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:agree:
Haha, yet another common problem in our jeeps... Not so hot air blowing out the vents.

If the heater core isn't leaking onto your passenger floorboard, and you're not smelling coolant inside, your heater core is fine!

Try "backflushing" your heater core with a garden hose in Both directions to remove all the crap that is preventing the coolant flow through the heater core.
;)
 

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Discussion Starter · #22 · (Edited)
No leaks...no 'sweet' smell. Good advice. I will flush, back flush and flush some more! The dash disassembly procedure looks about as retarded as most vehicles these days.

I guess the old saying about the heater core being the first component placed on the assembly line is true!

I don't think any of the vacuum doors are currently working, but the cable-actuated blend door from the temperature slide switch on the dash is working and it 'was' blowing decently hot air over the last several weeks of that wonderful bucking / timing issue you guys helped me resolve.

While I was trying to diagnose the heater vacuum control I did gain access to the heater core-air conditioner evaporator area to check the blower resistor.

I found one hell of a mouse nest in there! I dug everything out that I could, but it was sitting on the floor of the blend chamber and did not block the air flow from the blower or the movement of the blend door. I think I will re-visit their condo and see if anything shifted or moved down there...might be some loose debris that got to where it shouldn't be.

I'll take the air hose and blow down the dash defroster outlets, the dash vents, the floor vents and then clean out and blow out the blend chamber.
 

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Discussion Starter · #23 ·
Hot air restored.

You guys really know your Cherokee's!

I flushed and back-flushed the heater core after work yesterday and the output temperature at the defroster vents was immediately restored to what it had been running. Thank you!

The most difficult part of the job was getting the hoses that had probably been on the heater core inlet/outlet tubes for the last 26 years off without damaging the core.

I don't know whether this core is the through-flow type or has a by-pass passage, but I saw nothing come out of it in the way of sediment or grit. Only clean anti-freeze and then clear water flowed over my fingers as I held them in the output streams to feel and look for anything.

As far as I can tell nothing came out of that core, but yet the cure certainly worked! Kudos to all you Jeep guys! :thumbsup:

I ran out to time after topping off with anti-freeze, but I'll get back on the heater switch fix ASAP.
 

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Hot air restored.

You guys really know your cherokees . Thank you!
We just hang out on the forums and read a lot... :animlol:

Glad you got that part fixed!
:thewave:
 

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Discussion Starter · #25 · (Edited)
Forward of the firewall vacuum diagnostics are as complete as I can do.

I partially removed the RF plastic bumper cap and gained access to the OEM vacuum reservoir. It is the exact same Dorman spherical unit as the one that I bought for diagnostic purposes.

I unplugged the main rubber vacuum line from the intake manifold and the smaller hard plastic vacuum line that feeds the vehicle. I fished the two lines up through the core support and hooked them up to my brand new reservoir. The lines were in good shape with no cracks or rub-through areas.

Unplugging the small hard plastic line at various points showed I had vacuum draw right to the firewall pass through.

Reinstalling the two lines to the OEM reservoir showed I still had vacuum back through the small hard plastic feed line so the OEM reservoir must still be in operable condition.

So...

I guess I now need to see if everything on the passenger compartment side of the firewall is connected and in working order.

A fella has to know...is C4 the preferred dash disassembly tool or will plain old dynamite suffice? I do have a new Milwaukee cordless SawzAll that could have that slide switch out of the dash in under two minutes!
 

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Hey, just another thought... You might also try blowing out the check valves that are connected in the vacuum lines. I think they're designed to allow air flow in only one direction...


Tire Auto part Automotive tire Wire Finger

Still hoping it's something as simple as that.
:dontknow:
 

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Discussion Starter · #27 ·
The three check valves I blew through all appeared to function properly. I also tested the heater vacuum feed lines (the black hard plastic one and the white hard plastic one that pass-through the firewall as a pair at the rubber plug just above the heater blower motor area on the right side of the firewall) minus the check valve on the feed line.

I put an extension rubber line on the hard plastic pass-through lines and blew through them with my mouth. There was no resistance or back pressure felt other than that of the small diameter tubing. Almost as if they were not connected to anything or at least to an open port or something.

I'll take my inspection mirrors and a couple of good lights and see if there is anything I can spot with the dash assembly still in place.

That blue hard plastic line with the check valve on the end of it...the one I spoke about earlier that is just hanging loose and unused...does anyone have any idea where that one is supposed to run to or what accessory it is supposed to supply vacuum to?
 

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Discussion Starter · #28 · (Edited)
Well the das was surprisingly easy to tear apart. I removed the radio and pulled heater control out about an inch (max. clearance due to all the hoses and wire connectors).

What did I find?

ANOTHER damned mouse nest!!! Built right under the heater control. Out fell an AMC build tag from June of 1988 off some unknown component...I thought Jeep was a Chrysler product back in 1988, but this one does scream AMC from the door handles to the layout of the dash.

