1972 GMC tachometer problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tmarvin · 12-08-2013, 12:38 PM

Hey guys this is my first attempt at this forum so please bare with me. I have a 1972 GMC 4x4 and recently purchased a stock tach from LMC. I removed the old bezel ( Instrument panel) and installed the tach. On the rear of the tach are three electrical connections. The middle one is a nut and is obviously the ground. The other two one goes to the distributer and the other to a power source. I do not know which one goes where. I contacted LMC and the tach did not come with instructions. My concern is if I hook it up wrong I could damage the " expensive" tach. Can anyone help me a pic of the rear of the tach would help tks in advance

hugger6933 · 12-08-2013, 12:44 PM

I'll go dig out my service book. I had it on some notes here at my desk but can't find it. welcome

Coley · 12-08-2013, 12:52 PM

Ok, I've got one of mine sitting in front of me so here we go.
First, as you look at this from the back (important!!)
As viewed there are 3 connections.
The top centre one in the 'triangle is the chassis ground
The bottom RIGHT connector is the 12Volt power
The bottom LEFT connector is the COIL (-) connector.
Again, these directions are based on 'as viewed from the back'....so keep all of those orientations in mind when you spin it around and mount it up into the dash and then start to view things from the opposite 'needle' side as viewed from the drivers point of view.

Coley

hugger6933 · 12-08-2013, 01:02 PM

According to my book looking at the front of the tach, the lug in the 9 o'clock position goes to the neg side of the coil. The one in the 6 o'clock position goes to the # 3 spot in the fuse box. I have like I said another handwritten tach /dash wire diagram in my notes if I find it I will let you know if you have not been answered , but you will be. There are tons of people here to help you out. Jim

**TBONE1964** · 12-08-2013, 01:03 PM

Coley is right on the money. One other thing to remember is that the tach housing should ground to the cluster tin. How do the other gauges work? Does your truck have a factory wiring harness for a tach? The pink/red wire is the 12V wire and the brown (coil) wire should be on the negative side of the coil for a point style distributor or on the "tach terminal for HEI.

It is OK to add the ground to the center lug if you suspect a ground issue. Do you have a DVOM or a 12V test light to ensure power and ground?

What is wrong with the old tach?

Keep us posted.
Tom

Coley · 12-08-2013, 01:13 PM

Here's a quick pic for ya'.
If you look closely you can see the terminal/post I.D stampings that say 'COIL' and '12V'.
Have you removed and/or installed one before? (if not P.M me....I have a pretty good set of instructions that I did up to limit not only mistakes but also the time it can take if you haven't done it before)
Keep us posted on the installation.
All Good
Coley

...also, how' the weather up there in Oliver? I have relatives in that area that have a vineyard...nice place!

Andy4639 · 12-08-2013, 01:56 PM

Nothing hooked to the screw the tach is grounded through the dash. Just the factory v connector. This is mine.

**TBONE1964** · 12-08-2013, 02:01 PM

Quote:

Originally Posted by Andy4639

Nothing hooked to the screw the tach is grounded through the dash. Just the factory v connector. This is mine.

That is correct Andy and if the rest of the gauges work OK then the ground should be good. Don't over think this, it is not very complicated to figure out. make sure if you have an HEI that you are getting a good connection at the "tach" terminal on the distributor.

**Already Gone** · 12-08-2013, 07:20 PM

Subscribed

Andy4639 · 12-08-2013, 08:06 PM

Mine was in my 71 for over 20 years and it worked great straight out of a junkyard. Didn't do anything to it just ran the wires as suppose to. One to 12 volts and the other to the HEI tach connection. I used regular spade connector on the tach.

