Mystery Tachometer

[ButchM]

Hi everyone, after a ridiculous amount of online research and coming up with absolutely nothing I decided to join a forum and hope that someone else has seen this gauge before. So I'm in the process of "restoring" my 1986 Chevy K10 Scottsdale, after 8 long years I've finally found the time to install the engine I rebuilt 5 years ago and the transmission I rebuilt 2 years ago. The truck now runs and drives and to my surprise all of the factory gauges actually work, they just needed to be hooked up to sending units. I've got the voltmeter, temp, oil pressure, clock, and fuel gauge working and I have my eye on the factory tach now.

This tachometer has the fuel gauge built into it, I can't find anything about this gauge anywhere, it's like it doesn't exist. Every gauge cluster I've ever seen either has the large fuel gauge next to the speedometer or they have a single tachometer and a small fuel gauge where the clock is. I haven't actually pulled the cluster yet to see how the tach is wired, for now I'm just super curious as to where this gauge came from.

Also, this truck has the body of a 1986 C10 and the frame/drivetrain of a 1975 K10 but these gauges look like the 1980s style to me.

(sorry for the crappy pictures, it's a pretty sunny day)

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Pete
You have a hybrid/Frankendash. The clock is an option (don't have the option code) for "regular" pickups (C/K10-30). The tach you have is from a C50/C60 series medium duty truck with a 366 or 427 engine with air brakes. On those trucks there was an air pressure gauge where your clock is. The 5000 rpm top end is a clue, those engines had a much lower redline due to the heavier piston weight.
I have one of those tachs I plan to install in my truck.
As a side note: I was told by an old Chevy mechanic that the trucks that came from the factory with an air compressor (all air brake trucks) had forged steel crankshafts. May be a good source for that forged 427 crank at a cheap price.
Butch

 

[frickenbored]

Pete
You have a hybrid/Frankendash. The clock is an option (don't have the option code) for "regular" pickups (C/K10-30). The tach you have is from a C50/C60 series medium duty truck with a 366 or 427 engine with air brakes. On those trucks there was an air pressure gauge where your clock is. The 5000 rpm top end is a clue, those engines had a much lower redline due to the heavier piston weight.
I have one of those tachs I plan to install in my truck.
As a side note: I was told by an old Chevy mechanic that the trucks that came from the factory with an air compressor (all air brake trucks) had forged steel crankshafts. May be a good source for that forged 427 crank at a cheap price.
Butch

Yeah, this doesn't surprise me. The whole truck is Frankensteind together by the previous owner, now I'm doing my best to un-Frankenstein it. Hopefully I can pull the cluster this weekend and find out how that tach hooks up; however, I'm doubting it will play nice with my MSD 6AL box, but I should at least see it do something. I plan to ditch the MSD ignition soon in favor of a Progression Ignition digitally programmable HEI distributor anyway.

 

[Redfish]

I got mine from LMC with a new pod and printed circuit. All works great. I had to cut a bit of the speedo hole with a unibit.

This conversion is on The List. I just hate to give up my awesome Quartz Clock! I remember GM advertising being really proud of that Quartz Clock.

 

[bucket]

Yeah, this doesn't surprise me. The whole truck is Frankensteind together by the previous owner, now I'm doing my best to un-Frankenstein it. Hopefully I can pull the cluster this weekend and find out how that tach hooks up; however, I'm doubting it will play nice with my MSD 6AL box, but I should at least see it do something. I plan to ditch the MSD ignition soon in favor of a Progression Ignition digitally programmable HEI distributor anyway.

If the tach still works, it should work with the 6AL just fine. The tach should have 3 terminals. One ignition 12+, one ground and one tach signal. The 6AL should have a tach output.

 

[Keith Seymore]

I got mine from LMC with a new pod and printed circuit. All works great. I had to cut a bit of the speedo hole with a unibit.

This is basically what my '87 came with (no trip odo).

Clock was in the radio.

K

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[Keith Seymore]

The clock is an option (don't have the option code) for "regular" pickups (C/K10-30).

Clock is RPO U35.

K

 

[nvrenuf]

If the tach still works, it should work with the 6AL just fine. The tach should have 3 terminals. One ignition 12+, one ground and one tach signal. The 6AL should have a tach output.

This ^^

I ran a factory tach with a 6AL in my old K10 for years, worked great.

 

[frickenbored]

If the tach still works, it should work with the 6AL just fine. The tach should have 3 terminals. One ignition 12+, one ground and one tach signal. The 6AL should have a tach output.

This is good to hear. My 6AL does have a tach output. Back in the day I used to have a 1988 K5 blazer with a MSD street fire box and a summit racing brand tach, the tach signal was super lazy and inaccurate, I also tried a sunpro tach but had the same problem. I figured I needed the MSD tach signal adapter box.

 

[nvrenuf]

Just to say it, while the factory tachs are super cool I remember mine being slower than aftermarket tachs.

 

[Beercelo]

I got mine from LMC with a new pod and printed circuit. All works great. I had to cut a bit of the speedo hole with a unibit.

I got mine from LMC with a new pod and printed circuit. All works great. I had to cut a bit of the speedo hole with a unibit.

Anyone know which kit from LMC will fit on an 82? They only have kits for 78-80 and 83-87. Talked to 2 people from Lmc on the phone both said none of them fit the 81 or 82, period.

I got mine from LMC with a new pod and printed circuit. All works great. I had to cut a bit of the speedo hole with a unibit.

 

[frickenbored]

Got my cluster out tonight, that gauge is definitely a custom job as you can tell by the pictures but it's actually not too bad of a job. I hooked it up to my power supply through the circuit board and the tach snaps to zero. I'm not sure why it didn't work in the truck since the power and ground were hooked up, there must have been a bad connection somewhere but it's working now.

I've got some repairs to do on the circuit board but hopefully I'll soon post a picture of everything actually working. Eventually I'm going to install of of those Autometer GPS speedometers.

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[Camar068]

^^^

On the medium duty truck, the last gauge hole would have an air gauge for the brakes, or a vacuum gauge for the booster on juice brakes.

Thats one of the gauges I want for a quick look at engine vacuum.

 

[Turbo4whl]

More info, Some C50 etc. trucks have a taller fuel tank. If the medium duty small fuel gauge with the tach does not seem to read your fuel level correctly, replace the resistor on the back with the resistor from the original pickup gauge.

Also a blazer or suburban fuel gauge may also have a different resistor to match that tank. Most resistors are white with a color line. Different color, different value. The resister unbolts, can swap big to little gauge resistors, they are the same size.

 

[bucket]

More info, Some C50 etc. trucks have a taller fuel tank. If the medium duty small fuel gauge with the tach does not seem to read your fuel level correctly, replace the resistor on the back with the resistor from the original pickup gauge.

Also a blazer or suburban fuel gauge may also have a different resistor to match that tank. Most resistors are white with a color line. Different color, different value. The resister unbolts, can swap big to little gauge resistors, they are the same size.

I thought all the GM gauges were 0-90 ohm? Different height tanks used a sender with more or less swing in the float arm.

 

[Turbo4whl]

I thought all the GM gauges were 0-90 ohm? Different height tanks used a sender with more or less swing in the float arm.

There are different resistors on the back of the fuel gauges. That is why they bolt on. Old school tech. Squares are most all the same. MD trucks can be different.