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  #1  
Old 05-27-2005
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HVAC only works on high

well yesterday on the way to work i noticed that my HVAC only works when i have it turned up all the way, on the three lower settings it does nothing, but once i turn it up all the way its blows out air, i have never had this happen before, anyone know what it might be

thanks
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  #2  
Old 05-27-2005
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About the only thing I can think of is that those A/C controls are vacuum operated. Might a small leak in one of your lines.
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Old 05-27-2005
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You blower motor resistor block could be open circuited. The highest speed bypasses the resistors.
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Old 05-28-2005
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is there a way i can check the blower motor resister?
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  #5  
Old 05-28-2005
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Quote:
Originally Posted by 9XLT8
is there a way i can check the blower motor resister?
Unplug the 4 wire plug to the resistor block (under the HVAC blower motor in front of the pass side firewall). Then measure resistance across any two pins of the block, not the connector. There should be finite, measurable resistance between any pair of pins. Try several combinations. If the resistance on any or all combinations is maxed out on your highest analog scale or reads OL on a DVM, the resistor block has an open circuit or circuits and it's no good.

The blower can still work on the switch's highest speed because it bypasses the resistor block completely in that mode.


This is not a Ranger resistor but it is typical of what they look like:


Last edited by V8 Level II; 05-28-2005 at 04:47 PM.
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  #6  
Old 05-28-2005
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I would bet its the heater blower resister as Bob said, Same thing went out on my jeep and my dads dakota. Both of them only worked in high. Easy fix though, $20
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  #7  
Old 05-28-2005
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Yep. I concur also.

I already got rid of mine, but someone who did the EATC mod may have their old one and you could pick one up cheap or free.
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Old 06-13-2005
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Quote:
Originally Posted by rwenzing
Unplug the 4 wire plug to the resistor block (under the HVAC blower motor in front of the pass side firewall). Then measure resistance across any two pins of the block, not the connector. There should be finite, measurable resistance between any pair of pins. Try several combinations. If the resistance on any or all combinations is maxed out on your highest analog scale or reads OL on a DVM, the resistor block has an open circuit or circuits and it's no good.

The blower can still work on the switch's highest speed because it bypasses the resistor block completely in that mode.


This is not a Ranger resistor but it is typical of what they look like:

What do you mean by "finite"?? Does that mean a close or widely apart numbers?? My ac/heater fan started to only work on high today, and I came across this thread while doing a search.
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Old 06-13-2005
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By finite, I mean not approaching infinite. In other words, you should be able to find a range on your multimeter that gives a numerical value of the measured resistance other than close to zero or over scale (OL on a DVM).

This is just a final check you can make before you decide to purchase a new resistor block. I'm 95% sure that the resistance wire is broken inside yours - it is a common failure on this part.
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Old 06-13-2005
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Quote:
Originally Posted by rwenzing
By finite, I mean not approaching infinite. In other words, you should be able to find a range on your multimeter that gives a numerical value of the measured resistance other than close to zero or over scale (OL on a DVM).

This is just a final check you can make before you decide to purchase a new resistor block. I'm 95% sure that the resistance wire is broken inside yours - it is a common failure on this part.
I did a search over on RPS also, and I guess that these things are know to corrode inside and start to fall apart. So I guess that it's worth the $15 just to replace it then.

Thanks for the lightning fast response.
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  #11  
Old 06-14-2005
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[QUOTE=rwenzing]Unplug the 4 wire plug to the resistor block (under the HVAC blower motor in front of the pass side firewall).[QUOTE]

what do you mean "in front"? like in the cab?
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  #12  
Old 06-14-2005
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Quote:
Originally Posted by 9XLT8
Quote:
Originally Posted by rwenzing
Unplug the 4 wire plug to the resistor block (under the HVAC blower motor in front of the pass side firewall).
what do you mean "in front"? like in the cab?
Look under the hood on the front of the firewall on the passenger side and find the plug in this picture:

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  #13  
Old 06-14-2005
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cool deal i'll look for it tommrow
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  #14  
Old 11-02-2005
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Ok, I've got a weird one, must have been that trip to Jackson. :) My blower started to act up this morning. Low, Med Low and High are working just fine. Med High is not. When I have the fan speed *** on Med Hi it blows the same as Medium Low.

