How do I test my alternator? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72gmcshorty · 05-26-2003, 12:05 AM

I'm not sure if my alternator is charging my battery when truck is running. I'm not driving the truck yet just able to get it started and sit there idling. I have a multimeter tester. Is there some way to hook up my meter to tell if the alternator is actually charging when the truck is running?

sakohntr · 05-26-2003, 12:14 AM

With the truck off, check the voltage at the battery. Then start the truck and check the voltage at the alternator. You should have a slightly higher reading at the running alternator. Take your alternator reading from the larger terminal on the backside which has the heavy guage wire attached to it. Your alternator voltage should also rise with an increase in rpm's. good luck

72gmcshorty · 05-26-2003, 12:24 AM

When you say to check at the alternator terminal, do you mean the one that has the one single red wire attached to it? Is it supposed to be thicker than the other two in that clip into the alternator itself? I did the changeover to internally regulated alt and made all three wires the same thickness. Blue and White into the clip and red bolted onto back of alt.

mtdave2 · 05-26-2003, 12:25 AM

with the truck running your battery should read 14 volts or so.. anything about 13.5 would be good.

krue · 05-26-2003, 12:39 AM

I always just cranked the engine, then pulled the ground wire off the battery. If it dies, the alternator is toast, if it runs, your good to go. Of course I'm just a shade tree parts swapper!

**GMCPaul** · 05-26-2003, 04:16 AM

krue's method will tell you if the charging system is working, by pulling the positive cable with the motor running if the charging system is not supplying voltage the motor will die, if it does not you are getting a charge from the altenator, if it dies then either the wiring, voltage regulator or altenator is bad, when you have the positive wire off and if it stays running then turn on the headlights on bright, if it continues to run you know charging system is strong and puts out enough to supply recharging the battery during heavy electrical draw.
GMCPaul

SEVEND2 · 05-26-2003, 04:31 AM

Most Part stores have a tester too. If ya want you could go that way for the alt.

Class of 69 · 05-26-2003, 10:24 AM

When I worked as a repair tech, we alway's did what krue, and GMCpaul said. The testers stayed on the peg. Since you went to the integral alt, that eliminates testing the volt reg independently. Now, is the alt not charging since you made the conversion?? Maybe the wiring setup isn't quite right yet?

72gmcshorty · 05-26-2003, 12:34 PM

I can't drive my truck yet on the road. I've only been able to start it in the driveway. I've been doing a lot wire gremlin chasing and was turning the lights on more often than I should of I guess. I think that is what drained the battery. Haven't been running it long enough to check the battery.

Longhorn Man · 05-26-2003, 05:41 PM

Just pulling a cable off the bat will only let you knw a little bit. Your engine only needs a few volts and a couple amps to run. But the battery needs more than that to charge up and also running all the stuff like headlamps.
Take your multi-meter, and hook it to the batery. Set the tester to Volts DC and to the lowest scale that is over 15 most likely that will be the 100 volt setting.
Start the engine and rev it up a bit to kick in the one wire alt and make it start charging.
Look at your meter, anything over 13.4 or so is good.
Anything less than 12.5 and your not charging at all.
Anywhere inbetween those two readings means you are charging...but not enough.

05-26-2003, 12:05 AM	#1
72gmcshorty Registered User Join Date: Jul 2001 Location: North Delta, British Columbia Posts: 1,344	How do I test my alternator? I'm not sure if my alternator is charging my battery when truck is running. I'm not driving the truck yet just able to get it started and sit there idling. I have a multimeter tester. Is there some way to hook up my meter to tell if the alternator is actually charging when the truck is running? __________________ 1992 Gmc 2wd 1972 Chevy Blazer 2wd

05-26-2003, 12:14 AM	#2
sakohntr Registered User Join Date: Mar 2002 Location: Floodwood Minnesota Posts: 1,057	how to With the truck off, check the voltage at the battery. Then start the truck and check the voltage at the alternator. You should have a slightly higher reading at the running alternator. Take your alternator reading from the larger terminal on the backside which has the heavy guage wire attached to it. Your alternator voltage should also rise with an increase in rpm's. good luck __________________ 1967 K30 Napco 292/SM420 original paint 30k miles 1968 K60 Napco 396/5spd. dbl frame 52" michelins 1969 K10 SWB GMC 454/4spd 6" 35's posi,hooks 1972 C30 longhorn,BB/AT,A/C,Tilt,Toolbox,lokr 1987 K20 Chevrolet Suburban. 6" lift. 5.7L/TH400 1979 K60 GMC Coleman 4x4, 427/Allison.