Cleaned it up and found nothing obviously 'wrong' so far. I'll continue to dig into it this afternoon.

On the plus side, I did finally gain access to the long burned out tiny light bulb that illuminates that sweet LED screen clock!


FWIW, my Shade Tree Spidey Senses are still telling me the that blue vacuum line and check valve that run nowhere is the key to this mystery of no vacuum at the control switch.
 

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Discussion Starter · #29 ·
Success!

After finding nothing wrong, other than zero vacuum arriving at the distribution manifold that I had unplugged at the rear of the heater control switch, I decided to trace the vacuum source tubing (it IS the black hard plastic tube for any future folks reading this thread) one last time.

I don't know how I missed it...a 1/4" long hole was worn through the tubing about 6" from the firewall pass-through on the engine compartment side of the world! There is so little vacuum flow through that tiny tubing that no hissing noise was made nor did it affect idle RPM's.

The hole face the firewall so I could not easily see it, but I sure 'should have' felt it!

I cut, trimmed and sleeved the affected line with a 4" long piece of rubber vacuum hose, replaced the distribution manifold on the switch and instantly doors were opening and closing that had not worked (judging by the dust that flew out of the vent louvers) in the 14 years or so I have owned the Jeep.

In addition to replacing the clock's burned out light bulb, I also replaced the one that illuminates the heater control switch. Both use number 73 two-watt dash bulbs.

Thanks, again, to all who assisted and encouraged me! Regards and safe driving to you!
 

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Cool... Okay, better yet, WARM!

:thewave:
 

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Discussion Starter · #32 · (Edited)
Thank you, again!

I really appreciate the help and the schematic drawings you guys post are a BIG help to a shade tree wrench, like me.

My old Harleys and even the carbon fiber modern racing bicycles are dead simple compared to cars and trucks.

I heard from an A&P aviation mechanic friend of mine that the Renix engine management system used on my Cherokee is one of the most cursed at POS ever unleashed on the American car-buying public...and we all know there have been tons of components and electronics sold to us since 'they' decided to over-complicate our vehicles back in the late 1960's.

Anyone remember when Road Draft Tubes and Manual Chokes ruled the earth?
 

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Discussion Starter · #34 ·
M1A65, Good luck! Use a bright flashlight and inspect, then re-inspect every inch of vacuum tubing. Remove as much as you can. My leak was in a plastic tube that rubbed through the wall and no way could it be seen I
t faced the firewall. I could not hear the leak over the engine noise and I only found it by dumb luck after running my fingers over it for the third or fourth time.

That's why removal (and possibly replacement...the tubing is cheap) may be the smarter way out. I ended up replacing some of the rubber and hard plastic tubing that had become brittle with age and heat exposure. I also spliced a few kinda-sorta iffy looking areas when trying to track down that leak.
 

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Thanks Bob. Figured it was gonna be a pita!!! Looks like another weekend project added to the list!!!!! Gonna try blowing some cigar smoke into the lives and seeing iof anything leaks out.
 

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Discussion Starter · #36 ·
At least you'll get a good cigar out of the deal!

If your heat is not really 'hot' you might as well take the time to back-flush the heater core while you're at it. I flushed mine with a garden hose per the forum RKI's, reversing the flow thru both ports at the firewall, and that improved the temperature output quite a bit. The passenger compartment was much warmer and the defroster worked better.
 

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Success!

After finding nothing wrong, other than zero vacuum arriving at the distribution manifold that I had unplugged at the rear of the heater control switch, I decided to trace the vacuum source tubing (it IS the black hard plastic tube for any future folks reading this thread) one last time.

I don't know how I missed it...a 1/4" long hole was worn through the tubing about 6" from the firewall pass-through on the engine compartment side of the world! There is so little vacuum flow through that tiny tubing that no hissing noise was made nor did it affect idle RPM's.

The hole face the firewall so I could not easily see it, but I sure 'should have' felt it!

I cut, trimmed and sleeved the affected line with a 4" long piece of rubber vacuum hose, replaced the distribution manifold on the switch and instantly doors were opening and closing that had not worked (judging by the dust that flew out of the vent louvers) in the 14 years or so I have owned the Jeep.

In addition to replacing the clock's burned out light bulb, I also replaced the one that illuminates the heater control switch. Both use number 73 two-watt dash bulbs.

Thanks, again, to all who assisted and encouraged me! Regards and safe driving to you!
You just saved me from pulling my heater. I had checked all my vac lines and booster. Took them out and repaired several leaks. I knew the heater/AC control would work. But…no work. Still just blew out the defroster. Was ready to take out heater when I read your old post. Sure enough the short piece of black vac line that comes out of the firewall had two holes in it. The only place I hadn’t checked for leaks. It was about 15” long and tied into the small line leading to the canister via a small barbed coupling. I fixed the two leaks and now the air blows out the correct vents according to wherever I set the controls. I just had to thank you! You are the man!!!
 
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