Tach signal from crank position sensor

Hi all, my 96 burb is still sitting dead in the front yard. I have replaced that dreaded DS4 with the DB2 and the truck now runs. Since mine is 96 with single t-stat the throttle cable hook up was unbelievably painful. Now i am doing the trans control. I chose to use an aftermarket controller for various reasons. I have everything hooked up but the tachometer input. I am looking at a alternator converter that wi provided the needed signal, but was wondering if i can get the needed signal from the crank position Sensor, which as i have read is where the pcm gets the signal. I need a 12v square wave if i remember correctly. Any guidance is appreciated.
Neo

I think the best route would be to get an oil pump drive off an MFI 6.5 with the RPM adapter for use with an aftermarket controller.

J

Thanks for the input John. I believe, not claiming to be well versed in this, that the ESS you are referring to is basically the same style of hall effect sensor as the crank position sensor. There could be a different number of pulese on the ESS, but IIRC the CPS emits a low voltage 4 pulse per crank rotation signal to the PCM for engine speed/crank position. I think this is the same signal used to drive the tach (through the PCM, so I don't think I can just use the tach signal out of the dash.) I believe all I would have to do is amplify that signal with a transistor. I was hoping that someone else had doen this before. On top of that, I have the CPS, but an ESS is pretty pricy (>$200 as far as I can tell from the online shops.) Thanks for the input tho!
Neo

You shouldn't have to spend the cash to install an aftermarket ESS. You have other options available from the factory. I don't know of anyone who's used the crank sensor, but don't know it can't be done, either. As John said, the oil pump drive from a MFI 6.5 would be ideal. Another source is the alternator, where the IPC gets the signal for the tach. If your tach is working, the signal is already there and working.

That would hands down be the best alternative at ghis point. I tried to get the cps signal but mine if garbage. So old it came apart in my hands. Now i will have to extract the remains from the aluminum cover. Argh!!
So there is a plug on the bottom of the alternator. Two wires currentl come off it and go to a plug u der the coolant cross over. Is it one of those, or is there a other wire i need to put on what looks like a four position plug on the bottom of the alt that the two wires currently come from? Thank u very much for the guidance!

Trying to give a little back to the community that has helped me out so many times. So, as i suspected, you can and do get a very clean nice tach (5v square wave) from the crank position sensor. The wire that provides the signal is the light colored (yellow or white on the harness) one, it is on the end. The opposite (3 wire hall effect sensor) is grey iirc and that is source 5v, the middle (black?) is ground. The signal is super clean and perfect for the tci tcu. Here is the weird thing, i broke the three leads off my cps so did not have it hooked up while testing the truck earlier today - used the white wire from alt to drive tach signal but it does not work since it is 4x what u need. When i started using the cps, tach on tci tcu came in great, but now i get the mil AND the "check throttle" light on the dash. When i try and accelerate with the "check throttle" the pcm must intermittently kill the old ds4 fuel solenoid signal cause the engine bucks lite the fuel shut off on the db2 is being pulsed on/off.
I am suspecting the fix of the cps now tells the old pcm that the engine is running and tries to shut the fuel off over 900 rmp. This is pretty bizare. It now seems the best thing to do is keep the pcm in the dark and cut the cps signal so the p m doesnt know the truck is even running?!? Any suggestions? Thanks to all!
Neo

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Originally Posted by neo

So, as i suspected, you can and do get a very clean nice tach (5v square wave) from the crank position sensor. The wire that provides the signal is the light colored (yellow or white on the harness) one, it is on the end. The opposite (3 wire hall effect sensor) is grey iirc and that is source 5v, the middle (black?) is ground. The signal is super clean and perfect for the tci tcu.

I am suspecting the fix of the cps now tells the old pcm that the engine is running and tries to shut the fuel off over 900 rmp. This is pretty bizare. It now seems the best thing to do is keep the pcm in the dark and cut the cps signal so the p m doesnt know the truck is even running?!? Any suggestions? Thanks to all!
Neo

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Thanks for your followup. Most don't use the CKP (Crank Position) sensor for a tach signal because it requires a regulated 5vdc to operate. This 5vdc is generated by the PCM, which is sometimes disabled after a conversion to MFI.

The CKP generates 4 pulses per crankshaft revolution, where the oil pump drive generates 8 pulses per revolution, but it turns at half the speed of the crankshaft, so the net effect is the same. The oil pump drive is just a simple 2-wire sensor, so it is easier - usually - to adapt to an aftermarket transmission controller. That's what I did in 1999 when I installed a Jet 4L80-E in a non-electronic truck. Worked great.

Jim