12-08-2013, 12:38 PM	#1
tmarvin Registered User Join Date: May 2012 Location: Oliver, B.C Canada Posts: 12	1972 GMC tachometer problem Hey guys this is my first attempt at this forum so please bare with me. I have a 1972 GMC 4x4 and recently purchased a stock tach from LMC. I removed the old bezel ( Instrument panel) and installed the tach. On the rear of the tach are three electrical connections. The middle one is a nut and is obviously the ground. The other two one goes to the distributer and the other to a power source. I do not know which one goes where. I contacted LMC and the tach did not come with instructions. My concern is if I hook it up wrong I could damage the " expensive" tach. Can anyone help me a pic of the rear of the tach would help tks in advance

12-08-2013, 12:44 PM	#2
hugger6933 Registered User Join Date: Oct 2008 Location: Marianna Arkansas Posts: 7,280	Re: 1972 GMC tachometer problem I'll go dig out my service book. I had it on some notes here at my desk but can't find it. welcome

12-08-2013, 12:52 PM	#3
Coley Registered User Join Date: Oct 2011 Location: Victoria, B.C Posts: 3,794	Re: 1972 GMC tachometer problem Ok, I've got one of mine sitting in front of me so here we go. First, as you look at this from the back (important!!) As viewed there are 3 connections. The top centre one in the 'triangle is the chassis ground The bottom RIGHT connector is the 12Volt power The bottom LEFT connector is the COIL (-) connector. Again, these directions are based on 'as viewed from the back'....so keep all of those orientations in mind when you spin it around and mount it up into the dash and then start to view things from the opposite 'needle' side as viewed from the drivers point of view. Coley __________________ ....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500

12-08-2013, 01:02 PM	#4
hugger6933 Registered User Join Date: Oct 2008 Location: Marianna Arkansas Posts: 7,280	Re: 1972 GMC tachometer problem According to my book looking at the front of the tach, the lug in the 9 o'clock position goes to the neg side of the coil. The one in the 6 o'clock position goes to the # 3 spot in the fuse box. I have like I said another handwritten tach /dash wire diagram in my notes if I find it I will let you know if you have not been answered , but you will be. There are tons of people here to help you out. Jim

12-08-2013, 01:03 PM	#5
TBONE1964 Cluster King Join Date: Nov 2012 Location: Junction City, OR Posts: 5,263	Re: 1972 GMC tachometer problem Coley is right on the money. One other thing to remember is that the tach housing should ground to the cluster tin. How do the other gauges work? Does your truck have a factory wiring harness for a tach? The pink/red wire is the 12V wire and the brown (coil) wire should be on the negative side of the coil for a point style distributor or on the "tach terminal for HEI. It is OK to add the ground to the center lug if you suspect a ground issue. Do you have a DVOM or a 12V test light to ensure power and ground? What is wrong with the old tach? Keep us posted. 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

12-08-2013, 01:13 PM	#6
Coley Registered User Join Date: Oct 2011 Location: Victoria, B.C Posts: 3,794	Re: 1972 GMC tachometer problem Here's a quick pic for ya'. If you look closely you can see the terminal/post I.D stampings that say 'COIL' and '12V'. Have you removed and/or installed one before? (if not P.M me....I have a pretty good set of instructions that I did up to limit not only mistakes but also the time it can take if you haven't done it before) Keep us posted on the installation. All Good Coley ...also, how' the weather up there in Oliver? I have relatives in that area that have a vineyard...nice place! Attached Images __________________ ....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500

12-08-2013, 01:56 PM	#7
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: 1972 GMC tachometer problem Nothing hooked to the screw the tach is grounded through the dash. Just the factory v connector. This is mine. __________________ 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.

12-08-2013, 07:20 PM	#9
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,143	Re: 1972 GMC tachometer problem Subscribed __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

12-08-2013, 08:06 PM	#10
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: 1972 GMC tachometer problem Mine was in my 71 for over 20 years and it worked great straight out of a junkyard. Didn't do anything to it just ran the wires as suppose to. One to 12 volts and the other to the HEI tach connection. I used regular spade connector on the tach. __________________ 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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