Fixed Tach... YES!!!

CHarris · 07-18-2006, 11:42 PM

Thanks to the forum and Parish8 in particular!

I gave up on my factory 1991 vette cluster(frack) every working right and jumped on some autometer gauges. Got the tach all mounted and set up and it didn't work. No needle bounce no nothing. I was using the tach output on my 2004 GTO pcm to feed the signal but it just didn't work.

I saw some posts on the forum here talking about a pull-up circuit and how that would make it work fine. But I didn't really understand for sure how to do it. I PM'ed Parish and got the scoop. I ran a wire from a switched 12v source to a 680 ohm resistor and jumpered into my tach output wire between my pcm and my tach. Keyed the ingnition and the tach reads just like it should have all along. NICE!

Thanks, Parish! I would have been pulling my gauges out and really pissed had I needed to send it in. Instead I just used the forum and spent $5 with good ol radio shack

67pete · 07-19-2006, 04:28 AM

So this was just for the GTC PCM?

67pete · 07-19-2006, 04:28 AM

So this was just for the GTO PCM?

John B · 07-19-2006, 10:38 PM

The LS2 and LS7 PCM's require 12V with this resistor for the tach to work properly.

CHarris · 07-20-2006, 12:12 AM

I don't know what all it works on...

Parish did his on a truck with a couple of things that were running off the tach signal that were dropping the signal too low.

I did mine on a 2004 GTO pcm with nothing but a tach running on it.

John B (above) says the LS2's and LS7's need it too.

Anyone else got anything to add? I've no idea how many platforms this is needed on.

mzrico · 08-03-2007, 07:56 PM

Thank you. I had the same problem with my Ultra Lite Tach and my 2005 GTO LS2 set up. Did what you did and boy PROBLEM solve. This site and guys like you make life so much better.

89 formula350 · 08-03-2007, 11:27 PM

seems hit or miss on the ls1 pcm too. some 3rd gen f-bodys clusters work, some just work part of the time others do nothing until you add the resistor. it all on how sensitive the tach is.

s10mods · 09-09-2007, 03:18 PM

Quote:

Originally Posted by CHarris

Thanks to the forum and Parish8 in particular!

I gave up on my factory 1991 vette cluster(frack) every working right and jumped on some autometer gauges. Got the tach all mounted and set up and it didn't work. No needle bounce no nothing. I was using the tach output on my 2004 GTO pcm to feed the signal but it just didn't work.

I saw some posts on the forum here talking about a pull-up circuit and how that would make it work fine. But I didn't really understand for sure how to do it. I PM'ed Parish and got the scoop. I ran a wire from a switched 12v source to a 680 ohm resistor and jumpered into my tach output wire between my pcm and my tach. Keyed the ingnition and the tach reads just like it should have all along. NICE!

Thanks, Parish! I would have been pulling my gauges out and really pissed had I needed to send it in. Instead I just used the forum and spent $5 with good ol radio shack

anyone have a small diagram on this? do I run 12v to the resistor then to tie the other end of the resistor to the tach siginal?

G-Body · 09-09-2007, 10:30 PM

Yep, hook one end of the resistor to a wire that is IGN hot (+12v in the start and run positions)

Then "T" the other end of the resistor into the wire that goes from pin 10 on the red PCM connector to your tach.

Mike96z · 09-10-2007, 02:28 PM

how would i make the tach work on my 96 camaro

s10mods · 09-16-2007, 10:10 AM

Quote:

Originally Posted by G-Body

Yep, hook one end of the resistor to a wire that is IGN hot (+12v in the start and run positions)

Then "T" the other end of the resistor into the wire that goes from pin 10 on the red PCM connector to your tach.

Thanks

ls1_chevelle · 09-16-2007, 05:09 PM

Wonder if this would work on my 03 truck ecm, because I'm not able to get a tach signal from it either...

indiana warmrod · 09-17-2007, 08:28 PM

I did the trick GBody outlined with my 05 truck ECM and it worked like a charm.

V8 Supra Builder · 09-17-2007, 08:48 PM

I had posted here several months ago after I got my V8 MK3 Supra running that one issue I could not solve was the tach. It only worked when the car was shifted into reverse.

Here is my setup:
Engine: 2003 LM7 5.3L
Harness: 1998 F-body six speed
PCM: 1998 F-body, reflashed by HP Tuners
Tach adapter: Baker Electronix module (diagnostic test OK when disconnected, fails when connected to PCM)
Tach: Stock 1988 Toyota Supra
Trans: TKO600 manual

I figured this had something to do with the auto to manual swap I did but everything else worked, and the tach is getting a power feed while the engine is running. It works accurately when shifted into reverse, so a weak signal may well explain the problem. I was going to look into putting a resistor in the power feed to it to duplicate the b/u lamp load.

When I used the same stuff when I built the RX7 (exceptions- tach was a Racepak panel, harness was a converted 1998 A4, and motor was an L33), the tach works with no problems at all.

I will try the above and see if this helps with the Supra tach problem.

Thanks for posting this, yet another reason why this is a great forum.

EDIT-
I drew a diagram to simplify this. I will be giving it a try on my car & will update with results.

EDIT #2
Installed it today & the tach works great. Big thumbs up to the 99 cent solution to a long time bug.

Low91 · 02-13-2009, 01:29 AM

Hey everyone I'm new to this site, I wish I had found it when I started my swap

but now its near finished. I swapped an 05 l33/4l60e into a 91 silverado. I 've been driving it for a few weeks now but am still trying to get some kinks worked out. The main one being the tach. I'm using a cluster from a 98 silverado and I'm wondering if wiring in a resistor will fix my problem. So far everyone seems to be using an aftermarket tach set to 4 cylinder mode.Will wiring in a resistor work with my factory v8 tach? I have an autometer tach adaptor that just arrived today but I'd rather not use it if I can get away with just a resistor. Thanks

cam · 02-13-2009, 01:28 PM

The resistor 12v+ is only to boost the signal strength enough to excite the tach and make it work. The pulse frequency that comes off LS engines is 4cyl so the resistor trick does not affect this in any way whatsoever. In short you still need to either use a tach that can be set to accept 4cyl pulse or use a box such as you describe to change the tach out frequency from 4cyl to 8cyl or your stock tach will not read properly.

Stu Cool · 02-13-2009, 01:46 PM

You can re-program the PCM to give an 8 cylinder output for the tach. Speartech did mine that way.

Pat

Low91 · 02-13-2009, 02:13 PM

Thanks guys, I guess I'll just go ahead and use the tach adapter.
BTW if I were to have the signal changed in my PCM would I still need to add a resistor to boost the signal?

69 Ghost · 02-13-2009, 08:01 PM

I believe the S&P site has a little article on this also.

ad53030 · 02-27-2009, 05:25 PM

Ok... so now I'm having the tach not working issue. Autometer adjustable tach w/ 07 5.3 in 84 C10. Set to four cylinder mode and needle jumps when cranking during start but that's it. So am I reading this right that the resistor is so that you can reuse a factory type tach? I thought setting the tach to 4cyl mode would allow it to work if its an aftermarket tach. If I do still need the resistor w/ my Autometer Tach, I assume I still need to set it to 4 cyl mode. This correct?