Add Factory Tach or Not ?

[79 Silverado/454]

I have a pretty much survivor 79 short bed numbers matching big block truck. The truck is pretty much loaded up with A/C, PW, PL, tilt ,cruise, AM-FM radio, ralley wheels etc. The previous owner added the factory bed rails and aftermarket sliding rear window according to the SPID in the glove box.

I recently picked up another 79 gauge package with factory tach and I was thinking of changing out the current gauge package which has all the gauges with the clock to the one with the factory tach after I have it checked out and restored. I would be leaving my current speedometer in as that is the actual miles on the truck.

My question, do you think adding the tach will hurt the value of the truck or would it make it more appealing ?

I hate to give up the clock but, IMO the tach would be way cooler especially since the truck is a factory big block truck.

I would of course hold on to the original gauge package in case the next owner would want it.

[hatzie]

The Medium Duty tachs have the fuel gauge in the bottom of the tach. You could keep the clock with one of those.

[gmachinz]

The only deviation from its stock non-tach optioned package is that you will need to drill a hole in the firewall for the grommet. In all honesty, I think doing a clean, factory type tach cluster upgrade would increase the value and appeal of your truck. The 78-79 TACHS look cool with the yellow and orange redline imo-I put one in my father in laws 77 GMC and its looks killer!

To be correct, after drilling the firewall hole and popping the OEM grommet in place, spray the grommet down with undercoating to aid in sealing it. You did grab the factory tach harness too, right?

[79 Silverado/454]

Originally Posted by gmachinz

The only deviation from its stock non-tach optioned package is that you will need to drill a hole in the firewall for the grommet. In all honesty, I think doing a clean, factory type tach cluster upgrade would increase the value and appeal of your truck. The 78-79 TACHS look cool with the yellow and orange redline imo-I put one in my father in laws 77 GMC and its looks killer!

To be correct, after drilling the firewall hole and popping the OEM grommet in place, spray the grommet down with undercoating to aid in sealing it. You did grab the factory tach harness too, right?

Thanks for the tip on spraying the undercoating on the grommet to help seal it. It did come with the harness that goes to the fuse block & out to the distributor.

I did know about the gas gauge & tach being one on the medium duty trucks & I thought about going that way to keep the clock . I think that is a very neat way to do but, I would like for my truck to stay as close as possible to the way it could have been ordered from GM and my understanding is you couldn't get that on the light duty trucks.

Will I need to repin the main plug that plugs into the back of the cluster to feed the circuit board since that gas gauge is changing locations ?

[gmachinz]

No...the pinouts should be the same-only the printed circuit varies between tach and non-tach trucks.

[79 Silverado/454]

Originally Posted by gmachinz

No...the pinouts should be the same-only the printed circuit varies between tach and non-tach trucks.

OK, I just checked the harness for the tach. It looks like the grommet is missing. There looks to be a black wire that goes out to the distributor. A pink wire with a black stripe and a white wire that looks to pulled out of the plug that plugs into the fuse block

Should the white wire be connected to the plug that goes in the fuse block ?

The pink wire with the black stripe is connected to the plug that plugs into the fuse block.

I also would like to hear more opinions about if I should add the tach or not. It seems that we all think its a good idea so far.

[J Knight]

I think the tach cluster would be a nice addition to a well optioned truck. The key to it is that you have the "correct" 79 tach cluster that would have been offered for your 79 truck.

[nonstop]

It's your truck - do what you want with it. Just make sure the tach works before going through the trouble. You can also hold on to the factory gauges to put back in if it concerns you. Mine has the medium duty tach and a factory air pressure gauge where the clock was. I also have a factory vacuum gauge I was going to use, but the air pressure gauge was more useful to me.

[hatzie]

1979
White = TACH. Through the firewall to TACH on the HEI distributor
Pink/Black = IGN power. Plugs into the 2nd from the top IGN sockets in the fuse panel. They're just under the Hazard Flasher
Black = Ground. Plugs into the ground bar above the E-Brake pedal.

The fuel gauge wire may already loop over to both positions. Depends on the year. The 1979 wiring diagrams show it doing just that... It'll be a pink wire in one corner of the cluster plug with a short loop of pink wire around to 3rd position from the far end of the opposite side.

