about three times as much.
The other is that it needs to be adjusted, by turning the screw in the center of the back of the tach.
If it just started doing wacky stuff out of the blue? It’s probably the Electronic issue.

Kcorey · 04-01-2020, 06:25 AM

I can’t say if it just started doing it , because when I bought the truck there was no motor in it . Since I put the new engine in, it has been doing this. I’ll look at the adjustment

Quote:

Originally Posted by custom10nut

Two possibilities here.
As mentioned, the 50 year old electronics are probably worn out, but they will usually read way too low (mine read a consistent 225 when the actual rpm was 750).
The fix is a new tach or a rebuild, which will cost you about three times as much.
The other is that it needs to be adjusted, by turning the screw in the center of the back of the tach.
If it just started doing wacky stuff out of the blue? It’s probably the Electronic issue.

custom10nut · 04-01-2020, 07:58 AM

I think there’s an adapter you have to use with the HEI, to get it to read correctly.
I hope someone on here can give you better guidance

**kidatheart** · 04-01-2020, 08:24 AM

Is it possible it’s a 6 cylinder tach? How would it read if hooked up to a 8 cylinder?

Andy4639 · 04-01-2020, 09:13 AM

Checkout this post for a cheap way to fix your tach. It's less than $70.00 to fix it and it will work with HEI and read correctly. Either choice is good.
I just bought both and put the new one in the Ramp Truck. The rebuilt one will go in a cluster to sell maybe are keep as a backup.

http://67-72chevytrucks.com/vboard/s...23#post8706423

https://www.youtube.com/watch?v=epOYrxJV-a0

You Tube

gmc684x4 · 04-18-2020, 06:14 PM

Check this thread out a new board for your tach if it needs rebuild

http://67-72chevytrucks.com/vboard/s...d.php?t=803315

Andy4639 · 04-18-2020, 09:04 PM

You can pull the tach out of the cluster and hook it back up and run a ground jumper to it. Crank the truck up and set the tach per how it is running using a dwell meter if you have one that has the tach feature. If it's a factory tach it will be stamped on the back of the tin where the wires hookup either 8cyl or 6 cyl. All the ones I have ever owned did anyway.

03-31-2020, 06:42 AM	#1
Kcorey Registered User Join Date: Jan 2018 Location: Madison CT Posts: 64	Factory tachometer reading high Hey guys , I’ve got a 72 k20 with factory tachometer . I rebuild the engine last winter after buying truck , and ever since I installed it my factory tachometer reads roughly 3-400 rpm high at idle . And when I rev the truck up or accelerate , it seems to respond rather quickly ( meaning rpms show they are rising but definitely not matching the actual engine rpms). Just looking for some thoughts , it’s a 350 with a MSD hei distributor . Thanks __________________ 1970 Chevy c10 SWB twin turbo LS3. 1972 Chevy k20 super cheyenne.

03-31-2020, 03:11 PM	#2
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 11,350	Re: Factory tachometer reading high Mine is the same way. I believe the 50 plus year electronics have degraded and are causing the erroneous readings. I know that member Tbone1964 can fix the problem. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

03-31-2020, 10:03 PM	#4
custom10nut Registered User Join Date: Apr 2015 Location: East Tn (In the heart of the Smoky Mtns) Posts: 1,887	Re: Factory tachometer reading high Two possibilities here. As mentioned, the 50 year old electronics are probably worn out, but they will usually read way too low (mine read a consistent 225 when the actual rpm was 750). The fix is a new tach or a rebuild, which will cost you about three times as much. The other is that it needs to be adjusted, by turning the screw in the center of the back of the tach. If it just started doing wacky stuff out of the blue? It’s probably the Electronic issue.

04-01-2020, 07:58 AM	#6
custom10nut Registered User Join Date: Apr 2015 Location: East Tn (In the heart of the Smoky Mtns) Posts: 1,887	Re: Factory tachometer reading high I think there’s an adapter you have to use with the HEI, to get it to read correctly. I hope someone on here can give you better guidance

04-01-2020, 08:24 AM	#7
kidatheart Senior Member Join Date: May 2012 Location: Assaria, Ks Posts: 465	Re: Factory tachometer reading high Is it possible it’s a 6 cylinder tach? How would it read if hooked up to a 8 cylinder?

04-18-2020, 06:14 PM	#9
gmc684x4 Registered User Join Date: Jul 2012 Location: Whitehorse yukon Posts: 1,218	Re: Factory tachometer reading high Check this thread out a new board for your tach if it needs rebuild http://67-72chevytrucks.com/vboard/s...d.php?t=803315

04-18-2020, 09:04 PM	#10
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: Factory tachometer reading high You can pull the tach out of the cluster and hook it back up and run a ground jumper to it. Crank the truck up and set the tach per how it is running using a dwell meter if you have one that has the tach feature. If it's a factory tach it will be stamped on the back of the tin where the wires hookup either 8cyl or 6 cyl. All the ones I have ever owned did anyway. Attached Images __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.



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