whats wrong with my factory tach?

[matt_field]

well i just bought a tach from a guy in town, i think it came out of a bigger truck, it has the vaccum and all that. well i got it hooked up great and i went for a ride yesterday and the thing reads high, ideling is good i think but doing 60mph it reads 4500, im thinking the truck is actually like 3200 or 3500, anyways could this be a 6 cyl tach? how do i know? and my ignition on my truck is a mallory unilite with a mallory coil could this be the problem if the tach is probably set up to be with points? well itd be great if someone could help thanks

Matt

[kwmech]

If it came out of a large truck there is a good chance that it was a GMC with a V6. Usually the rear of the tach will say 6 cyl in black ink.

[walker]

I am not sure if there is a difference in I6 and V8 tachs. If you have a portable, tune up tach, I would install it, rev it up and compare the two. Other way is to use an RPM calculator. Need to know tire size, rear gears and finaldrive ratio on the tranny. I have 4.11's and I think I turn 4500rpm at about 75mph.

[matt_field]

how would i tune that tach, i have a monster tach that is right im sure i could compare it to

[chipflyer]

I have the same problem with the tach in my 71, it reads way high. Is there a fix for this?

Jeff
71 C-10

[ericguy320]

my 72 is doing the same thing. I know it is idles at 650 rpm but the tach says 1500. I took it out and cleanned the connector on the back it had a little crud on it, that was good for about 150 rpm but it is still there.

[BurnoutNova]

on my camaro tach, i noticed the needle wasnt on zero when the key is on, and the motor is off. i turned the key on, and with the gauge lens out pulled the needle off and repositioned it on the zero. i was very careful when i did it so i didnt break the needle. thi may not be your probleom though


ALSO: it doesnt matter id it for points or hei, the rotor spins the same and the coil output is the same.

[GARMCo]

...those that are broke, and those that will be soon. I was having the same symptoms as everyone else in this post. I spent a couple hours carefully taking mine apart and cleaning it and trying to troubleshoot-- to no avail. These things just weren't intended to be worked on. I gave up and ordered a 5000 rpm generic tach from Summit (has the same 270deg. needle sweep as the OEM tach), and plan to install its guts behind my OEM face. I'll report back next week if anyone's interested in the outcome. -- Tommy/GARMCo

[ckhd]

I had the same problem with the tach on my '72 'burb. I hooked up a Sun tach along with the factory tach, and when it was really 3000 rpms, the factory tach ran at about 4500 rpms.

There is a screw on the back of the tach covered by a sticker. I adjusted it and it's good now.

I basically would rev it up to about 3000 rpms, adjust the factory tach, let off, rev it up again, and adjust a little, etc. It took several adjustments to get it consistant.

It's pretty awkward to get both the Sun tach and the factory bezel where I could adjust on it, and make sure that the grounds were good.

[GARMCo]

That's interesting-- my '73 tach had no such adjustment feature--wish it did. TM

[busterwivell]

When I finally got around to hooking up my tach, it was inconsistant. That was last year when I was blowing modules in the HEI. When I finally switched HEI's, the problem went away. It's consistant now, and using the formula, it's right on the money.

[mrein3]

I've been looking at that same tach in the Summit catalog to install in my bezel.

Please do report back.

Originally Posted by GARMCo

...those that are broke, and those that will be soon. I was having the same symptoms as everyone else in this post. I spent a couple hours carefully taking mine apart and cleaning it and trying to troubleshoot-- to no avail. These things just weren't intended to be worked on. I gave up and ordered a 5000 rpm generic tach from Summit (has the same 270deg. needle sweep as the OEM tach), and plan to install its guts behind my OEM face. I'll report back next week if anyone's interested in the outcome. -- Tommy/GARMCo

[67chevemall]

Use a tune up tack and compare!
If there isn't a switch to change from 4 - 6 -8 cyl. on the Tach
Then just use the other Tach for reference and make your own marks
Or make a new set of numbers!

Or buy a new one!

[matt_field]

thanks guys, im gonna hook up my other tach tonight and try to tune it !! seems like its pretty common then that they are off

matt

[GARMCo]

On my '73 K5-- I just completed the conversion of installing the guts from an Autometer model#2697 tachometer (0-5000rpm, 270 degr. needle sweep) behind my factory tach face. Must say it turned out great--finally I have a reliable tachometer again while retaining all the OEM cosmetics. The entire process took about 2-3 hours, and did involve some mild hurdles, but with patience anyone can do this retrofit. I took photos along the way, and can email the process & description to you if anyone is interested. Cheers--Tommy

[bluec10]

When you say a bigger truck, do you mean a 3/4 ton or a 1 ton, or are you referring to a heavy GM truck from that era?

I'd be interested in knowing if the heavy truck gauges work in 1/2 tons.

