Changed Alternator....

[vernj4]

The old alternator had a hard time making the gage read 14v; Bought a new alternater and installed; Low and behold the new alternater also had a hard time making the gage read 14v; Also the relationship between rpm and speedometer reading seemed different from what I was used to.

Went to the parts store and retrieved the old pully; measured the diameter at the top of the grooves - 2.402 inches (61mm). Removed the "new" pulley - measured 2.225 inches. Changed the pulleys and now I see the rpm and speedometer reading I was used to.

Did a search of the data base: found 2 diameter pulleys discussed - 62mm & 63mm. Somehow I ended up with a 61mm pulley. Not sure what pulley diameter was concidered the OEM size. Anyone know?

Based on this experience, I would guess that as our trucks got old and we changed alternators, the replacement units did not necessarily have the correct diameter pulley. Another variable to be concidered in the MPG equation.

[GMC Hauler]

You are correct. The alternators are the same for gas and diesel, and the parts store don't differentiate. I ended up with a 67mm pulley. I had to go to an alternator shop to get the pulley, though. Seems to work for me.

[tom.mcinerney]

>>behold the new alternater also had a hard time making the gage read 14v

Vern-
Be advised that a lot of the 'rebuilt' and even 'reman' electric stuff isn't too reliable.
The instrument panel voltmeter may or may not be accurate. Measure your battery voltage at the terminals on each battery with a digital handheld meter. A $18 job from Radio Shack is fine, much better as a test than the OEM gage, which is OK for trends. A load test on the batteries is a good thing.

Also keep in mind that in hot weather batteries are liable to fail, and in hot weather the regulator cuts back on the alternator output somewhat.