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Heater stuck on defrost


99xlt4.0

Member
Joined
Jul 21, 2024
Messages
5
City
Chilliwack
Vehicle Year
1999
Transmission
Manual
Hey guys. I'm new to the forum and greatful to be here! We recently got my wife's old truck back from family farm. Her pap bought this 99 Dark green 4.0 stepside manual transmission truck new for 25 years ago. Got a Lil issue with the heater not switching from windshield to the centre dashboard ducts. I been digging around on the net and I see guys have been fixing the vacuum lines. I didn't see to much info on the 4.0 vacuum lines. Any help is very much appreciated as it freaking hot. THX
 
there is one that comes from the passenger side fire wall.....follow that critter and see whats going on.
 
Thanks yes I will. I was thinking I'll mooch around in that area again and check connections etc,as fir a while I noticed that it would sometimes be venting correctly and then randomly change to defrost so I thought it may be a connection?
 
hmmm....could be a few things. maybe something else is opening and causing a vacuum variation ect....some systems have electric blend doors. though a 99 i am guessing is a combo.
 
It does have the electronic blend door actuator.
20240721_183956.jpg
I have seen videos on cleaning these things maybe a clean could fix the issue?
 
they pop gear ect....fairly cheap and usually not a total horror show to replace.....but sometimes...ewwwww
 
i have had to pull straws and pens out as culprits dropping down from the defrost duct on the dash quite a few times... especially in the fullsize gms that were jammin up the doors.
 
I see the screws holding the actuator at the rear are difficult to get at. This thing is now blowing cold through the vents, the problem comes n goes.
 
does it also have a heater hose bypass?
 
so it may be more then 1 issue...... ballz.
 
Could be a leaking vacuum reservoir or something. If the reservoir is bypassed it could also be the issue. 99 vacuum bulb/reservoir is on the passenger side underneath where the splash guard is underneath your coolant and windshield wiper fluid. You would need to remove the splash guard to get to it.

When exactly are you losing the air to the defrost vents, and when does it come back. This is very relevant to trying to diagnose cause. Also is it a 4x4 as this expands possibilities and placement of specific relevant parts.

As a side note I don't see why it would be the blend door since that should not affect where the airs final destination is actually going, just the mixture of the air. Here is a schematic I only spent two minutes looking at it, but it should tell you most or all you need to know about the system.
Funny thing is I have had to deal with this or a similar issue on all five Rangers I have owned. Smoke testing is typically the fastest check.

Good luck, and tell me if any of this helped.
 

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