Where does tach signal come from?

[Rob4]

I have a 1989 chevrolet truck and I would like to install a tach. My 1994 6.5 diesel has a tach and the instrument cluster for it looks like it is an exact fit to go into the 1989. The connector is different but I should be able to rewire the connector so that all the gauges will work. I thought that the signal for the tach in the 1994 came from the alternator. Is my thinking not correct? Couldn't the signal also come from the alternator in the 6.2 as it is the same alternator? I say this because when I last replaced the alternator I was told that the replacement alternator had the the same number as in the 6.5. I also read somewhere in the forum that a signal could be taken from the vacuum pump and conditioned (higher amplitude and converted into a square wave--Dr. Lee). If a signal can come from the alternator does it also need to be conditioned? Where is the vacuum pump located? I have looked alll over the engine and can't find it. Was it available on all engines or just the ones with emission control? Please share your thoughts.

[john8662]

Interesting question!

Generally, for these GM's, the tach signal comes from the alternator, it never really came from the vacuum pump speed sensor.

Your engine being an '89 in a pickup won't have a vacuum pump mounted on the rear of the engine with the type of speed sensor you're looking for. The vacuum pump is belt driven on those models.

There may be an oil pump drive with a sensor in the area where the vacuum pump used to go. If there are no wires then it's not a sensor.

At any rate, I've often wondered what it would take to swap instrument clusters with one from a later model (92-93).

The '94 one may or may not work, all bets off here.

If the alternators are the same and the amount of wires on the connector are the same, it MIGHT work, try it and see.

J

[N.E. TMRPR]

Just a quick question. The sensor on a 91' motor that is in place of the vac pump. Can it be removed on the 91' and put the vac pump from the 87' in place of the sensor? I am restoring a earllier Blazer and would like to put the 91' engine in it. It has the serpentine belt on it without AC but would also like to find out how to put the AC on this engine.
Thanks, Ted

[Rob4]

To John8662----Thanks for the reply to my post

[Inspector]

One thing that should be remembered is that if you replace the alternator the drive pulley has got to be swapped to the new alternator. A larger or smaller pulley will change the tach reading.
Denny

[Robyn]

To add the AC you will need the proper brackets and such for the particular engine you are using.
The nice part is that you can mix and match all this stuff to get just about anything you want.

To get the AC on the Blazer you will have to make new AC hoses to suit the need. The Blazer originally had the AC pump on the drivers side of the engine. The later style with the serp drive placed the pump on the pass side.

Its no big deal though. Just a little fooling around and some creative hosing :D

The tach drives I believe come from the Alt as mentioned. I would take a peek at the factory electrical schematic and se what gives.

OR just yank the belt off the 94 and run it to see if the tach works, if not then its a definate yes that your there with the alt supplying the signal.

Good luck

Robyn

[Rob4]

To Inspector. I'm okay with the pulley size as when I replaced the alternator awhile back I made sure the pulley size for the 1989 6.2 was the same as in the 1994 6.5. I had heard that just a small difference in pulley size can mess up the tach reading. Thanks for the info.

[Rob4]

Robyn-- I hadn't thought about taking off the belt on the 94 to see if the tach still works. Beats digging for the wiring diagrams. Seems like a fast way to check it. Gosh I love this forum.

[DmaxMaverick]

Or, just unplugging it would be faster.

[Rob4]

DmaxMaverick-- Thanks for the input. that's even quicker than removing the belt. Thanks.