goatcrazy

Hi, I'm trying to get my original factory style tach (new reproduction) working with the tach output signal from my PCM. This is a '69 LeMans with a 2004 LQ9 powertrain.

I'm picking up the signal direct from the PCM and I've tried it two ways:

1) Pull up resistor (560 ohms) to +12v direct to tachometer

2) IC circuit (from one of the posts)

I've got two things going on
A) The tach is inaccurate (roughly 1/2 actual engine RPM) and
B) Above 1800 Tach RPM it "cuts out" and goes to zero.

A) is most likely because the tach output from the PCM is a 4 cyl signal so that can be re-programmed.

But I'm struggling with B)...Do I need to try a different resistance for the pull-up circuit? Is this just not compatible with a factory tach? What could I be missing it seems that most people have no issues once they go with the pull-up circuit..I've never seen a tach "cut out" before??What would cause this?

Any help would be appreciated!!

zipster

iTrader: (5)

I am throwing this out there because I heard about it. Set the tach output to "2X" I hope someone corrects me on this.

zipster

iTrader: (5)

I did a little searching,

Tach output in HPtuners for an autometer tach in 8cyl mode needs to be 3/3

I hope this is correct, I need to do it to.

goatcrazy

Do you think that the reason why it's cutting out is related to the 4 cylinder output? I was thinking that it may be a different issue?

gofastwclass

iTrader: (2)

I'm running my Thunderbird with a factory tach at the 4cyl output through a signal converter (installed before I had HPT) so that isn't the issue. I'm assuming you have good solid connections? Are you able to scan the PCM and see what the PCM is reporting when your tach drops? Does the engine cut out or anything at that point? Hopefully you simply have a loose connection on your tach that hits the proper harmonic at that point.

Yes, HP Tuners (and I believe EFI Live) has an adjustment for tach output called "Tach Output Resolution" which has a high and low setting. On 24x engines 6 / 6 would be a 4 cyl factory output while 3 / 3 should be 8 cyl but I don't have an 8 cyl tach laying about to try it with.

There is a writeup on LT1swap.com about the pull up resistor, I've never had to do that but some have problems with it.

goatcrazy

[QUOTE=gofastwclass;14433018]I'm running my Thunderbird with a factory tach at the 4cyl output through a signal converter (installed before I had HPT) so that isn't the issue. I'm assuming you have good solid connections? Are you able to scan the PCM and see what the PCM is reporting when your tach drops? Does the engine cut out or anything at that point? Hopefully you simply have a loose connection on your tach that hits the proper harmonic at that point.


Yes, I'm able to connect to the PCM and the tach cuts out at 3800 engine RPM, but the engine is still going it's not being commanded off.

I was thinking about running a direct connection to the PCM just to eliminate wiring as an issue. I wasn't originally leaning in that direction because the speedo signal works fine and theyr'e both wired in a similar manner...But I suppose it's worth a try..

G-Body

iTrader: (4)

Yep, 3/3 is the resolution you need with an 8cyl tach.

goatcrazy

I've done some more experimenting with grounding by taking a ground wire directly from the tach to the chassis ground and saw no improvements.

Then I changed the pull-up resistor from 588 ohms to 688 and the "cut-off point" changed from 2000 rpm to 1800 rpm [tach running at half speed]

Then I lowered the resistance to 488 ohms and the tach cut off at 2300 RPM. I then took it to 388 ohms and it went to 2600 RPM (did not cut out but I stopped revving the engine as I was close to 5200).

I think that my next step will be to purchase HP tuners and change the tach output to 8 cyl and see what happens. Hopefully I'm going in the right direction...

gofastwclass

iTrader: (2)

It sounds like you are.

If you're going to buy HPT, get the pro. The additional $150 spent on the initial purchase is worth it because you can hook up a wide band (or other external sensor) at a later date and run logs using only the interface without your laptop (which I use a lot). If you go standard the upgrade cost for pro is $289 + shipping which makes buying the big dog from the start a better idea in my opinion.

Another forum member talked me into going pro and I'm glad I spent the extra $ up front. Knowing what I know now I would have regretted my initial choice and spent double the difference purchasing the pro upgrade later.

Link to HP Tuners site explaining the differences.