[gmachinz]

The white wire does go out to the tach BUT-on HEI trucks, that white wire goes out through the firewall grommet to one side of the tach filter, the other side of the filter is brown and goes to the HEI TACH terminal on the distributor. The white wire should also have a single female Packard 56 connector which snaps into the male connector from the tach filter. If you need a NEW tach harness w/filter PM me-I build reproduction tach harnesses which unlike all other aftermarket offerings, I include the tach filter with mine as well.

[79 Silverado/454]

Originally Posted by gmachinz

The white wire does go out to the tach BUT-on HEI trucks, that white wire goes out through the firewall grommet to one side of the tach filter, the other side of the filter is brown and goes to the HEI TACH terminal on the distributor. The white wire should also have a single female Packard 56 connector which snaps into the male connector from the tach filter. If you need a NEW tach harness w/filter PM me-I build reproduction tach harnesses which unlike all other aftermarket offerings, I include the tach filter with mine as well.

Thanks Hatzie for telling me where the wires go. That's a big help. gmachinz PM sent.

[Gregski]

I upgraded my 74 no Tach to a cluster from a 76 I think with Tach, here's what I learned

1. Used Tachs don't work, as a matter of fact all old gauges don't work, or work poorly, I would recommend a brand new one, or even a good quality aftermarket one if I was to do it again

2. I also wanted to retain my old speedo with the new Tach and guess again, can't or at least not with some doing, the non Tach Speedo is different believe it or not than the Speedo that comes with the Tach I think the front plate facia won't fit, you would need to do a swaparoo not sure it is worth it, I may be attempting it cause my newer used Speedo stopped working, see #1 above

[hatzie]

Originally Posted by Gregski

I upgraded my 74 no Tach to a cluster from a 76 I think with Tach, here's what I learned

1. Used Tachs don't work, as a matter of fact all old gauges don't work, or work poorly, I would recommend a brand new one, or even a good quality aftermarket one if I was to do it again

2. I also wanted to retain my old speedo with the new Tach and guess again, can't or at least not with some doing, the non Tach Speedo is different believe it or not than the Speedo that comes with the Tach I think the front plate facia won't fit, you would need to do a swaparoo not sure it is worth it, I may be attempting it cause my newer used Speedo stopped working, see #1 above

1 You can fix the old Tachs by repairing the circuit board. Fix Squarebody GM Truck Tach

Wilcox Corvette among others sell a replacement circuit board.
More discussion on this here --->67-72 Chevy truck forums. 7K 73-87 tachs

2 There are not with-tach and without-tach speedometers. You can move your speedometer to the tach cluster housing. I've done it. It fits fine.
The tach doesn't drop into the large fuel gauge cluster housing. You need a Tach cluster housing or extensive modifications to your non-tach gauge housing. The tach printed circuit is also quite different to accommodate the connection on the back of the housing. It also must accommodate the small fuel gauge connection in the lower LH part of the pod.
You can get into trouble with the warning lamp slot in the bottom of the tach and the speedometer. It's possible to have 0, 1, or 2 warning lamps in the bottom of both 5" gauges.

[ray_mcavoy]

Originally Posted by Gregski

I upgraded my 74 no Tach to a cluster from a 76 I think with Tach, here's what I learned

1. Used Tachs don't work, as a matter of fact all old gauges don't work, or work poorly, I would recommend a brand new one, or even a good quality aftermarket one if I was to do it again

From what I've encountered, the electronics in the 73-75 & 76-77 tachs aren't all that great. And the 73-75 circuits are potted / epoxy encapsulated, making the circuit boards non-repairable.

The electronics in the 78+ tachs are based around a National Semiconductor LM1819 air core meter driver and tend to be much more reliable. They do sometimes have issues with the calibration resistor that causes the tach to read too high or peg the needle. But that's an easy fix as hatzie pointed out --> Fix Squarebody GM Truck Tach

Originally Posted by Gregski

2. I also wanted to retain my old speedo with the new Tach and guess again, can't or at least not with some doing, the non Tach Speedo is different believe it or not than the Speedo that comes with the Tach I think the front plate facia won't fit, you would need to do a swaparoo not sure it is worth it, I may be attempting it cause my newer used Speedo stopped working, see #1 above

Originally Posted by hatzie

You can get into trouble is the warning lamp slot in the bottom of the tach and the speedometer. It's possible to have 0, 1, or 2 warning lamps in the bottom of both 5" gauges.

Yes, I believe that is where Gregski probably ran into an issue with his speedometer swap.

