Defroster without AC

[tkleonard71]

I got a 1985 GMC Sierra with a reman 350. The AC is not hooked up and missing some components but I was wondering could I run just the defroster? The fan and heater box is there but not working.

 

[Bextreme04]

I got a 1985 GMC Sierra with a reman 350. The AC is not hooked up and missing some components but I was wondering could I run just the defroster? The fan and heater box is there but not working.

yes, but it won't be nearly as effective as it would be with the AC functional. Elk hunting in November here in Oregon was a little rough without the AC functional the one year I tried it. It takes a long time to clear the windshield of moisture without the AC to dry the air out. If you live in a drier climate, it likely won't be a major issue. Cold and moist is the worst for this.

 

[75gmck25]

Also check under the hood and make sure you still have the vacuum lines and vacuum reservoir that connect to the HVAC dash controls. Without vacuum you can’t switch the air flow between dash, heat, defrost, etc.

Many guys start pulling out vacuum lines and devices under the hood because they think it’s all “emissions crap”, and then find out their A/C and cruise control no longer work.

 

[tkleonard71]

Got any pics of what I should be looking for?

 

[tkleonard71]

yes, but it won't be nearly as effective as it would be with the AC functional. Elk hunting in November here in Oregon was a little rough without the AC functional the one year I tried it. It takes a long time to clear the windshield of moisture without the AC to dry the air out. If you live in a drier climate, it likely won't be a major issue. Cold and moist is the worst for this.

Thanks for the response. Yeah I’m in central Texas and it gets pretty cold during winter! Would you recommend trying to piece back the original ac or after market?

 

[75gmck25]

The box under the hood has two key electrical components.
- The resistor pack for lower blower speeds is inserted into the plenum (held with two screws) and it uses the dash switch to provide the lower speeds on the dash switch.
If you have no low speeds, start there with troubleshooting.

- the high speed relay (sliver box shape) provides the blower high speed. It has its own separate 12 volt supply and fuse ( I think they still did it that way in ‘85), so the full power does not go through the dash switch. That relay is activated if the dash switch is on high, or if the A/C is set to max recirculate.
If high speed does not work, start with this relay.

Under the hood there is a vacuum reservoir for the vacuum actuators used in the HVAC, and I think in ‘85 it is the same vacuum pod used with the cruise control. It will have a vacuum supply line running from the intake to the reservoir to provide engine vacuum. Then there will be a small diameter hard plastic vacuum line that runs from the reservoir and through the firewall over near the HVAC plenum. Under the dash that hard plastic vacuum line connects to the dash controls.

When you move the dash air control slider it provides vacuum to different vacuum actuators, and that moves the doors that control air flow. IIRC, if there is no vacuum applied, the default air flow will be about 1/2 to floor heat and 1/2 to defrost.

The amount of heat is controlled by a cable running from the dash to the heater box. It does not rely on vacuum. If it does not work well, it’s often because junk fell down the defroster vents and it’s blocking module movement of the flapper(s) inside the box. Mice also love to build nests inside the box.

 

[Bextreme04]

Thanks for the response. Yeah I’m in central Texas and it gets pretty cold during winter! Would you recommend trying to piece back the original ac or after market?

I was stationed at Ft. Hood for quite a few years. I would definitely try and get the old system working again. It isn't very expensive even to just buy new replacement major components. The only thing you would need that I haven't seen available new would be some of the steel lines and the interior/exterior HVAC boxes. The evaporator, condenser, compressor, switches, soft hoses, filter/dryer, blower motor, relays, etc... were all available as new parts last time I checked. I fixed my system a few years ago and it wasn't too bad. Still works fine and will freeze you right out of the cab in the summer time. You just need to flush all lines and components that have been in there and open to the atmosphere. Then replace all O-rings with the green ones, replace filter/dryer and orifice tube, then fill components with appropriate amounts of PAG 150, pull a vacuum, and charge with R134 at 75% of the specified R22 amount.

 

[Ricko1966]

I got a 1985 GMC Sierra with a reman 350. The AC is not hooked up and missing some components but I was wondering could I run just the defroster? The fan and heater box is there but not working.

Yes you can run just the defroster,cars came new without A/C but still had defrosters they'll keep Ice and snow off the wind shield,they will not defroster near as well as a car that has working A/C to dehumidifier the Air. I have and have had several vehicles that came new without A/C and still appreciate my defrosters,but lots of times I've have to wipe the windshield with a rag,depending on weather conditions. To add relevance my daily never had air and I appreciate having my defroster working with just heat.

 

[Grit dog]

Thanks for the response. Yeah I’m in central Texas and it gets pretty cold during winter! Would you recommend trying to piece back the original ac or after market?

You’re getting ahead of yourself I think.
Right now you’re somewhere between a bunch of diagnosis and fixing what’s broke to get a fan blowing hot air on the windshield vs spending $1000 to $2500 to rebuild OE AC or gut the whole mfer and install aftermarket heat and AC.
The stories about days gone by and such are largely irrelevant. It’s more about what are you using the truck for and how soon before you need working heat/defrost because now you got nothin according to the info you provided.
And no you don’t need AC for defrost as stated above by others.
Again…goals. Do you need a daily, fully functional all weather jump in turn the key and go, hit the freeway about the time the temp needle comes off of cold with 4 clear windows, after brushing wet snow off the windshield on a 33 deg wet day. Or ??

None of this matters though until you fix the “fan and heater box is there but not working” part. Either by getting it working in a minimal fashion, or a full rebuild of everything or charging about $2500 in parts today on the ole American Express so you can start installing vintage air and rebuilding the heater.

 

[Ricko1966]

Got any pics of what I should be looking for?

How about some pics of what you have. The we can tell you what you are missing.

 

[Moodyalaskan]

Open a door window an inch or so to let the moist warm air out of the cab, helps speed up the defrost time. Did it for years in vehicles with no a/c.

 

[Ricko1966]

Open a door window an inch or so to let the moist warm air out of the cab, helps speed up the defrost time. Did it for years in vehicles with no a/c.

Never knew that will get a chance to try it tomorrow.

 

[electoqute]

I got a 1985 GMC Sierra with a reman 350. The AC is not hooked up and missing some components but I was wondering could I run just the defroster? The fan and heater box is there but not working.

If your near Fort Worth let me know. Just got done rebuilding my a/c boxes and controls. Got wiring and vacuum diagrams and parts lists to help out.

 

[Dejure]

Look on the bright side: Without AC sucking the moisture out of the air, as pointed out, you might have to keep a towel or something around to remove moisture. At the same time, off-gassing in the cab will leave a film on your window interiors, even if no one smokes in the rig. That messes with your night time driving by contributing to glare. By wiping the window down, even if you don't wash it, some of that film will be removed, improving night time vision.



On a side note, like others, all my rigs were AC free and they worked just fine. Even in the higher humidity of the Pacific Northwet.

 

[Dejure]

Open a door window an inch or so to let the moist warm air out of the cab, helps speed up the defrost time. Did it for years in vehicles with no a/c.

Yeah, AC, wing vent, same thing on these years, right?