LS swap alternator issues - The 1947 - Present Chevrolet & GMC Truck Message Board Network

todd7498 · 04-12-2019, 09:28 PM

Hi, I did an LS swap in my 72 last year and have had alternator issues. Alternator charges fine until I turn on the A/C and it sucks the voltage down unless I rev it up, head lights or electric fans don't bother it but the A/C really seems to pull the voltage down. If I rev it up a bit and keep it around 900 plus rpm it seems to be fine. Any ideas? Thanks

Possibly I need a bigger alternator? I think mine may be the 105amp, I am running a stereo set up with a couple small amps, and dual electric fans.

BR3W CITY · 04-12-2019, 09:55 PM

Are you on factory AC or one of the aftermarket AC setups? In the tune there are settings that will use the input trigger from the +12v to the AC and bump the RPM to accommodate the load/draw on the engine.

If its purely an issue of draw, I'd check your grounds and wire gauges (i.e. no 24awg on big 12v supplies).
Also might be worth seeing the the donor setup was running GmBmh charging.

todd7498 · 04-12-2019, 09:59 PM

Factory A/C, PSI harness has the wire to hook up to my A/C to bump idle and it is hooked up.

todd7498 · 04-12-2019, 10:02 PM

I was running a 4.8 last year now have a 5.3 in it, wire sizes are not small but I will check that stuff over again.

BR3W CITY · 04-13-2019, 11:13 AM

Did the same stuff happen on the 4.8?

todd7498 · 04-13-2019, 05:29 PM

Yes, same issue with the 4.8 last year. Today I installed another frame to chassis ground. I have a ground from the battery to the block, and it splits to the frame (ground cable from the donor van with the 4.8 so it is adequate). Original frame to chassis ground too. The alternator is from the 2005 van which had A/C but mechanical fan and pretty basic cargo van so I am wondering if it is to small of amp alternator. Thanks again for any help.

I never had any issue until mid summer last year when I got the A/C working and at that time I also installed the dual electric fans.

04-12-2019, 09:28 PM	#1
todd7498 Registered User Join Date: Dec 2017 Location: Brandon Manitoba Posts: 10	LS swap alternator issues Hi, I did an LS swap in my 72 last year and have had alternator issues. Alternator charges fine until I turn on the A/C and it sucks the voltage down unless I rev it up, head lights or electric fans don't bother it but the A/C really seems to pull the voltage down. If I rev it up a bit and keep it around 900 plus rpm it seems to be fine. Any ideas? Thanks Possibly I need a bigger alternator? I think mine may be the 105amp, I am running a stereo set up with a couple small amps, and dual electric fans. Last edited by todd7498; 04-12-2019 at 09:51 PM.

04-12-2019, 09:55 PM	#2
BR3W CITY meowMEOWmeowMEOW Join Date: Sep 2009 Location: MKE WI Posts: 7,128	Re: LS swap alternator issues Are you on factory AC or one of the aftermarket AC setups? In the tune there are settings that will use the input trigger from the +12v to the AC and bump the RPM to accommodate the load/draw on the engine. If its purely an issue of draw, I'd check your grounds and wire gauges (i.e. no 24awg on big 12v supplies). Also might be worth seeing the the donor setup was running GmBmh charging. __________________ *'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING* MY BUILD THE H8RDCPTR //\\ MY YOUTUBE CHANNEL REV J HD

04-12-2019, 09:59 PM	#3
todd7498 Registered User Join Date: Dec 2017 Location: Brandon Manitoba Posts: 10	Re: LS swap alternator issues Factory A/C, PSI harness has the wire to hook up to my A/C to bump idle and it is hooked up.

04-12-2019, 10:02 PM	#4
todd7498 Registered User Join Date: Dec 2017 Location: Brandon Manitoba Posts: 10	Re: LS swap alternator issues I was running a 4.8 last year now have a 5.3 in it, wire sizes are not small but I will check that stuff over again.

04-13-2019, 11:13 AM	#5
BR3W CITY meowMEOWmeowMEOW Join Date: Sep 2009 Location: MKE WI Posts: 7,128	Re: LS swap alternator issues Did the same stuff happen on the 4.8? __________________ *'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING* MY BUILD THE H8RDCPTR //\\ MY YOUTUBE CHANNEL REV J HD

04-13-2019, 05:29 PM	#6
todd7498 Registered User Join Date: Dec 2017 Location: Brandon Manitoba Posts: 10	Re: LS swap alternator issues Yes, same issue with the 4.8 last year. Today I installed another frame to chassis ground. I have a ground from the battery to the block, and it splits to the frame (ground cable from the donor van with the 4.8 so it is adequate). Original frame to chassis ground too. The alternator is from the 2005 van which had A/C but mechanical fan and pretty basic cargo van so I am wondering if it is to small of amp alternator. Thanks again for any help. I never had any issue until mid summer last year when I got the A/C working and at that time I also installed the dual electric fans.



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