Heater issues 1987 4.0
10-13-2023, 10:55 AM
#16
Junior Member
Thread Starter
After hours and hours of googling, before and during my troubleshooting I finally found someone with the same problem… https://www.cherokeeforum.com/f2/low...2/#post3179296
Unfortunately it looks like I have to pull out the AC and HVAC and replace my heater core… 
I finally got around to (or more accurately, decided to pay for) figuring out the problem on my '01. As a reminder, I had good airflow with the blend door in the cold position, but the flow dropped off as the door was moved to hot.
The solution started as CCKen said to golftoad. Taking out the blower resister block showed a mouse house on the evap coil. After removing the heater box we could see that the evap coil was about half obstructed. But that would reduce air flow at any temp setting, so that couldn't be it (although it certainly needed to be cleaned out).
There was a little bit of debris (that had snuck past the evap coil) on the air inlet side of the heater core, but not enough to make a difference. But holding the core up to the light you realized that you couldn't see light through it! A bunch of fine debris had come past the evap coil and worked its way deep into the heater core fins. I decided to replace the heater core and the (leaking) evap coil with new while everything was apart. Now there's good air flow at any temp setting!
Just thought I'd close this out for anyone who stumbles on it later.
The solution started as CCKen said to golftoad. Taking out the blower resister block showed a mouse house on the evap coil. After removing the heater box we could see that the evap coil was about half obstructed. But that would reduce air flow at any temp setting, so that couldn't be it (although it certainly needed to be cleaned out).
There was a little bit of debris (that had snuck past the evap coil) on the air inlet side of the heater core, but not enough to make a difference. But holding the core up to the light you realized that you couldn't see light through it! A bunch of fine debris had come past the evap coil and worked its way deep into the heater core fins. I decided to replace the heater core and the (leaking) evap coil with new while everything was apart. Now there's good air flow at any temp setting!
Just thought I'd close this out for anyone who stumbles on it later.
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AW-100 (10-13-2023)
10-13-2023, 08:01 PM
#18
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Year: 1990
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rats nest in the blower motor/ ducting?
10-14-2023, 06:11 AM
#19
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Thread Starter
Pulled the blower motor, all empty.
AC evaporator
10-14-2023, 06:14 AM
#20
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Thread Starter
10-14-2023, 07:34 AM
#21
Senior Member
Yeah, that's the hair-brained idea; somehow adapt the fittings on the AC Evaporator to connect to the Heater Hoses.
Pressure in the Low Side of an AC system is around 30 psi, so roughly twice that of the cooling system.
As with every other hair-brained idea, who knows until you try it...
Pressure in the Low Side of an AC system is around 30 psi, so roughly twice that of the cooling system.
As with every other hair-brained idea, who knows until you try it...
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AW-100 (10-14-2023)
10-14-2023, 07:52 AM
#22
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Year: 1990
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If both hoses are about the same temp, the heater core is flowing and you should have heat!!
It would be nuts to change the heater core at this point.
Heat is controlled by the blend air door which is cable operated.
Where the heat goes is controlled by vacuum, like dash or defrost for the most part.
The movement of the blend air door can be monitored by poking your noggin up under the passenger side dash and moving the Heat/Cold lever with your hand at the same time.
It would be nuts to change the heater core at this point.
Heat is controlled by the blend air door which is cable operated.
Where the heat goes is controlled by vacuum, like dash or defrost for the most part.
The movement of the blend air door can be monitored by poking your noggin up under the passenger side dash and moving the Heat/Cold lever with your hand at the same time.
10-14-2023, 08:02 AM
#23
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Thread Starter
I have checked all the vacuum switches in the driver side footwell, and the wire operated lever in the passenger footwell. Everything works perfect, except when I switch from cold to warm on the temp selector the flow of air stops. Only logical explanation I can find is that the air channels through the heater core is clogged, so when the heat selector plate direct air towards the heater core air cannot penetrate the heater core.
10-14-2023, 08:10 AM
#24
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Sure looks clean in the photo though.
What comes out the defrost?
What comes out the defrost?
10-14-2023, 08:35 AM
#25
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Good flow of cool when the selector is to cool and upper set to DEF. Barely anything if I switch to warm.
10-14-2023, 10:18 AM
#26
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That's a good piece of info.
10-15-2023, 04:19 AM
#27
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I probably should replace the evaporator when I remove the dash, but it seldom gets so hot here that I need AC. Is it possible to replace the heater core without disconnecting the AC, or do I need to take out the entire heater box for access?
10-15-2023, 05:41 AM
#28
Senior Member
The Heater Core is at the front, up against the firewall, directly over the transmission tunnel... There's a screw securing the Heater Core to the Box, right out at the firewall. It's down between the two tubes which are stacked on top of each other, and the other wall of the box is maybe an inch away. It's not fun getting that screw out when the HVAC box is out of the vehicle, let alone if it was in the vehicle. I don't have every tool, thankfully, and it was just easier to split the two halves of the box than to try and finagle that screw out otherwise. With something akin to an ratcheting Ford Ignition Wrench, you may have decent luck. Necessity is the mother of invention and I'm not saying it's impossible to get that screw out while the Box is in the vehicle, but it's going to be a PITA.
But first, I should note, that to even get to the point of considering whether you can get to that screw or not, you'll need to remove the Dash Panel entirely after disconnecting the Dash Wiring Harness from it, meaning the Gauge Cluster, Switches, Stereo, etc, need disconnected. You'll also need to remove the Transmission Shifter to give it room to swing out. With the 90 degree fittings on the ends of the AC hoses, I think you'll be limited to how far out you can pull the box, but you may need to unbolt some of the AC components under the hood to get enough slack in the lines.
But first, I should note, that to even get to the point of considering whether you can get to that screw or not, you'll need to remove the Dash Panel entirely after disconnecting the Dash Wiring Harness from it, meaning the Gauge Cluster, Switches, Stereo, etc, need disconnected. You'll also need to remove the Transmission Shifter to give it room to swing out. With the 90 degree fittings on the ends of the AC hoses, I think you'll be limited to how far out you can pull the box, but you may need to unbolt some of the AC components under the hood to get enough slack in the lines.
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AW-100 (10-15-2023)
10-15-2023, 07:31 AM
#29
Senior Member
A maybe silly idea but if you think the heater core might be blocked externally by mouse nest or whatever I would try to blow air backwards thru the system with a leaf blower or similar before I pulled the HVAC box out. At least see if anything flies out at the fan housing.
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cruiser54 (10-15-2023)
10-15-2023, 08:06 AM
#30
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That’s not a bad idea at all! I won’t be able to see anything blow out as the AC evaporator is in the way, but at least I can see if I can get any debries pushed loose…