I'm looking at the wiring diagram and the rheostat is in series, goes through 3 resistors for low, 2 for med low and 1 for med high. If my resistor was bad I shouldn't be able to get low or med low at all. The circuit would be open.

That leaves two things. Short/open circuit on circuit 752 or the blower switch is bad. Anyone else have any suggestions for other things to check?
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  #15  
Old 11-02-2005
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Where can you actually get these resistors?
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  #16  
Old 11-02-2005
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You can't get them seperately they are part of the blower resistor assembly. You have to replace the entire assembly.
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  #17  
Old 11-02-2005
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no you dont, i just found a site that sells them seperately.
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  #18  
Old 11-02-2005
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Quote:
Originally Posted by BrianB31
Ok, I've got a weird one, must have been that trip to Jackson. :) My blower started to act up this morning. Low, Med Low and High are working just fine. Med High is not. When I have the fan speed *** on Med Hi it blows the same as Medium Low.

I'm looking at the wiring diagram and the rheostat is in series, goes through 3 resistors for low, 2 for med low and 1 for med high. If my resistor was bad I shouldn't be able to get low or med low at all. The circuit would be open.

That leaves two things. Short/open circuit on circuit 752 or the blower switch is bad. Anyone else have any suggestions for other things to check?
You're right, Brian, it can't be the resistor elements. If it's an open on 752, it looks like it should revert to the low setting, not medium low. Maybe you just have a high resistance connection somewhere in 752 that's slowing the motor down, The connection right at the resistor block is known to corrode.

Last edited by V8 Level II; 11-02-2005 at 12:34 PM.
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  #19  
Old 11-03-2005
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Quote:
Originally Posted by FoMoCoStang
no you dont, i just found a site that sells them seperately.
what might this web site be, because i still need to fix mine i have just been to lazy to do it but with it getting cold and all i would like my heat to work on something other than HIGH
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  #20  
Old 11-03-2005
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Quote:
Originally Posted by 9XLT8
what might this web site be, because i still need to fix mine i have just been to lazy to do it but with it getting cold and all i would like my heat to work on something other than HIGH
You can get a resistor block assembly (#7 )at a Ford dealer or parts house. $16~20 list.

Online for about $12 + S&H from FordPartsCheap

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  #21  
Old 11-04-2005
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well it looks like i will be taking a trip to the ford dealer sometime
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  #22  
Old 08-26-2006
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I realize this is an old thread. I figured I would tell about my experience. My blower motor resistor failed. I could not un plug the reistor from the harness. It was rusted together. After I finally got it apart the pins were stuck in the harness. I figured I am screwed now. I was going to cut the connector off and just put individual connectors on. I went to the auto store to get the connectors I needed and found the connector that goes to the wiring harness just like the one my pins were stuck in. It was even called a ford truck blower motor resistor plug. Must be a common problem.

What I am trying to say is dont spend an hour trying to get the plug off when it will be screwed anyway. Resistor at ford dealer 18.02. wiring plug 10.00. Total 28.02. Now my fan works on all speeds. Thanks to all who have posted.
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  #23  
Old 08-26-2006
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Thanks for posting about the replacement connector. It's good news because corroded terminals at that plug are fairly common. A dab of dielectric grease on each pin should help to keep it from recurring. Dielectric grease is also a good preventative maintenance step for the connectors of new trucks, too, especially those that will see road salt.
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  #24  
Old 12-18-2008
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My fan only works on the lowest and highest settings so I replaced the blower motor resistor. It was so corroded I had to destroy the resistor in order to separate it from the connector. The pins from the resistor were stuck in the connector and I had to pull them out with needle nose pliers. I plugged in the new resistor and my fan still only works on the highest and lowest settings. Do I need to replace the connector? Is there a way to test the connector I have?
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  #25  
Old 12-18-2008
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