The speedometers in the "regular" 73 through early 76 clusters had a solid face plate without any provisions for a warning lamp. When the tach (or clock) option was added in those years, the "brake" warning light was re-located to the bottom of the speedometer. So those clusters used a speedometer with a narrow "window" or "slot" in the face plate to accommodate that light.

Sometime during the 76 model year, GM changed the clusters to include a wide "window" or "slot" at the bottom of the 5" gauges. With this configuration, the speedometers easily swap among the different cluster styles.

Originally Posted by Chaplain

I apologize in advance for the simi-highjacking of your thread. Any of this familiar?

Yup ... that appears to be a 73-74 or 75-77 or so tach harness. I could be more specific if I could see the end that attaches to the coil / HEI TACH terminal.

Those years used brown (instead of white) for the tach signal wire that passes through the firewall & connects to the coil. And a solid pink (instead of pink w/black stripe) wire for the IGN power connection to the fuse box. The one pictured has Packard/Delphi 56-series connectors that plug into the older style fuse boxes that take glass fuses. A Pack-Con connector was used on the newer tach harnesses that plug into the fuse boxes with the plastic blade style fuses.

[ray_mcavoy]

Originally Posted by Chaplain

More pics of the tach connector. Can an older harness be used with a 79 or later?

Yes, if you were to change out the connector on the pink wire that plugs into the fuse box you could use the older harness in your 79. As hatzie said, you'll need to obtain a #12004892 Pack-Con connector to fit the IGN port on your 79's fuse box.

Also, I'm still not seeing the far end of the brown wire in your photos. If it's a 75+ harness, no changes to the connector on the brown wire will be necessary. But if it's a 73-74 harness with a fork/spade terminal on the end, that will have to be changed to work with HEI.

Originally Posted by hatzie

I've seen narrow window 76 tachs with just a seatbelt light in the tach...

Yeah, I've come across a couple of those and think I might still have one in my spare parts collection. I believe the large fuel gauge used in the non-tach 76 clusters had a similar narrow window with just a seatbelt light as well.

[hatzie]

Originally Posted by ray_mcavoy

Yeah, I've come across a couple of those and think I might still have one in my spare parts collection. I believe the large fuel gauge used in the non-tach 76 clusters had a similar narrow window with just a seatbelt light as well.

Yup. Would've had to be there no matter what gauge was in the bucket.

[hatzie]

^^^^ Forgot about the early potted boards. ^^^^
The 73-75 tachs have a different font and no colors on the face... And the $&%* potted board. The usual suspects will replace the board and calibrate from 5K to 7K.
I've seen narrow window 76 tachs with just a seatbelt light in the tach...

The fairly rare late 1980 and later tachs have a different color needle, a light brown ring and, a larger font.

I do believe he has the wrong year tach harness too... Packard 56 instead of Pak-Con I is the thing that leaps out at me. Most of the Pak-Con I fuse panel connectors for the accessory power ports on the ATC fuse panels are obsolete. #12004892 can still be had for this harness... #12004888 directly above this one is obsolete. The clock power connector #12004886 is obsolete as well so don't loose it.

American Autowire has the correct harness with the right connectors but there's no filter capacitor included. They may be doing what I have contemplated for some of the obsolete connectors in their arsenal. 3-D laser resin printer. They are a fairly large business or I bet they had a run of some connectors made. They make many of the, extremely expensive to make, molded rubber connectors for their harnesses like the 67-83 taillight plugs.

The 73-78 harness could be used but it won't plug into the IGN port on the 79 fuse panel or the ground buss bar.

[motornut]

I found a tach cluster when i did my idiot to gauges swap in my 78.
if your servicing the cluster,consider painting the inside.
I even added extra LEDS,kept the stock white.
I would do it (Tach) to my 79, but for now it's painted with green bulbs.
Tach is on the wish list lol.
I ran a second tach a while to confirm the stock was accurate,it was.
I use a trailer plug for non stock tach,but got the stock one wired into the cluster.

[motornut]

Mine installed
and here is the later style I saw,wish I'd got it all....

[N2TRUX]

Originally Posted by motornut

I found a tach cluster ....
I even added extra LEDS,kept the stock white..

,More details on the LEDs please. I am looking for part #s to do an LED update on my 84 Sierra.

[79 Silverado/454]

Can someone post a pic of where the harness comes through the firewall and also show the routing of the wire going to the distributor ?