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1973 - 1979 F-100 & Larger F-Series Trucks Discuss the Dentsides Ford Truck

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  #1  
Old 12-11-2011, 08:16 AM
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Heater not getting warm

Hey all, my heater is not getting very warm. I have added cardboard to the radiator, helped some. I have also checked both hoses coming from the heater core and they are hot. I just picked this truck up from my dad last week, so I will have a lot more questions, but since it is COLD in Wyoming right now, heater is next priority. Any ideas?

1979 F250 Ranger Lariat 4X4 , 400 6.6L, no factory a/c.
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Old 12-11-2011, 08:39 AM
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check the thermostat it could be broke. the spring will be broke if it is.
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Old 12-11-2011, 09:44 AM
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78/79s used a big ol vacuum switch operated by the dash levers. The big switch then directed vacuum to actuate small switches to operate the various doors and ports. It'll likely be a disconnected vacuum hose, a bad big switch, or a bad actuator.

It looks something like this but this diagram is for a 75. Yours will differ slightly. Dennis Carpenter sells yours for $50:

Click the image to open in full size.
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Old 12-11-2011, 10:01 AM
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If the hoses to the heater core are getting hot, that implies the heater core itself should be getting hot in which case I agree with HIO Silver, take a look at your ducting & blend doors and see if something is amiss there.

In the 80s models, the door & hinge assemblies have been known to break over time, the 70s models may or may not share that characteristic but you can crawl under the dash and investigate.
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Old 12-11-2011, 11:16 AM
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are BOTH hoses hot?
if so then see above
if one is hot and the other not really
then the core could be partially blocked

take off both hoses
get your garden hose and take turns blasting water into both sides of the heater core back and forth for a few seconds each
the water will come out milky white at 1st , then gradually clear up

i've seen some people have good reults, using an air hose with a rag around the tubes and use lower airpressure to help loosen the scale up along with the water
but you need to be careful doing so
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Old 12-11-2011, 01:54 PM
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Mine blows constantly through the defrost, vacuum switch behind the controls is bad. You can slide the heater control **** through the different positions and feel no detents where it should stop. I got under the hood the other day and manually operated the vacuum motor inline on the heater hose that allows flow into the heater core and now it will blow heat. I have the switch on order from Dennis Carpenter.
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Old 12-11-2011, 03:08 PM
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On the earlier (like my 74) models with A/C, the "temp" lever controls a cable-operated blend door. My 80 model is also setup this same way, and it has the non-a/c heater (dealer added a/c). Would a 79 also have the cable operated blend door? I ask, as my 95 also has a cable operated blend door (dealer added A/C yet again)......

I, too, had to manually open the vacuum operated heater control valve to get hot water to flow thru the heater core on the 74
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Old 12-12-2011, 10:44 PM
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Quote:
Originally Posted by jmm View Post
check the thermostat it could be broke. the spring will be broke if it is.
Pulled the Thermostat and tested it, it opened to about 1/3" at 200 degrees. Replacing it since I am already in there. Spring was not broken.

Guess I need to start checking the vacuum switches and heater internals. Was sure hoping it would be easier than that.
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  #9  
Old 12-15-2011, 09:05 AM
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OK, not all the results are in, but I pulled the old thermostat and it was a 180 rated, OEM was 192, which I replaced it with. Drove my truck last night with a bit of a warm-up and the heater seemed much better (and engine temp got up to normal range). Won't know until a really cold day, or driving over the pass, but it is an improvement so far.

On a side note, when I was digging around in the vents I found a bolo tie that I dropped thru the defrost vents over 20 years ago, finally worked it's way thru the floor vent.
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Old 12-15-2011, 01:38 PM
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Quote:
Originally Posted by Rogue_Wulff View Post
On the earlier (like my 74) models with A/C, the "temp" lever controls a cable-operated blend door.

I, too, had to manually open the vacuum operated heater control valve to get hot water to flow thru the heater core on the 74
1973/79 F100/350 & 1978/79 Bronco with factory installed integral A/C. The A/C is combined with the heater, the A/C controls are in the same panel as the heater controls.

D5TZ-19B888-A .. A/C Vacuum Control Selector Valve Assy (Motorcraft YH-285).

This valve bolts to the backside of the A/C Heater control panel. All the vacuum hoses for the four blend door vacuum motors and heater water valve connect to it.

When you slide the lever on the control panel, this valve opens one blend door, closes another. This valve also has the "kicker" switch for the A/C compressor mounted to it.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
There are two other A/C systems offered for 1973/89 F100/350 & 1978/79 Bronco. Both were dealer installed, one is combined with the heater (Deluxe), one was not (Economy).

The A/C controls on both Deluxe and Economy A/C are separate from the heater controls.
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Old 12-15-2011, 01:53 PM
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Quote:
Originally Posted by NumberDummy View Post
1973/79 F100/350 & 1978/79 Bronco with factory installed integral A/C

D5TZ-19B888-A .. A/C Vacuum Control Selector Valve Assy (Motorcraft YH-285).

This valve bolts to the backside of the A/C Heater control panel. All the vacuum hoses for the four blend door vacuum motors and heater water valve connect to it.

Also has the 'kicker' switch for the compressor mounted to it.
Maybe so. But, this truck has factory (integral) A/C, even says so on the original window sticker, but the temperture selector operates a cable that goes to the blend door, on the top of the A/C/heater box under the hood. I'd take a pic, but my camera died. Everything else is vacuum controlled.

As for the vacuum heater valve, it was just stuck. Likely due to having not moved in over 3 years and the associated corrosion that formed around the level's exterior. Perfectly understandable.....
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Old 12-15-2011, 02:02 PM
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There is no cable shown in the pic or listed in the text section of the parts catalog. The blend doors are vacuum operated, controlled by the selector valve.

The vacuum controlled heater water valve (D4AZ-18495-A ~ Motorcraft YG-136) is a notorious POS. Gets stuck, usually in a closed or partially open position.

It's available from Ford since it has been a "best seller" for over 40 years (D4AZ-18495-A replaced D1AZ-18495-A).
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Old 12-15-2011, 03:30 PM
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D3TZ-18518-A A/C (temp) Regulator cable fits 73-79 F100/350 with factory integral A/C and 78-79 F100/350 with deluxe hi/low ventilation system.
Opens/closes a door to direct air thru or around (and anywhere between) the heater core.

Source: 1999 Obsolete ford parts inc. catalog (no longer in business under this name) covering 48-79 pickups and 66-77 Bronco, they were located in OKC, Ok.

Please verify this number. Any other vehicles it might fit would not be listed in my dusty old book, as it was only concerned with the specific models listed.
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Old 12-15-2011, 04:20 PM
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Mine has the cable as well, hooks to a blend door in the fact A/C box under the hood.
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Old 12-15-2011, 04:22 PM
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"As for the vacuum heater valve, it was just stuck. Likely due to having not moved in over 3 years and the associated corrosion that formed around the level's exterior. Perfectly understantable..."

Same with mine, I bought a cheap plastic valve from the parts store but mine started working after being manually operated and with the help of some PB blaster.
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Old 12-15-2011, 04:22 PM
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