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Old 03-29-2018, 03:37 PM   #1
BFD305
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GM 5.7 L31 Vortec - Tach Wiring??

Hello - I did a full swap with a 2000 L31 engine/trans/computer/harness.
Everything is working great.
Does anyone have any info how to wire in a tach to this setup? I have searched everywhere and cant find the info.

If anyone can help thanks.

Running the stock harness, coil and distributor
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Old 03-30-2018, 12:30 PM   #2
LH Lead-Foot
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Re: GM 5.7 L31 Vortec - Tach Wiring??

Going back to August of 2017 with no answers? Wow. Have you found the answer? If not, the default answer is usually the PCM generates a 4 cylinder output signal on a separate circuit. Sometimes, a pull-up resistor is added to increase signal strength so the tachometer you have can read it.

In electronics, there are "Pull-down" and "Pull-up" signals. If a two resistors of the same value are connected to B+ on one end with the other resistor wired in series to ground, the middle connection will be half of B+.

Many digital dash vendors indicate the use of a pull-up resistor to allow the signal strength to increase the signal so the tach can read it. This type of circuit is a type of a voltage divider.

If this answer is not clear or what wire the signal is on, please give me the model, year (2000 I think), but I can look it up on GM electronic service information as my stand-a-lone setup goes back to the 90's when paper service manuals were still printed, but the change over to digital / internet based GM eSI was the only way going forward. Mine goes to 2004. It's on CD's.
Hope you have worked it out by now. Best of luck.
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Old 03-30-2018, 12:36 PM   #3
LH Lead-Foot
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Re: GM 5.7 L31 Vortec - Tach Wiring??

Going back to August of 2017 with no answers? Wow. Have you found the answer? If not, the default answer is usually the PCM generates a 4 cylinder output signal on a separate circuit. Sometimes, a pull-up resistor is added to increase signal strength so the tachometer you have can read it.

In electronics, there are "Pull-down" and "Pull-up" signals. If a two resistors of the same value are connected to B+ on one end with the other resistor wired in series to ground, the middle connection will be half of B+.

Many digital dash vendors indicate the use of a pull-up resistor to allow the signal strength to increase the signal so the tach can read it. This type of circuit is a type of a voltage divider.

If this answer is not clear or what wire the signal is on, please give me the model, year (2000 I think), but I can look it up on GM electronic service information as my stand-a-lone setup goes back to the 90's when paper service manuals were still printed, but the change over to digital / internet based GM eSI was the only way going forward. Mine goes to 2004. It's on CD's.
Hope you have worked it out by now. Best of luck.
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Old 03-30-2018, 03:24 PM   #4
SunSoaked
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Re: GM 5.7 L31 Vortec - Tach Wiring??

X2
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Old 03-30-2018, 11:03 PM   #5
BFD305
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Re: GM 5.7 L31 Vortec - Tach Wiring??

I actually just posted that yesterday. I am hoping to just use an old school style mini tach. It is from a 2000 GMC savana van. L31 engine. Any info would be great.
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Old 03-31-2018, 12:35 PM   #6
LH Lead-Foot
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Re: GM 5.7 L31 Vortec - Tach Wiring??

According to what I could find about an L31 engine, your 2000 GMC Savana "L31" is the last gen I Vortec and uses a distributor with an external coil. The ignition module is mounted on a heat sink near by the coil I would guess. Look for the WHT/BLK wire and make a waterproof connection onto it. The signal should be 8 cylinder and match you tachometer. Some tachometers have small "DIP" switches for 4, 6, 8. If your does, adjust a needed. If not, I would assume it will read correctly for an 8 cylinder.

Sorry I miss read the forum post and made a mistake seeing an older date. Maybe I am rushing a little or not thinking right, but I will admit when I make a mistake.
I had long days of hard work since 7 on the farm that paid nothing, and nothing is what I got when I left. I worked 16 hour days, sometimes 2 jobs to support a family of 6. I taught 4-gas analysis to shop owners and tech at night for Snapon. I have spent 105-110+ hours a week commission in dealerships. Then after 28 yrs. had to quit. Hired next day on contract labor by Raytheon and held 4hr. automotive technical seminars and 30 some classes per quarter for ACDelco. I averaged 242 days a year on the road. After my elbow joint wore out, pinched ulnar nerve in both elbows, torn rotator cuff, 3 lumbar fusion, arthritis in a knuckle that feel broken and a failed radio frequency never oblation in my lumbar. Bad pain every day!!!

Then last week had to put down my 7 year black lab, born with one eye, because of someone gave him antifreeze laced food over the fence, causing his liver, kidneys to shut down and lost almost all white blood cells in 3 days...and now nothing! Sad what people will do! Now Everything is Silent...Screeching halt! TOO DAMM MUCH!

Just trying to help. I am new to these forums, don't know everything about them, nor about X2-drunken chickens as well explained by hombre estupido senior member. I would put a cork in it, maybe the chickens will go away.

Look at the schematic for circuit 1847 0.8mm / 18 AWG wire, connector position "D", tap into that. I don't know what an L31 is, just the 8th digit in the VIN tells me the engine code. I am just a dealer tech, not an experience swapper.

I joined a few forums to help research my last build, an Gen-III 5.3L LS2 with my 4L65 (5 Pinion gear Sets, front and rear) non-electric, 3.08 final into my daily driver...67 C10 That's all!
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Old 03-31-2018, 01:02 PM   #7
BFD305
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Re: GM 5.7 L31 Vortec - Tach Wiring??

Thanks. I will give that a try. I figured beings I had a coil and distributor there had to be some wire that would work. I appreciate the info, I figured someone would know.

Sorry about your dog, that sucks.
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