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Where does the 6.5 gets it's tach signal ?

Dash Tach? from the Alt. Timing (the tach the PCM sees)... from the CPS.

on edit: That's why when the alternator dies, the dash tach quits but the motor keeps running.

See? You were right, no matter what I said! :D
 
Yep. The PCM needs to know where the IP position is compared to the crank position.
 
Acesneights1 said:
That sums it up. Is it from the CPS or is it off the Alt ?
Click to expand...

Well like most things, it depends. First I would say, the 6.5 doesn't get any tach signal, it sends out several RPM signals. Depending on the engine and spec, they come from several sources. The mechanical pump models send an analog signal from the engine speed sensor on the oil pump drive. I'm not sure, maybe the manual trans models do not even have this, there would be no reason to but GM may have built them all with the ESS sensor, I would be curious about that. The electronic pump models have a digital speed sensor on the crank, this is where the PCM gets its RPM signal from. AFAIK, the tachometer is driven off the alternator on all models.

This is something I have always wondered why GM did. They have a very accurate engine speed signal for the PCM, yet drive the tach from the alternator. Why would they do this? It would have been no problem at all to use this signal for the tach. They did the same with the engine temperature. The PCM uses a sensor in the crossover, the gauge uses a separate sensor in the head. What were they thinking?
 
Do the 6.5 alts have a lug for the tach signal ? Where I'm going with this is wanting to hook a tach up to my 6.2. was thinking about ways to do it. Don't really wanna mess with magnets on the balancer. I may go with teh ESS OP drive but not sure how to wire it to say an Autometer tach.
 
Acesneights1 said:
Do the 6.5 alts have a lug for the tach signal ? Where I'm going with this is wanting to hook a tach up to my 6.2. was thinking about ways to do it. Don't really wanna mess with magnets on the balancer. I may go with teh ESS OP drive but not sure how to wire it to say an Autometer tach.
Click to expand...

No they really don't. I had a big disagreement with my alternator guy - who is very good and has years of experience - about this. He said the Diesel alternator cannot run the tach as it lacks the proper circuit. Apparently some Delco alternators have a special tach signal circuit, but ours do not. Even his book stated the Diesel tach is not from the alternator, but I was able to show him it did run the tach. I can't tell you how its done, but from what I know, its just a standard alternator and somehow it signals the tach. I think you are installing in an older truck, I don't know how easy it would be to do as the pulley must be a certain size and not sure how many aftermarket tachs can run off the alternator.

I might have the instructions on using the ESS to run the tach at home, I will try and have a look when I get the time.
 
The 6.5 diesel tach is run off of the stator output terminal in the voltage regulator. It simply reads the stator pulses like it would those of an ignition charge pulse since they are both similiar in wave length. GM didn't make the tach run off of the PCM in the 6.5's as there gasser tachs are driven directly off of the ignition charge circuit of the coil just like most all of your old style tachs were. The PCM couldn't produce this signal so they would have had to of made a tch generator to have run it or changed the tachometer, so they instead used the alternator stator output which produces a coil type wavelength and signal. Saved quite a bit of money on there part actually and provides a decent tach when you have the right pulley on it. As for adding a tach to your 6.2, what style alternator does it have now? The older SI series would involve tapping into the stator to get the tach signal out of it unlike the CS series where the stator output is part of the regulator plug. It is possible though as the marine industry still uses the SI series of alternator and makes tachs just for hooking up to them. I would reccomend going with a marine style tach as they have a broad adjustment range in most all of them via a set of dip switches in them. And also they are specifically made to read the stator pulse that the alternator produces.
 
THEFERMANATOR said:
The 6.5 diesel tach is run off of the stator output terminal in the voltage regulator. It simply reads the stator pulses like it would those of an ignition charge pulse since they are both similiar in wave length. GM didn't make the tach run off of the PCM in the 6.5's as there gasser tachs are driven directly off of the ignition charge circuit of the coil just like most all of your old style tachs were. The PCM couldn't produce this signal so they would have had to of made a tch generator to have run it or changed the tachometer, so they instead used the alternator stator output which produces a coil type wavelength and signal. Saved quite a bit of money on there part actually and provides a decent tach when you have the right pulley on it.
Click to expand...

Thanks, I always wondered why they did that. Makes sense.
 
Well...That could be my other problem...I have 2 alts. Truck is 24v start CUCV. Also they are v belt so I'd have to swap a pulley and get it right but I gotta start somehwere.
 
Tach Exasperation

Changed out the OEM Alternator to get higher output optional unit installed for increase amps. Had to change plug, but tried to reconnect wires to new connector pigtail. I have three leads. Which of the F I S P do I need to hook up the tach signal from. I am sure the pulley is the same diameter, so shouldn't have a problem. Been over a year since change and truck waited for new IP. Now back up a running, please help with correcting a probable mis-wire of the new pigtail. This is on a 1997 Chevy K2500 6.5l diesel. Thanks.
 
Kenny, you're hooped.

Paint one of your fan blades orange, take off your hood, and count the flashes while you drive :D
 
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