Jake Harsha

Hey all, I've read all the threads about creating a pull-up circuit to get my tachometer working with the LQ4 in my Jeep, but when i check the output with the engine running, with my digital voltmeter I see next to NO voltage (like .02 volts or so).

I read that, since this is a 9 (or so) volt square wave signal, I should be seeing around 4.5 with my voltmeter, but like I said...I'm only seeing .02 or so.

I have no reason to suspect a problem with the PCM as the tune was done by SpareECM and they do this sort of thing all the time, but I suppose it still could be... (Can you even shut the engine speed output off in the tune?) The Tune in question is just a basic 2007 Silverado 2500/MW3 (NV4500 5-speed) with the VATS, EVAP, secondary O2 sensors and Torque management turned off. The engine seems to run perfectly.

Could this be a problem with my digital voltmeter? It's nothing fancy--just a cheap Radio Shack one.

Thanks,
Jake

Pop N Wood

iTrader: (1)

Yeah, a cheap meter could cause issues.

See if you can borrow a proper tach. Maybe your local parts store has a loaner.

Jake Harsha

I have a tach but I get nothing on that either. It's a VDO Cockpit Series. I live up in Northern MN so I don't know where I'd ever borrow a tach that would be compatible with the unmodified signal.

I'm assuming that even if everything was working correctly I'd need a pull-up circuit to get it to register on my tach because it needs a 12V input. Perhaps I'll just try that and see if I get anything. From what I've read on here, you just connect a key-switched power source/680 Ohm resistor to the tach wire...Right?


Thanks,
Jake

Jake Harsha

To anyone else having the same problem that finds this thread... It was the Multimeter. I hooked the pull-up circuit (680-ohm Resistor to B+) and my VDO Cockpit Series tach (set to 4-banger mode) worked like a charm!

Hope this helps someone else,
Jake

rotor vs. piston

iTrader: (3)

I hope this helps, I didn't find this thread when I was in the garage, but will try this tomorrow.

I have the same tach, and problem.

rotor vs. piston

iTrader: (3)

Okay, I'm about to go get the resistor, re-read this and want to be sure I understand. I need to put the resistor in series w/ the 12v power to the tach, or put an additional 12v source w/ resistor in parallel w/ the tach wire coming from the PCM?

96gt4.6

iTrader: (5)

Take the resistor and put it between the tach's 12v source and the tach pickup wire, or if you want you can run an additional 12v feed wire, but i've always found it's easier just to tap into the tach's 12v feed as you are already there.

Here's an article to guide you, scroll down till you see the pull-up resistor part, ignore the diagram labeling as this isn't for a tach hookup, but will help illustrate what needs to be done:

http://www.msextra.com/doc/ms3/ignition.html

rotor vs. piston

iTrader: (3)

My worry is this, what's stopping the voltage from back feeding into the PCM and killing it?

96gt4.6

iTrader: (5)

Unfortunately I can't answer that one. I just know that when I started doing swaps years ago, I needed a tach and this is how I was told also.

I have used this technique multiple times, with no PCM failures to date. The oldest swap I have running around is around 3 years, daily driven.

That instruction link I provided was straight from Megasquirt. Surely if this causes problems with the PCM they would have known by now.

rotor vs. piston

iTrader: (3)

Thanks for the help, I got some more resistors today. I should have time to do this tomorrow and just wanted another opinion.

Thanks again.

rotor vs. piston

iTrader: (3)

Just wanted to report back. I wired in two 680 ohm resistors in series for my pull up circuit, works perfectly. I admit, I said a prayer, "please don't fry my PCM", and turned the key and all worked great.

Thanks for this thread and the help 96gt'.

supershift67

iTrader: (1)

What pin number are you hooked up to. I also have a lq4 and need a tach hooked up.thanks

rotor vs. piston

iTrader: (3)

I'm running a GTO LS1, I think the tach out put is the same pin though, #10 on the C2 connector.

coltboostin

Bumping an old thread....

I have a VDO tach I brought back to life with a 1k ohm resistor. The issue is the tach reads low (abotu 750rpm low @6k), and is slower to react than my Summit gauge I have as a back up (Summit is too large to fit the space though) I have seen a few threads where people with VDO tachs are experiencing the same issue.

SO my question-

1) Would wiring MORE resistors to bump the signal help more? Would it have potential to damage the ECU?

2) Has anyone with a VDO tach gotten them to work 100%?

3) Do people seem to truly have ZERO issues with the Autometer or Autogauge units? I dont know who made the Summit brand unit I have, but it reacts fast. and is dead nuts on at redline.

Thanks!