Heater core change. Now no charge - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chrismoore701 · 06-21-2020, 08:56 PM

I am finishing up the heater core change. Now no charge in the alternator. Where should i start looking? Is there something under the dash i could have hit? Unplugged battery to change cable routing while running and it died. Checked gauge. Under 12 volts.
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chrismoore701 · 06-21-2020, 09:14 PM

Measured voltage on back of alternator and it is 11.70. Measured to a header bolt. Any chance it could be something other than alternator? What are the odds?

Dead Parrot · 06-22-2020, 08:36 AM

Type of Alt? If one of the 10/12SI internal regulated models, unplug and replug the two wire plug at the Alt. The brown wire is the excite wire and needs to have voltage from somewhere to start the Alt once things start spinning. On an idiot light setup, it gets the kick start voltage via the Gen/Alt bulb. On a gauge cluster, IIRC, there is a resistance wire that serves the same purpose.

chrismoore701 · 06-23-2020, 07:27 AM

Here is what I found out last night, when I fire the motor up the volt meter reads below 12 volts. Step on the gas, it raises to about 14 and stays. I was playing around with it and disconnected the battery and it died, I fired it back up and then I step on the gas and disconnected it again and it stayed running. You just need to wake it up I guess. I took the alternator off and had it tested and it passed. Is this normal?

Dead Parrot · 06-23-2020, 10:01 AM

Yes. Normal behavior. My CUCV has dual 12V alts in a weird 24V setup. Two idiot lights. If on start the 6.2 diesel doesn't rev enough, one or both Gen lights stay on and the voltmeter stays in the yellow (no charging) zone. Bump the throttle and the light(s) go out and the voltmeter quickly climbs into the green zone.

Having working idiot lights or matching resistors are important. Without the small excite current, the alt won't start generating. That was why I suggested doing the remove/replace on the two wire plug. Replacing a heater core has plenty of chance to bump or jerk on those wires and plug.

Bandit130 · 06-23-2020, 05:28 PM

Sounds to me like the regulator is intermittently dropping off. Start truck, measure voltage while tapping on alternator to find out if there is change. Don't tap too hard.

06-21-2020, 08:56 PM	#1
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,319	Heater core change. Now no charge I am finishing up the heater core change. Now no charge in the alternator. Where should i start looking? Is there something under the dash i could have hit? Unplugged battery to change cable routing while running and it died. Checked gauge. Under 12 volts. Posted via Mobile Device

06-22-2020, 08:36 AM	#3
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,521	Re: Heater core change. Now no charge Type of Alt? If one of the 10/12SI internal regulated models, unplug and replug the two wire plug at the Alt. The brown wire is the excite wire and needs to have voltage from somewhere to start the Alt once things start spinning. On an idiot light setup, it gets the kick start voltage via the Gen/Alt bulb. On a gauge cluster, IIRC, there is a resistance wire that serves the same purpose.

06-23-2020, 07:27 AM	#4
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,319	Re: Heater core change. Now no charge Here is what I found out last night, when I fire the motor up the volt meter reads below 12 volts. Step on the gas, it raises to about 14 and stays. I was playing around with it and disconnected the battery and it died, I fired it back up and then I step on the gas and disconnected it again and it stayed running. You just need to wake it up I guess. I took the alternator off and had it tested and it passed. Is this normal? __________________ Chris _______________________________ 1979 Chevy Stepside SPORT 2018 Chevy Silverado, 39 years apart My build 2008-2016. http://m.imgur.com/a/bURrE

06-23-2020, 10:01 AM	#5
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,521	Re: Heater core change. Now no charge Yes. Normal behavior. My CUCV has dual 12V alts in a weird 24V setup. Two idiot lights. If on start the 6.2 diesel doesn't rev enough, one or both Gen lights stay on and the voltmeter stays in the yellow (no charging) zone. Bump the throttle and the light(s) go out and the voltmeter quickly climbs into the green zone. Having working idiot lights or matching resistors are important. Without the small excite current, the alt won't start generating. That was why I suggested doing the remove/replace on the two wire plug. Replacing a heater core has plenty of chance to bump or jerk on those wires and plug.

06-23-2020, 05:28 PM	#6
Bandit130 Registered User Join Date: Dec 2014 Location: Euless, Texas Posts: 511	Re: Heater core change. Now no charge Sounds to me like the regulator is intermittently dropping off. Start truck, measure voltage while tapping on alternator to find out if there is change. Don't tap too hard. __________________ 1974 Cheyenne 10 LWB Stock 350 W/HEI /TH350/AC/4 BBL Quadrajet Mopar by Birth Chevy by Choice

06-21-2020, 09:14 PM	#2
chrismoore701 Registered User Join Date: Dec 2004 Location: Highland, IN Posts: 2,319	Measured voltage on back of alternator and it is 11.70. Measured to a header bolt. Any chance it could be something other than alternator? What are the odds?



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