Written by: Dave Cross (Crossy)

Tach Dash Differences

Factory tachometers were offered in all 6 & 8-cylinder GM pickups from 1973 to 1981, and beyond in some cases.  Tachometers were offered on 1981 and up models, but they are not listed in the "options" catalogs and are extremely hard to come by.   The gasoline powered light trucks came with a 5,000 RPM tachometer, while the medium/heavy duty diesel trucks came with 4,000 or 5,000 RPM units. The medium/heavy duty trucks are easily identified by a tiny fuel gauge under the tachometer and anything from a clock, blank, air pressure, or vacuum gauge can be found in the lower left corner where the clock usually resides in non-tachometer equipped trucks. The tachometer itself is easy to wire, but if you are unsure you can look closely at the back of the tachometer it will tell you where the wires go. Also, getting at least a short piece of the factory pigtail on the back of it is nice to have…but not a necessity. Many people don't know that there was a 'filter' (looks like a condenser) installed in the factory wiring on most of the trucks for radio interference reasons, it was mounted on the firewall behind the distributor…but this is not a necessity for the tachometer to operate correctly. Speaking of distributors, it does not matter if you have and old point ignition or HEI, the tachometer will still work. For the most part, all the tachometers & gauges look alike, but there was a slight change in the gauge numbering starting in 1980, as well as the needle colors. Although all 73-87 (88-91) clusters look strikingly similar, there are many differences…

73-75: Amp gauge instead of volt gauge. This cluster will only work for these three years only and are NOT serviceable since they are completely sealed.  Just plugging one of these clusters in a 76 and up truck will result in instant meltdown upon turning the key.

1973 Tach Cluster (Full View)
1973 Tach Cluster (Close Up)
1973 Tach Cluster (Close Up)
1975 Tach Cluster (Full View)
1975 Tach Cluster (Close up)
1975 Tach Cluster (Close up)

76-77: Volt gauge, but mechanical oil pressure gauge. This cluster will only work for these two years unless a complete wiring change is made in the later model wiring connector. It is not just a simple matter of using the mechanical pressure gauge instead of the electric. If you are brave and think you can rewire it to be compatible and use the mechanical oil pressure gauge, then something to keep in mind here is to get at least a little piece of the factory oil line and fitting that was on the back of the gauge. I say this because the factory gauge has a unique type of ferrule and fitting on the back of the gauge that is good to save and reuse. I have made aftermarket lines/fittings that work but have also had them blow off the back and make one heck of a mess. The factory used a steel oil pressure line and I prefer that to plastic, or you can buy 1/8" copper.

1977 Tach Cluster (Full View)

1978 Tach Cluster (Full View)

The following tachometer pics are of a 1980 Canadian version, the speedometer is in KPH.
1980 Tach Cluster (Full View)
1980 Tach Cluster (Close up)

81-84: ‘78 and up tachometer cluster will work as a “plug in and go”, but you will not have use of your factory cruise control if the truck is an ’83 or newer (the green box behind the speedometer).

85-86: ’78 and up tachometer cluster will work as a “plug in and go”, as long as you have removed the factory feedback carburetor, ESC distributor, etc.   Most 85 models do not have the feedback carburetors, all 86 models do.  The feedback carburetors require feedback from the VSS behind the speedometer that you are not able to reuse.  (The VSS sends input to the ECM, a.k.a. the "computer", behind your glove box) 

87 EFI trucks, 87-91 Blazers/Suburbans: Cannot install tachometer cluster because of speed sensor (VSS) feedback needed for the factory EFI computer and ABS brakes (90-91).

The big things to get from all of this are…

1. AMP or VOLT gauge mix-ups could cause a fire.

2. Know the gauge cluster works otherwise it will cost a small fortune to fix it. If your tachometer needs to be repaired, it will usually set you back a minimum of $125, and on top of that, most repair shops do not carry the parts to fix them.

3. Is it an 8-cylinder or 6-cylinder tachometer? Tachometers were also offered in both 6-cylinder and 8-cylinder applications, be sure to get the correct one since a 6-cylinder unit will read slightly higher in an 8-cylinder powered truck  6-cylinder tachometers are pretty rare and are normally found in the large trucks.

4. Big truck (like C-60's and such) tach dashes can be made to work in a pickup, but offer many of there own unique problems because of optional gauges that were available in them (see above).  Also, all the years mentioned above do not follow suit on large trucks...your on your own there!

5. If you try to make a dash out of parts & pieces, you had better know that you need the CORRECT printed circuit for your year truck/dash also...even the plastic housing itself was different.

6. Before you go hunting, know this...75% of sellers on E-bay don't care about your concerns.  I Email them all the time hoping they will correct their ad and they don't, only 25% are grateful.  Most are advertised "untested" or "as is" for a good reason, and it's most likely because they are bad.

Tachometer Testing

Look closely at the back of the tachometer, you can see the markings as to where each lead needs to go (COIL, GND, 12V). Tachometers are very easy to hook up and test on an V-8 engine, especially if you have one around with an HEI distributor (HEI's have a 'TACH' terminal right on the distributor cap). If you have anything around with the old points distributor, then the terminal marked COIL on the tachometer goes to the (-) side of the coil. The 12V terminal of the tachometer and GND terminal are self explanatory, you need any 12V power source and a ground. If you do not have original wiring pigtail then a .250 female spade crimp-on connector on the end of a wire works fine. When tachometers start to go bad they read high, like 1200 RPM at idle, and get worse as the RPM’s increase. Having a known good tach in the vehicle to compare it with is a real plus. At idle you should see 500-800 RPM and a quick tap of the throttle should see 2500 RPM or so. As some go bad the needle responds very slow to RPM increases or it will “stick” as it moves forward. When they really get bad they will bury immediately or not register at all.

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