79 C-10 Tach Questions

[79 Silverado/454]

Is there a difference in the tach for big block engine trucks verses small block engine trucks ?

Also I see some of the tachs have the gas gauge built into them. Was this an option for the pickups or was this more for the heavy duty trucks ?

My truck has the full gauge pkg, with clock and dual gas tanks. If I were to want to add the tach to my truck is it difficult to do and everything still work ?

[rich weyand]

No difference between SBC and BBC for tach.

The stock tach w/fuel gauge was on medium-duty trucks. The tach was set up for 4000 rpm red-line as I recall.

I went this route. http://67-72chevytrucks.com/vboard/s...d.php?t=629131

[Chevy1966]

I need to say that the link to Rich's earlier post concerning the gauge clusters is about the best, most detailed explanation of digging into a custom gauge modification I have seen. You need to read it. However I should mention that the combo tach/fuel gauge did come in the 5000 RPM version as the attached picture shows. --Rob

Note to Rich: for a long time I wanted a Stinson Station Wagon, but ended up with a Bellanca Cruisemaster instead. The Bellanca, with three fuel tanks also had fuel gauge challenges, which can result in an adrenalin rush if you are not careful and which I found out about firsthand. I understand your reason for wanting gauges for all tanks!

[johnwaynejeep]

I agree that Rich's post is very useful in making this change. If you want I can give you the pin outs for the 1980 dash cluster pin. It should help if you need to move any pins around, also the tach did not go through the cluster harness, it goes through an independent 3-wire harness. You have one line that plugs into an accessory power position on the fuse panel, a ground and a line through the firewall to the distributor.

there was some kind of resistor in line with the distributor link. Do a search for wiring diagrams and you should be able to find them and see what this resistor was and if you need it.

[79 Silverado/454]

Thanks for the replies guys. I think the dual fuel gauges is cool. I will definetly use your write up if I decide to do this. I was thinking of doing the tach with the built in fuel gauge so I could keep the clock for a more factory "full gauge pkg." type look.

Robert : Where did you get the 5000 RPM tach with the built in fuel gauge ? With it being a 5000 RPM tach is it from a light duty truck or ?

[Chevy1966]

To my knowledge the tach/fuel combo gauge was used only in the medium duty line, possibly in the heavy duty trucks, but definitely not available in the light duty models from the factory. I think the 4000 RPM tach was used for diesels and the 5000 RPM version was for gas. I found the one I have on eBay and paid the price for it. I have seen others advertised on CL for very reasonable prices, but you need to be quick to catch these deals. As Rich pointed out there is someone out there making and selling the 7000 RPM version but there is just something wrong placing a gauge on my dash that would even suggest you could spin up the old 350 that high. You might want to contact Tom, TBONE1964, and see if he can help you find one. He is in the gauge restoration business and is building up his inventory to do more with the squares.

[J Knight]

GM sports salvage is also making the older font to match the 73-79 gauges. I have one in my 82 with the newer font, super easy install.

[rich weyand]

Huh. I don't see the older font on their web page. I went with the newer font in my '78.

I kind of like the bigger numbers. Not sure which way I would have gone had I had a choice, or knew I had a choice.

If someone has a definite preference for the older font, I would ask them about it when you call.

[hatzie]

Originally Posted by 79 Silverado/454

Is there a difference in the tach for big block engine trucks verses small block engine trucks ?

Also I see some of the tachs have the gas gauge built into them. Was this an option for the pickups or was this more for the heavy duty trucks ?

My truck has the full gauge pkg, with clock and dual gas tanks. If I were to want to add the tach to my truck is it difficult to do and everything still work ?

Gasoline engine tachometers read pulses per rev so they are cylinder specific displacement doesn't matter.

The GM truck factory tachs I've worked on are National Semiconductor LM181 based units.

You can test any tach with a DC power supply, a frequency generator like the HP209A or HP205C Audio Oscillators, and a frequency counter for accuracy... I use my TEK 2236A Scope.

RPM = [frequency(Hertz) X 60(seconds)] ÷ [(Number of Cylinders) ÷ 2]

If your tach is out of an 8 cylinder;
100Hz = 1,500 RPM
200Hz = 3,000 RPM
300Hz = 4,500 RPM

If your tach is out of a 6 cylinder;
100Hz = 2,000 RPM
200Hz = 4,000 RPM
300Hz = 6,000 RPM

Diesels may require a bit more work as they typically read an output from one pole of the alternator.

I'd love to show both tanks on the dash at all times.

One option for showing both tanks all the time would be one of the big truck combo tachs and the Light truck small fuel gauge in the lower left corner instead of a clock.

This is my ideal diesel tach... would have 2 fuel gauges in the bottom half and glow indicator... Chopped photo with GIMP no such thing exists.

[79 Silverado/454]

Thanks for the leads on TBONE1964 and GM Sports Salvage. I would prefer the older font for my truck in a 5000 RPM version. I guess I would have to move the fasten seat belt and brake warning indicator lights over to my speedometer in place of the choke and 4 wheel drive indicator lights. My truck isn't 4X4 so I don't really need that anyway.

I'm a little nervous about cutting out the housing for where the fuel gauge would be in the tach. GM Sports Salvage says you can use your existing harness for the fuel gauge. That would be nice... I guess you would have to buy the extra harness that runs from the tach to your distributor thru the firewall.

Would you have to change the circuit board on the back of the housing ?

[rich weyand]

No. You need to wire the indicator lamps over to the other positions (just the two hots -- the indicator bulbs all have common ground), and run one wire through the firewall to the tach. That wire comes with the unit. The tach picks up its hot and ground from the fuel gauge connections.

I think the GM Sports Salvage units are all 7000 rpm, though, although with the correct redline.