05-26-2003, 12:24 AM	#3
72gmcshorty Registered User Join Date: Jul 2001 Location: North Delta, British Columbia Posts: 1,344	When you say to check at the alternator terminal, do you mean the one that has the one single red wire attached to it? Is it supposed to be thicker than the other two in that clip into the alternator itself? I did the changeover to internally regulated alt and made all three wires the same thickness. Blue and White into the clip and red bolted onto back of alt. __________________ 1992 Gmc 2wd 1972 Chevy Blazer 2wd

05-26-2003, 12:25 AM	#4
mtdave2 newly minted old timer Join Date: Dec 1999 Location: Hudson WI Posts: 3,508	with the truck running your battery should read 14 volts or so.. anything about 13.5 would be good. __________________ ____________________________________________ 72 c20 longhorn 65k org miles and counting! 69 k10 Suburban

05-26-2003, 12:39 AM	#5
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	I always just cranked the engine, then pulled the ground wire off the battery. If it dies, the alternator is toast, if it runs, your good to go. Of course I'm just a shade tree parts swapper! __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

05-26-2003, 04:16 AM	#6
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,719	krue's method will tell you if the charging system is working, by pulling the positive cable with the motor running if the charging system is not supplying voltage the motor will die, if it does not you are getting a charge from the altenator, if it dies then either the wiring, voltage regulator or altenator is bad, when you have the positive wire off and if it stays running then turn on the headlights on bright, if it continues to run you know charging system is strong and puts out enough to supply recharging the battery during heavy electrical draw. GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

05-26-2003, 04:31 AM	#7
SEVEND2 Registered User Join Date: Aug 2001 Location: Menifee, Calif, USA Posts: 2,145	Most Part stores have a tester too. If ya want you could go that way for the alt. __________________ With a 1972 Chevy Custom-10 Long bed w/350cid th350, (under-construction). Progress on truck so far (follow link): http://67-72chevytrucks.com/vboard/s...ad.php?t=51051 and stage 2 http://67-72chevytrucks.com/vboard/s...ad.php?t=58132 ~ "Why couldn't you have just put the bunny back in the box?" ~

05-26-2003, 10:24 AM	#8
Class of 69 Registered User Join Date: Mar 2003 Location: Stillwater MN Posts: 1,166	When I worked as a repair tech, we alway's did what krue, and GMCpaul said. The testers stayed on the peg. Since you went to the integral alt, that eliminates testing the volt reg independently. Now, is the alt not charging since you made the conversion?? Maybe the wiring setup isn't quite right yet? __________________ 69 GMC K10 ..some restoration required....still..

05-26-2003, 12:34 PM	#9
72gmcshorty Registered User Join Date: Jul 2001 Location: North Delta, British Columbia Posts: 1,344	I can't drive my truck yet on the road. I've only been able to start it in the driveway. I've been doing a lot wire gremlin chasing and was turning the lights on more often than I should of I guess. I think that is what drained the battery. Haven't been running it long enough to check the battery. __________________ 1992 Gmc 2wd 1972 Chevy Blazer 2wd

05-26-2003, 05:41 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Just pulling a cable off the bat will only let you knw a little bit. Your engine only needs a few volts and a couple amps to run. But the battery needs more than that to charge up and also running all the stuff like headlamps. Take your multi-meter, and hook it to the batery. Set the tester to Volts DC and to the lowest scale that is over 15 most likely that will be the 100 volt setting. Start the engine and rev it up a bit to kick in the one wire alt and make it start charging. Look at your meter, anything over 13.4 or so is good. Anything less than 12.5 and your not charging at all. Anywhere inbetween those two readings means you are charging...but not enough.



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