Vintage Air voltage drop - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jchav62 · 03-20-2020, 09:23 PM

Hi all! Not too long ago i purchased a used vintage air setup from a fellow member. It's been installed for a while. I'm finally going to take it in for some r134 tomorrow.

Anyhow, i just notice that when I turn the fan on, the voltage drops to just under 12 volts (aftermarket gauge) and the Gen light faintly comes on. With the multimeter, it still read about 14 volts at the battery, but about 11-12v at the fuse box. The higher the fan speed, the lower the volts. I have converted to an internally regulated alternator. Any ideas on what's causing this voltage drop?

Thanks... and wish everyone all the best during these crazy times.. stay healthy

JC62

vin63 · 03-21-2020, 02:46 PM

What is the amp rating of your alternator? Intalling my Vintage Air AC system was one of the reasons I went with a 160 amp alternator.

sstock · 03-21-2020, 03:02 PM

You have a large enough feed wire to your fan motor?
Feed wire size from battery to fuse panel?

jchav62 · 03-22-2020, 03:11 PM

Quote:

Originally Posted by vin63

What is the amp rating of your alternator? Intalling my Vintage Air AC system was one of the reasons I went with a 160 amp alternator.

I'll have to double check. I upgraded but i don't remember the expat amperage.

jchav62 · 03-22-2020, 03:13 PM

Quote:

Originally Posted by sstock

You have a large enough feed wire to your fan motor?
Feed wire size from battery to fuse panel?

I put in a new under hood harness. Under the dash is still original wiring. Factory fan wiring.

**1971 GMC SHORTY** · 03-22-2020, 10:33 PM

Ac Compressor Pulling To Many Amps, You need A 160 Amp Alternator

jchav62 · 03-24-2020, 12:19 PM

Quote:

Originally Posted by 1971 GMC SHORTY

Ac Compressor Pulling To Many Amps, You need A 160 Amp Alternator

It's actually the heater/ac fan. It was doing it with just the heater, before I even put Freon in the system. Now that the system is charged and the compressor is cycling on and off, it doesn't drop the voltage.

I'll upgrade the alternator. thanks!

**The Rocknrod** · 03-24-2020, 12:40 PM

I think the compressor doesn't pull amps does it? It just grounds and engages the clutch. Is that correct? Unless a computer controlled vehicle has something else?

sstock · 03-24-2020, 12:41 PM

Quote:

Originally Posted by jchav62

It's actually the heater/ac fan. It was doing it with just the heater, before I even put Freon in the system. Now that the system is charged and the compressor is cycling on and off, it doesn't drop the voltage.

I'll upgrade the alternator. thanks!

Before you buy another alternator, run a 12 guage jumper wire directly from the battery to the fan lead and check for voltage drop there. The fact that you still have the original fan motor feed wire sends alarm bells to me, just not big enough to carry the load. You know ohms law higher the resistance the more the voltage drop. Quick analogy, the wire from the fuse panel to the fan motor is like you are trying to pass a four lane road full of cars down a two lane street.

jchav62 · 03-25-2020, 02:56 PM

Quote:

Originally Posted by sstock

Before you buy another alternator, run a 12 guage jumper wire directly from the battery to the fan lead and check for voltage drop there. The fact that you still have the original fan motor feed wire sends alarm bells to me, just not big enough to carry the load. You know ohms law higher the resistance the more the voltage drop. Quick analogy, the wire from the fuse panel to the fan motor is like you are trying to pass a four lane road full of cars down a two lane street.

That makes all kinds of sense! I'm definitely going to try that. Great idea...THANKS!

**jayoldschool** · 03-25-2020, 09:22 PM

Rule out a loose/slipping alternator belt before worrying about anything else.

69 C10 LWB · 03-25-2020, 10:42 PM

It sounds like you have a voltage drop or bad ground to the cab. If you still have 14 V at the battery your alternator is probably fine. If you had a voltage drop on the fan wire your fan would be slow but your voltmeter should not drop. It sounds like you may have a bad connection where the main power wire goes into the cab or possibly at the horn relay if you still pull cab power from there. Could also be a bad ground to the cab.

jchav62 · 03-31-2020, 12:59 PM

Quote:

Originally Posted by 69 C10 LWB

It sounds like you have a voltage drop or bad ground to the cab. If you still have 14 V at the battery your alternator is probably fine. If you had a voltage drop on the fan wire your fan would be slow but your voltmeter should not drop. It sounds like you may have a bad connection where the main power wire goes into the cab or possibly at the horn relay if you still pull cab power from there. Could also be a bad ground to the cab.

