Another Tach Question. (Did search first!)

2005 5.3L (Tahoe motor) in a 93 Silverado. Got almost everything working so far. What I'm having trouble with is the Tach signal. I'm using an Auto Meter tach. I tapped into Pin 10 (white wire) on the green connector on the PCM. I also set the tach to 4 cylinder (2 pulse I believe). With the tach plugged in it moves ever so slightly when it first gets power. When I connect the "signal" wire to the white wire on the PCM nothing happens. The tach doesn't work. If I take the "signal" wire on the tach and repeatedly tap it to a 12V power source (simulating an AC signal) the needle does move around, up to 7k RPM. I've checked the ground and power sources with a digital multimeter. They seem fine. Does anyone know if the white wire on pin 10 still have the RPM output? Was there a change in '05 or earlier? I'm so frustrated with this. Any help will be greatly appreciated. Thanks again!

You need to change the tach resolution in the PCM from 6 to 3 or get a module from Dakota Digital to convert the signal. Who did your tune? Send it back to them and have the resolutiuon changed if you want to make things easy.

Thanks again NITROUS. I'll check out Dakota Digital. Wait4me Performance did the tune. I'll also contact them tomorrow and see what they say.

BTW... is this what I need?
Dakota Digital - digital instrumentation and accessories automotive and motorcycle enthusiasts.

I'll be doing this soon too, thanks for the info.

That or you can contact BakerElectronix.com and get one of his adjustable resolution tach modules.

RedHeartbeat... is this the adapter you are referring to?
http://mysite.verizon.net/vze7erz1/s...files/atsd.pdf

Also, If I understand this correctly I need to convert the 4 cylinder signal to an 8 cylinder signal? Shouldn't the aftermarket tach be able to do this with the dip switches? I'm not arguing I'm just trying to make sense of this. Thanks again.

You can use that to change the signal count to your tach module if you don't have a PCM program to modify it.

Ya, I forgot about Baker Electronics........They will probably save a little money over Dakota but the cheapest way is the resolution change with HPTuners or EFI Live.

i have a problem with my 98 and i have module could the tune cause it not to work i have tried a diffrent tach. and i do have the baker module.

i just spoke to a tech rep at auto meter. He says that the RPM output of the PCM on the LSX based engines is carried on a 5V square wave signal. The Auto Meter tach required a 12V square wave signal. Autom Meter offered to changed the input requirement on the tach at no charge if I send it out to them. But who knows how long that would take. They also mentioned that they have an adapter for that exact application. Auto Meter part number 9117 Tach Adapter. "This adapter senses the current from the positive coil source wire and converts it into a 12-volt square wave signal that can be read by Auto Meter Tachs and accessorie."

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Originally Posted by beater6.0

i have a problem with my 98 and i have module could the tune cause it not to work i have tried a diffrent tach. and i do have the baker module.

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would this be an issue with a nelson made harness? and a manual tranny?

i think it is in my pcm, im saving some money right now to get a nelson tune done

I'll be picking up the tach adapter tomorrow. I'll also install it tomorrow. Beater6.0 I'll let you know if it works or not.

I don't think it's the harness but if the resolution is off one way or another it can affect the output.

FWIW, I have a 2002 5.3L tuned by Nelson in a 92 fullsize.
It has an AutoMeter tach wired as described in the initial post that works well, I have it in 4 cylinder mode.
I guess there must be differences in pcm

Hmmm. That's weird. Well, the Technical Support Supervisor at Auto Meter wanted to know specifically what year vehicle the engine was out of. Well, I'll know for sure if it worked by late tomorrow. Thanks.

Let us know when you find something out.
I also have a harlan shift light hooked up, and it is exactly the same as the tach.
The two are in unison also as the light comes on as soon as the tach hits 6500.

i think my problem is the pcm, hope fully i can get a nelson tune in the next week or two and if that aint the problem im burning the truck down

Well, the Auto Meter 9117 tach adapter did work, but not in the way I thought. It thought it would take the 5V output of the PCM and convert it to a 12V output that the tach could read. Instead the tach adapter is installed in-line with the power supplying voltage to the ignition coils. It basically reads the current going through that line and supplies the needed 12V square wave signal that the tach is looking for. Does it work? Yes, but the needle on the tach is a little fluttery. Other than that it's fine.

glad it works i would like mine to work from the pcm. bu tglad yours is working

Thanks. Let me know how yours turns out. I'd like to have the tach working from the PCM output as well... eventually.

Does the needle flutter throughout the RPM range or is it just at idle?
Most aftermarket tachs are much more sensitive than a factory unit, and thus will pick up small changes in RPM which results in a flutter on the tach.
Mine flutters at idle mostly d/t the cam, but once the engine comes up off idle, the needle smooths out.

The majority of the fluttering is mostly during idle. As the needle advances through the RPM range is smooths out a bit. However, it never gets entirely smooth. I don't have a manual shift transmission, nor do I have a shift light. I use the tach mostly for engine management. So it works for my purposes.

beater6.0, I just found this. I'm going to try it this week and i'll let you know how it goes.

Fixed Tach... YES!!! - LS1TECH

thanks trunknuts did u try it i will be at radio shack tommrow morning beating there door down

well i just got done at radio shack got the resistor and hooked it up to an aftermarket tach and guess what it worked so im all giddy like a school girl.

Just got home from trying the resistor and what do you know? It freaking works! I spent $80 on a crappy tach adapter and a 99 cent resistor fixed my problem. Go figure...

thats sucks. glad u got it fixed im happy mine is finally working

beater, how did you wire your check engine light? I have the brown/white wire from the pcm going to the ground side of my light. The other end I have going to an ignition powered 12V wire. It won't light up when I go to start the engine, but I know it works cause I tried to start my truck today with a low battery and when it slowly cranked, the light turned on. Am I wiring it wrong?