LS 376 Tacho

Has any one pulled the tach signal out of an Chevy LS engine? The manual says to combine the Engine speed with Ignition voltage. They recommend a 5000 ohm pull up resistor at the igniton wire before merging, it says this is a 15A signal. It also say the engine speed (TACH out) signal is low voltage square wave.

Question how many watt resistopr do i need? I don't think i have all the infor and have called 2 chevy service departments. Has anyone doen this so i don't have to re-create the wheel?
 

Terry Oxandale

Skinny Man
No expertise here, but a "low voltage" signal could be anything between 5 to 12 VDC. So at "15A" (which I assume is 15 amps, which seems extreme for a "signal", I'd figure on a 180 watt resistor, which again is extreme, and huge. Something is not right on the amp assumption for the signal or my interpretation. My guess is you'd be dealing in milliamps, so the wattage would be very small, and easily available. You'll know if you're wrong with an easy feel for heat.
 

Seymour Snerd

Lifetime Supporter
Has any one pulled the tach signal out of an Chevy LS engine? The manual says to combine the Engine speed with Ignition voltage. They recommend a 5000 ohm pull up resistor at the igniton wire before merging, it says this is a 15A signal. It also say the engine speed (TACH out) signal is low voltage square wave.

Question how many watt resistopr do i need? I don't think i have all the infor and have called 2 chevy service departments. Has anyone doen this so i don't have to re-create the wheel?

I'm not familiar with LS engines but am an EE. I found a thread
http://www.ls1tech.com/forums/conve...onnect-your-autometer-tach-working-ls1-2.html
that corroborates what you are saying in general. What's probably happening is that the as-is "engine speed" signal in its high state is not a high enough voltage for the tach, so people fix that by providing a "pull-up" resistor to pull up the high side of the square wave. The thread seems to revolve around resistors in the ~500 ohm range, but you won't hurt anything by starting with 5,000 ohm. It just might not supply enough current to drive your tach (or whatever). So my suggestion would be start with a 1/4 or 1/2 watt 5,000 (a more common size in that range is 4,700) and if that doesn't work drop to a 680 or 470.

The statement that the igntion wire is a 15A signal almost certainly means that it is fused at 15A. This would have no bearing on what you are trying to do.

As for the particulars of the power rating for the resistor: if you put in a 5,000 ohm resistor the power dissipated will be v-squared (12-squared = 144 nominally, but it's really 14-squared = 196 ) over 5,000 over 2 (because it's a square wave), or about 0.02 watts. If you use a 470 ohm resistor it will be about 0.2 watts. In that case I would suggest a half-watt resistor just to be safe. Half watt resistors are also physically bigger and so possibly easier to deal with regardless of resistance value.
 
Last edited:

Craig Gillingham

Banned because I can't follow the forum rules.
from memory the digidash requires a standard 5V Sq wave pulse input and the st GM has a 2v..3V ?? pulse so indeed a pull up resistor is required - cant remember what size though. The digidash has a tolerance band tough i am sure 5v-12v??

I now have a piggy back controller over the GM ECU "plug and play" as Efi live isnt a live system, each change takes up to 30secs and its constant state to return to original hard settings is a pain. The piggy back now holds a tune and is a live system so was easy to on road map.
I managed to achieve complancy on my Ls376 which the Uk distributor dropped its euro4 tag that it was originally promoted as.
Along side dropping the digidash, Race logic traction control was added and variable speed eps which also added which also requires a 5v sq wave tacho input and 4 wheel speed sensors.
 
Normally, on the GTM builds, using the Autometer tachs, I use a 4.7k or 5k ohm resistor. I do this right at the tach. When I crimp on my terminal ends for the 12v "ign" wire and "signal" wire for the tach, I crimp the ends of the resistor in with the wires.
 
Alan and Shane, thanks for the great info. i took your advice and found a 1/2 watt 5.6k ohm resistor at Radio Shack. I spliced it at the GM bulkhead fitting, because i'm splitting it to 2 devices, a DigiDash gauge and a heads up display. Would splitting the tach signal influence the level of resistance?
Can't wait to test it but first i've got to get the engine started to genreate some RPM's. Once again thanks for the top notch info!!
 

Attachments

  • P5310018.jpg
    P5310018.jpg
    41.3 KB · Views: 237

Seymour Snerd

Lifetime Supporter
Would splitting the tach signal influence the level of resistance?

There's no way to tell without knowing the input resistance of both devices. So if it doesn't work with 5.6K, try half that, then half again, etc. until it does work. BTW, best to get it working with one of the devices first, and then add the other.
 
Back
Top