The Stovebolt Site Forums The Shop Area The Electrical Bay Alternator help.

Is there a easy way to check out a alternator? my batt goes dead or low. after charging it stays up awhile . i have a volt meter and today it was showing 12v and not going up when i rev the engine. or should i drive to the parts store for professional help?

What kind of alternator?

oops! its a gm old style with the two wire plug and a + lead to the batt. I have the exciter wire tied to the + lead but could put it back to the starter if thats a better idea. Its a 235 eng with stomp starter.

"GM old style"- - - - - -there were two of them, one with an extrnal regulator, and the other (10-SI) with a built-in voltage regulator. Does the 2-wire plug enter from the back of the housing or is it on the outer perimeter of the alternator?

The early model 2-wire plug is almost square, and has a semicircular indexing tab. That alternator must have an external 4-wire regulator which mounts on the firewall or the radiator support. The 10-SI alternator has a rectangular plug on its outer perimeter and one wire needs to connect to the large BAT terminal, and the other one should be connected to a switched ignition voltage source.

If you've got a 10-SI alternator, it's possible to bypass the regulator and put it into full output by inserting a shorting pin like a small-bladed screwdriver or a cotter pin into a D-shaped hole in the back of the housing near the 2-wire plug's location. This eliminates the regulator from the circuit and gives the alternator full currrent flow in the field coils. Don't run the engine above fast idle, as there will be no voltage control.
Jerry

Must be the 10-si alt. there is no external reg on the truck. is there a way to tell by looking at how many watts it puts out? i see lots of sizes in the parts book. I think i see the hole to bypass the reg. will try that.I charged the batt all nite and it starts up but the meter just shows 12v

I sent you an e-mail to the address in your profile with my notes on this issue.

Larry

George had a nice class on this subject in Kansas city
at the reunion... maybe he will chime in..

The voltage should come up when the engine is started. If your connections are correct like HRL shows then the unit is not working.

Thanks for the info. I charged the batt last nite and it started ok this a.m. But when I put a meter on the batt post of the ald all I saw was 12v. unhooked the batt wire and read 0v. took it to the autoparts and they checked it out. ng. came home with a new alt and all looks well. The parts guy said the old and new were 78 amp alts. he used the pn on alt to tell. wonder if there is a list to check the size of my other alts. just courious.

back in the day one could remove a cable,, not good idea now though.. I usually cheat and take it to the local FLAPS and let them zap it...

Pulling a battery cable with the engine running can create voltage spikes of over 100V in a fractrion of a second- - - -bad juju on transistors and diodes! I've seen it on oscilloscopes on numerous occasions. The regulator bypass trick will usually determine whether the problem is with the basic alternator or the regulator, but beyond that, the unit needs a bench test.
Jerry

diodes and transistors you are dating yourself. use a o scope. ? where would you find one . lastone i saw was a textronic 435 almost as big as my 235, went to the autoshop got a new one garonteed for life. mine or its ?

That alternator you just bought has six high-current rectifier diodes in it, plus a bunch of solid stare circuits in the regulator. When I was a training manager for Snap-On Sun we sold a handheld lab scope that would run two real-time scope patterns at the same time, on a split screen. It was used to monitor things like fuel injector pulse width and various other signals that any auto technician worthy of the name needed to do good diagnostic work.

In 1957, my father's shop purchased the first ignition scope in Nashville Tennessee. I grew up using it on virtually every tune-up, then spent a lot of years using a lab scope in USAF electronics shops. I spent over 30 years teaching auto mechanics students to interpret scope patterns, and a lot of them are shop owners now. Yes, scopes are very relevant to the people who know how to use them.
Jerry

with the truck a little above idle, take a screwdriver and stick it to the back of the alternator bearing...its a pressed in inclosed needle bearing so your not going to hurt anything.....stick it to this bearing....if the screwdriver sticks then your alternator is putting out a charge....You should get a pretty strong magnetic pull . This is just a quick check and no it doesnt tell you all the specifics but lets you know its putting out a charge. Best bet is to take the alternator off and have it bench tested at a starter and alternator shop if you have concerns with it.