Heater not blowing much?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

teeitup · 03-10-2003, 10:55 PM

My heater and defroster doesn't seem to blow very much. It has the ac control but ac is not hooked up. The fan only has 2 speeds also. I've blocked off the ducts going to the ac vents but it doesn't seem to blow high and not very hot. Any ideas??? Thanks Randy

Hutch · 03-10-2003, 11:27 PM

Broken Ground wire from the high speed fan relay to the cab.
Located behind the glove box near the top of heater box close to passenger cowl.
Also My high speed quit working the other day and I replaced one of the dash fuses (not sure which one it was) and wallah high speed again.

You probably also have a birds nest inside the a/c box restricting the air flow.

Have fun

Gene · 03-10-2003, 11:31 PM

Or it may be a bad high speed relay itself from a leaking windshield. I had this same problem, the high relay is located directly behind your glove box. Took mine off and it was completely rusted inside.

JimKshortstep4x4 · 03-11-2003, 08:38 AM

Is your diverter box door working properly? If it does not move with the control than it shuts the heater/defroster ducts off. I noticed that you had blocked the ac vents off according to your post and it sounds like the air flow may be directed to them instead of the heater/defrost.

Out of the (5) trucks with ac that we have, all (5) diverter box doors had to be repaired.

Jim

mrein3 · 03-11-2003, 08:50 AM

A week or two ago I topped off and burped the radiator on my truck. Holy cow is the heater hot now!

I think I'm going to try to find a recovery system from a new vehicle. I think the truck simply puked out enough and got enough air into the system that the heater started to get weak.

On the high-speed fan GM cars from this vintage had a seperate wire feeding the fan with its own fuse. Look for an orange wire with an in-line fuse. You may have blown that.

1971LT1SWBPOSI373 · 03-11-2003, 09:57 AM

My truck has 3 fan speeds, lo, med, and high. Sometimes I have to push the lever over to the extreme right to get it to blow on high. There is a switch on the back side of the control panel that is notorious for going out (3 speed switch) and is pretty easy to change, and available from any of the truck places. I guess the first thing to check would be to see if you can get it to blow in the high position, or jumper the switch to verify it's bad.

oldcarbuff · 03-11-2003, 11:02 AM

My fan has three speeds, but I would put my guess on something blocking the flow through the heater core. Leaves, mouse nests, you know--that kind of nice stuff.

Mine randomly blew out leaves for about 6 weeks after winter hit. I don't know where the previous owner had parked it, but it was apparently in the shade.

OCB

19mike69 · 03-11-2003, 11:28 AM

mine wasnt working on high and i traced it to a bad relay,the one behind the glovebox everyone is talking about

03-10-2003, 10:55 PM	#1
teeitup Just like to part out trucks!! Join Date: Nov 2001 Location: Meriden, Ks-North of Topeka 10 miles Posts: 27,396	Heater not blowing much?? My heater and defroster doesn't seem to blow very much. It has the ac control but ac is not hooked up. The fan only has 2 speeds also. I've blocked off the ducts going to the ac vents but it doesn't seem to blow high and not very hot. Any ideas??? Thanks Randy __________________ 67 Chev swb stepside 4x4 283 4 speed SOLD 72 blazer 4x4 4 speed Cheyenne 72 GMC swb 4x4 fleet 4 speed, original paint 1958 Cameo (one of 1405 built, it was my dads) 2020 chevy Apex 4x4 crew cab 1969 Z28 with the 302 1980 chevy swb 4x4 1964 chevell Malibu SS 1976 K20, sold new originally out of my dads chevy dealership Members that I've met, over 200 probably closer to 300 in the 12 years I've been on this site, everyone I met "It was a pleasure"

03-10-2003, 11:27 PM	#2
Hutch Registered User Join Date: Oct 2001 Location: Grapevine, Tx. Posts: 2,142	Broken Ground wire from the high speed fan relay to the cab. Located behind the glove box near the top of heater box close to passenger cowl. Also My high speed quit working the other day and I replaced one of the dash fuses (not sure which one it was) and wallah high speed again. You probably also have a birds nest inside the a/c box restricting the air flow. Have fun __________________ John Hutchison Grapevine,Texas 68 1/2 t swb 68 Mustang 69 Jaguar 71 1/2 t swb 88 Supra CompTs6m 93 Escort GT 2005 Corolla XRS

03-10-2003, 11:31 PM	#3
Gene Registered User Join Date: Aug 2002 Location: Muskogee, Oklahoma Posts: 2,914	Or it may be a bad high speed relay itself from a leaking windshield. I had this same problem, the high relay is located directly behind your glove box. Took mine off and it was completely rusted inside. __________________ 1971 SWB 4X4 Build BOARD MEMBER #5448 BOOMER SOONER!!!!

03-11-2003, 08:50 AM	#5
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,253	A week or two ago I topped off and burped the radiator on my truck. Holy cow is the heater hot now! I think I'm going to try to find a recovery system from a new vehicle. I think the truck simply puked out enough and got enough air into the system that the heater started to get weak. On the high-speed fan GM cars from this vintage had a seperate wire feeding the fan with its own fuse. Look for an orange wire with an in-line fuse. You may have blown that. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

03-11-2003, 09:57 AM	#6
1971LT1SWBPOSI373 Registered User Join Date: Mar 2002 Location: Springfield, VA Posts: 460	My truck has 3 fan speeds, lo, med, and high. Sometimes I have to push the lever over to the extreme right to get it to blow on high. There is a switch on the back side of the control panel that is notorious for going out (3 speed switch) and is pretty easy to change, and available from any of the truck places. I guess the first thing to check would be to see if you can get it to blow in the high position, or jumper the switch to verify it's bad. __________________ 1971 Shortbed 350/350 w/ Posi-traction 3:73's 1996 Chevy Silverado TJ in Springfield, Virginia Likes old Chevy trucks Drinks Miller MGD

03-11-2003, 08:38 AM	#4
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Is your diverter box door working properly? If it does not move with the control than it shuts the heater/defroster ducts off. I noticed that you had blocked the ac vents off according to your post and it sounds like the air flow may be directed to them instead of the heater/defrost. Out of the (5) trucks with ac that we have, all (5) diverter box doors had to be repaired. Jim

03-11-2003, 11:02 AM	#7
oldcarbuff Registered User Join Date: Sep 2002 Location: Muncie, IN U.S.A. Posts: 60	My fan has three speeds, but I would put my guess on something blocking the flow through the heater core. Leaves, mouse nests, you know--that kind of nice stuff. Mine randomly blew out leaves for about 6 weeks after winter hit. I don't know where the previous owner had parked it, but it was apparently in the shade. OCB

03-11-2003, 11:28 AM	#8
19mike69 Registered User Join Date: Jul 2002 Location: pottsboro,tx Posts: 513	mine wasnt working on high and i traced it to a bad relay,the one behind the glovebox everyone is talking about

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