I replaced the original wire from the fuse box to the fan switch with a 10ga wire. That seemed to do the trick. The voltage still drops with the fan on, but no lower 13v with the fan on. Around 12.5 on high. Thanks for all you input...

Stay safe! JC62

03-20-2020, 09:23 PM	#1
jchav62 1962 C-10 Join Date: Apr 2005 Location: Las Vegas, New Mexico Posts: 4,978	Vintage Air voltage drop Hi all! Not too long ago i purchased a used vintage air setup from a fellow member. It's been installed for a while. I'm finally going to take it in for some r134 tomorrow. Anyhow, i just notice that when I turn the fan on, the voltage drops to just under 12 volts (aftermarket gauge) and the Gen light faintly comes on. With the multimeter, it still read about 14 volts at the battery, but about 11-12v at the fuse box. The higher the fan speed, the lower the volts. I have converted to an internally regulated alternator. Any ideas on what's causing this voltage drop? Thanks... and wish everyone all the best during these crazy times.. stay healthy JC62 __________________ Joseph Chavez "See the USA in a Chevrolet!!" ......................_____ .........________//__{\_____ ,,,,,,,/__(O)___//___/__(O)_/ 1962 C10 Shortbed Fleetside 327/th350 GV overdrive 1974 C10 Longbed Fleetside 350/th350 "The Trash Truck" 1999 Chevy 2500 OBS, Longbed 5.7/4L80E 4x4 2015 GMC Sierra SLE 1500 4x4

03-21-2020, 02:46 PM	#2
vin63 It's Better With Nitro Join Date: Apr 2008 Location: Chino Hills, CA Posts: 2,261	Re: Vintage Air voltage drop What is the amp rating of your alternator? Intalling my Vintage Air AC system was one of the reasons I went with a 160 amp alternator. __________________ 1963 C-10: Deluxe-optioned cab, shortbed, fleetside Pontiac 462 ci, Kauffman D-Port alum. heads 4L80E, narrowed sheetmetal Ford 9-inch Tubular front and rear suspension Custom 6-piston front disc and 4-piston rear disc brakes

03-21-2020, 03:02 PM	#3
sstock Registered User Join Date: Mar 2014 Location: Tacoma Wa Posts: 146	Re: Vintage Air voltage drop You have a large enough feed wire to your fan motor? Feed wire size from battery to fuse panel? __________________ 1953 3100 261cu in, sm420, 3.55 torque tube, omaha orange 1964 GMC 1000 series,custom, 305E, sm420

03-22-2020, 10:33 PM	#6
1971 GMC SHORTY Senior Member Join Date: Apr 2009 Location: SAN FERNANDO VALLE SOL CAL Posts: 1,064	Re: Vintage Air voltage drop Ac Compressor Pulling To Many Amps, You need A 160 Amp Alternator __________________ 1966 GMC K25 V6 351E Original Utility Truck 2000 C3500 Hot Rod Utility Truck 2003 ROAD KING 100th 124ci 6 Speed 145CI M/P Pro Street FXR 2002 FXDP 2007 2500HD DURAMAX. DD Studio Teamsters / Mechanic Retired GM Master Tech 20 Years Retired Ford Tech 15 Years

03-24-2020, 12:40 PM	#8
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: Vintage Air voltage drop I think the compressor doesn't pull amps does it? It just grounds and engages the clutch. Is that correct? Unless a computer controlled vehicle has something else? __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

03-25-2020, 09:22 PM	#11
jayoldschool Moderator Join Date: Dec 2012 Location: ON, Canada Posts: 2,154	Re: Vintage Air voltage drop Rule out a loose/slipping alternator belt before worrying about anything else.

03-25-2020, 10:42 PM	#12
69 C10 LWB Registered User Join Date: Jan 2012 Location: St Louis Mo Posts: 214	Re: Vintage Air voltage drop It sounds like you have a voltage drop or bad ground to the cab. If you still have 14 V at the battery your alternator is probably fine. If you had a voltage drop on the fan wire your fan would be slow but your voltmeter should not drop. It sounds like you may have a bad connection where the main power wire goes into the cab or possibly at the horn relay if you still pull cab power from there. Could also be a bad ground to the cab.



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