[mrein3]

Originally Posted by GARMCo

On my '73 K5-- I just completed the conversion of installing the guts from an Autometer model#2697 tachometer (0-5000rpm, 270 degr. needle sweep) behind my factory tach face. Must say it turned out great--finally I have a reliable tachometer again while retaining all the OEM cosmetics. The entire process took about 2-3 hours, and did involve some mild hurdles, but with patience anyone can do this retrofit. I took photos along the way, and can email the process & description to you if anyone is interested. Cheers--Tommy

Garmco,
The summit on-line catalog shows autometer tach 2697 to be a 2 1/16" tach. Is the the one you used? I assumed the factory tach hole would accept the guts from a 3 1/8" tach.
Please post or email the process and description.
Thanks,
Matt

[ckhd]

Originally Posted by bluec10

When you say a bigger truck, do you mean a 3/4 ton or a 1 ton, or are you referring to a heavy GM truck from that era?

I'd be interested in knowing if the heavy truck gauges work in 1/2 tons.

Yes, the large trucks (c-60, etc...). They have 8cyl tachs in them. Work great with your old bezel (the truck bezel is notched different than a pickup bezel).

One word of warning, use your old fuel gauge. The big truck fuel gauges have different warning lights in the fuel gauge (different wording).

[GARMCo]

I swapped in the guts from a (ordered through Summit, approx $100) Autometer brand 2 & 1/16" dia. model #2697 tachometer. Mated it to my original OEM tachometer face and OEM needle to maintain the original appearance. I chose the Autometer 2697 since it has the same 0-5000rpm range and 270 degree needle sweep as the OEM tach, and also the same wire connections. Also, it has a tiny red calibration screw on the back which allows you to easily make small adjustments in the indicator. Here's the process I used. It took about 3 hours. Start with the new tach--remove the lightbulb from the back and discard, then 1) On the new #2697 tach -use pliers to gently snap off the one-piece glass & bezel. 2) Right away you'll be confronted with the only real hard part of this job--getting the needle off of the tach without damaging it. The folks at Autometer either glued the needle to its shaft, or else it's just a tight fit, but mine was quite hard to remove. Thinking that there must be some glue involved--I used a hollow plastic Q-tip shaft to carefully apply some lacquer thinner to the union. After a few minutes of very careful prying, the needle finally came off (with pliers no less). (I spent the rest of the process worried that I had damaged the tach in the quest to remove the needle, but later found it was fine.) 3) Break off each end of the 2697's needle until you are left with just the tiny brass shank collar that was attached to the tach's needle shaft. The 2697's tiny needle shank collar has a slight hump on the back side. I used a fine power grinding wheel to flatten this out to make for a better fit with the OEM needle that you will be attaching it to. 4) On your OEM needle--bend the four retainer tabs to release the needle shank collar which actually is a small aluminum piece about half the size of a dime. This piece is a carrier for the actual shank collar, which is pressed in rivet-style at the factory. Carefully grind off the OEM needle shank collar until you have a flat carrier surface to affix the 2697's needle shank collar to. Using Elmer's glue -- mate the 2697's needle shank collar to the OEM needle shank carrier. Set aside and allow glue to dry. You now have your OEM needle mounted to the new tachometer's shank/collar. 5) On the 2697--remove the 2 tiny phillips screws (save these screws) and remove the faceplate. Remove the 2 nuts on the back of the tach case, and remove the white plastic tach cup housing also. You should now have just a circuit board with wires coming out the back, with the bare needle shaft surrounded by a white plastic mounting yoke coming out the front. 6) Disassemble your OEM tach until you have just the bare face plate. This may require carefully drilling out 2 factory rivets that hold the housing onto the back of the faceplate. BE CAREFUL not to scratch your OEM faceplate !! 7) Place your OEM faceplate down over the needle shaft of the 2697 tach's guts. Carefully measure how far apart your OEM rivet holes are. Using a 1/16" drill bit--drill holes in the 2697s white plastic mounting yoke to match the width of the OEM faceplate's rivet holes. In my case, they were only off about 1/8". I had to drill a hole at the extreme edge of each side of the 2697's plastic mounting yoke. After that, using the 2 tiny screws from the 2697's faceplate -- mount your OEM faceplate to the 2697's guts, by running the tiny screws thru the OEM faceplate's rivet holes, and into the holes you drilled in the 2697's white plastic mounting collar. 8) DON'T INSTALL YOUR NEEDLE YET. Instead, attach a tiny strip of paper to the needle shaft as a tempoirary needle (I used a thin slice of a decal). Give everything a test run prior to final installation. On my truck, the 2697's black wire (ground) was attached to the black wire in the OEM pigtail, the 2697's red wire was attached to the OEM's pink wire (power) and the 2697's green wire was attached to the OEM's brown wire (neg. side of coil). Using a tiny screwdriver--run your motor and calibrate the new tach using a dwell meter as cross reference. 9) When you are sure you have it callibrated, turn off your motor-- the temporary paper needle will stay right where it was when the truck was idling. Now carefully install your OEM needle in the same RPM position--being CAREFUL not to rotate the tachs needle shaft in the process. Press the OEM's needle gently onto the new tach's shaft. It will be a tight fit and the new installation will ride slightly higher off the fact that it did originally. But be careful not to press the needle on too hard or you may damage the tach. Now you'll be within a couple hundred rpm--fire up the motor and re-calibrate one more time. 10) Slide the white plastic tach cup housing back on, making sure to route the wires with care. Button it all up. Have I forgotten anything ? I hope not -- I have several pics I can email anyone who needs them-- good luck-- Tommy

[Slick67]

This may be a good one for FAQs section!!!!!!