Factory Tach Failure? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Keyblazer · 03-17-2015, 01:11 PM

Working on my '73 Blazer, and working thru its issues.

The Factory Tach, is not working and is stuck on 1500 rpm.
It jumps when I turn on the ignition, but nothing.

Wondered if this is a standard failure mode of these or I have a wiring issue, as I have other instrument that are not working?

Pic.

ray_mcavoy · 03-17-2015, 01:47 PM

The factory tachometers have their own wiring harness that is independent of the other gauges.

On the back of the tach you'll find a 3-terminal connector. Check the condition of the terminals to make sure they're not dirty or corroded. You can measure the voltage between the pink (+12V ignition) and black (ground) wires on that connector to verify the power & ground connections. The +12V feed is connected to a receptacle on the fuse box. The ground wire (for 73-77) is often just clipped to a metal portion of the dash and the type of clip used is subject to a poor connection if it or the dash develops a bit of surface rust.

The brown wire on the tach's 3-terminal connector is the signal from the coil. You can hook a 12V test light to this wire and it should be fairly bright with the engine idling and get dimmer as the engine speed increases. If not, check the condition of the wire and make sure it has a good electrical connection to the negative coil terminal (or TACH terminal if you've changed to an HEI distributor).

Unfortunately, there isn't much you can do beyond that. All of the factory 1973-1975 tachometers that I've seen have their internal circuitry encapsulated in an epoxy type material. That makes it pretty much impossible to do any troubleshooting or repair of the individual components.

Keyblazer · 03-17-2015, 02:08 PM

Awesome Info Ray, Thanks.
Will do as you say, and report back.

**hatzie** · 03-18-2015, 12:01 PM

If you have an automotive multimeter with Dwell and Tach you can actually set it to 6 or 8 cylinder tach and plug directly into the tach lead.

I have an old Sun analog Dwell tach but the digital versions are pretty cheap now and double as a basic multimeter.

http://www.harborfreight.com/lcd-aut...kit-95670.html

http://www.amazon.com/Tool-037-Didital-Tachomete-M...

03-17-2015, 01:11 PM	#1
Keyblazer Registered User Join Date: Feb 2015 Location: Irvine, CA Posts: 36	Factory Tach Failure? Working on my '73 Blazer, and working thru its issues. The Factory Tach, is not working and is stuck on 1500 rpm. It jumps when I turn on the ignition, but nothing. Wondered if this is a standard failure mode of these or I have a wiring issue, as I have other instrument that are not working? Pic.

03-17-2015, 01:47 PM	#2
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,366	Re: Factory Tach Failure? The factory tachometers have their own wiring harness that is independent of the other gauges. On the back of the tach you'll find a 3-terminal connector. Check the condition of the terminals to make sure they're not dirty or corroded. You can measure the voltage between the pink (+12V ignition) and black (ground) wires on that connector to verify the power & ground connections. The +12V feed is connected to a receptacle on the fuse box. The ground wire (for 73-77) is often just clipped to a metal portion of the dash and the type of clip used is subject to a poor connection if it or the dash develops a bit of surface rust. The brown wire on the tach's 3-terminal connector is the signal from the coil. You can hook a 12V test light to this wire and it should be fairly bright with the engine idling and get dimmer as the engine speed increases. If not, check the condition of the wire and make sure it has a good electrical connection to the negative coil terminal (or TACH terminal if you've changed to an HEI distributor). Unfortunately, there isn't much you can do beyond that. All of the factory 1973-1975 tachometers that I've seen have their internal circuitry encapsulated in an epoxy type material. That makes it pretty much impossible to do any troubleshooting or repair of the individual components.

03-17-2015, 02:08 PM	#3
Keyblazer Registered User Join Date: Feb 2015 Location: Irvine, CA Posts: 36	Re: Factory Tach Failure? Awesome Info Ray, Thanks. Will do as you say, and report back.

03-18-2015, 12:01 PM	#4
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Factory Tach Failure? If you have an automotive multimeter with Dwell and Tach you can actually set it to 6 or 8 cylinder tach and plug directly into the tach lead. I have an old Sun analog Dwell tach but the digital versions are pretty cheap now and double as a basic multimeter. http://www.harborfreight.com/lcd-aut...kit-95670.html http://www.amazon.com/Tool-037-Didital-Tachomete-M... __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 03-18-2015 at 12:06 PM.



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