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Discussion Starter #1
My 1988 Cherokee's heater control slide switch has been stuck on Defroster for years. Working the slide will turn off the system (far left position), but no other air flow position works...no dash output, etc.

After reading through some old threads it sounds like Defroster is the default position for air routing when no vacuum is present to the switch and that a white vacuum line visible near the firewall in the engine compartment (behind the charcoal can for the fuel evap system) is the line that feeds vacuum to the switch.

Am I correct so far?

Well, there is no vacuum at the 'T' when I removed the white line. One end of the 'T' goes to a dead end and a fitting on the passenger fender wall...on a bracket of some sort. The other side of the 'T' fitting seems to be the feed side and the rubber hose runs into an 1/8" steel dog leg piece of tubing that runs under the charcoal can. I think there are 3 steel dog leg tubes grouped together in that location and it is the one nearest the front of the vehicle that has no feed line on it.

Yeah...I think I found the source of the 'lack of vacuum' problem...there is NO vacuum line at the other end of that steel dog leg! That would be the end facing the engine.

Can someone tell me where that vacuum line is supposed to run? Is it supposed to be 1/8" rubber hose and 'where' does it terminate for its vacuum tap?

Anyone have a link to the vacuum system diagram for the 1988 XJ?

Thanks in advance for any assistance.
 

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I think you're right on track with locating the culprit.

I don't know about on the Renix, but on the later H.O. models, there is a Catalyst Sticker on the firewall next to your brake booster with the vacuum diagrams printed on it.
Something like this...


That said, there isn't a whole lot of vacuum going through the lines. This is the main reason for the vacuum canister to actually "store" needed vacuum to help run the vents.

From what you've mentioned, it seems that from the 'T' forward, the only thing missing is the connection to the vacuum canister. The canister "should" be inside the front bumper on the passenger side. If it is no longer there or damaged, you should be able to locate one at your favorite local junkyard and relocate it inside the engine bay.

I grabbed a smaller round ball off a later cherokee for my '95.
vvv

ForumRunner_20141015_145758.jpg

Hope this helps
;)
 

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here, this may help:
 

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Discussion Starter #4
I can find all kinds of vacuum maps on the web, but nothing year-specific.

Something like this 'might' be close, but I hate to guess. The dash switch picture looks like my 1988, but the feed line into the switch from the reservoir is black and the line off it is not color coded.

 

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You connected to a vacuum canister? :unsure:

Can you post a pick of this "dogleg"?
 

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here are pics of the 88 vacuum to the HVAC. with and without AC.
 

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Discussion Starter #7
jfxj67, I'll try and get you a picture of the three steel tubes (what I'm calling a dog leg assembly). They are located mostly under the fuel system evap charcoal can (the plastic cylinder located on the passenger side inner fender), so getting a clear picture might be impossible.

They are used as a junction to go from rubber vacuum hose to a securely mounted piece and then back to rubber hose. Three ends point towards the engine and the opposite ends point up or towards the right side of the vehicle.

The white plastic vacuum tubing that supplies the dashboard slide switch terminates in a 'T'. The 'T' has a line that runs to the front most steel tubing dog leg. That is where I think there is a missing vacuum hose (going to the vacuum reservoir/canister?)...nothing further on that supposed vacuum supply.


gms, THANKS!

I'm going to stop at the parts store and pick up some rubber hose, plastic tubing, caps to block off any found open terminations/leaks (can't hear any, but I did find more 'missing' tubing connections that I will block just to be safe). None were drawing vacuum.

The two main sections of the vacuum harness on the right side of the engine were in good shape other than one cracked rubber factory splice boot and one small hole rubbed thru one of the 1/8" plastic lines. Both are temporarily sealed and will be booted with rubber tuber this evening.

Ah! the 1980's! When the manufactures were scrambling to meet changing emission standards with miles of vacuum lines!
 

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Discussion Starter #8 (Edited)
Questions:

What is the device in the lower left of the A/C equipped schematic that the white line runs to?

Any idea where is it located?

A black line runs from the vacuum reservoir to this device and a white line runs off the other side of it to feed the slide switch on the dash if I'm reading the drawing correctly.

Again, thanks!
 

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It's in both schematics. Almost looks like it turns vacuum to the heater control switch on our off, not really sure.
 

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Discussion Starter #10
Meh! No love!

I spent 2 hours getting greased up to my elbows courtesy of Jeep's no extra charge free engine compartment rustproofing system.

Other than getting filthy, I got no where.

The 'dog leg' 3 vacuum lines run down to the passenger side-rear face of the front axle. Something to do with the 4-wheel drive system and locking in the hubs?

The white hard plastic vacuum line that goes through the firewall beside a black hard plastic line in a rubber plug was the line(s) I 'think' runs the das slide switch...maybe? It runs to a 'T' and that line is dead...no vacuum.

I plugged the white line into a temporary vacuum tap I made into the main vac line running from the intake to the reservoir. No change. The slider never came off defrost regardless of the position the slider was moved to.

I found another white line, but it controls a rotary valve on the heater hose.
I think that one plugs into a yellow hard plastic hose inside a length of wiring loom cover (4 hoses in there...green, red and blue also). The blue hose terminated in some sort of plastic check valve and is just hanging loose! No vacuum on it so no leak.

I don't think the system has any leaks because the minute I unplug any active vacuum line the idle really picks up as the motor leans out.

I tried plugging a few 'dead' lines into my temporary tap, but nothing got the slider switch to come off defrost.

Anyone know with certainty which hose powers the slider and where it comes through the firewall?

I'm lost, but I'll get on it again tomorrow after work. :brickwall:
 

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I am still trying to figure out what the lower left thing is. :dunno:

The vacuum lines that go to the axle are for the disconnect on your front axle and to the t-case. They engage and disengage the front axle, most guys put a solid axle on the passenger side and do away with the vacuum disconnect because of all the problems. but your MPG's will go down.
 

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I'm still not fully understanding the need for all of the technicalities... Schematics, vacuum diagrams and whatnot?

Again, my question is "IS YOUR VACUUM CANISTER HOOKED UP"?

If it is NOT, it would seem to be your only problem.

Please clarify so we can help better?
;)
 

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Discussion Starter #13
gms, the 'lower left' thing...no idea, but it's key to figuring out the physical routing under the hood.

jfxj67, yes...the primary vacuum tap hose (3/16" I.D.) that runs from the intake, behind the engine along the firewall and forward along the top of the right inner fender to the reservoir (located behind the right/front bumper area) is connected and the line has good vacuum pull. No leakage in that line.

There is also a small (1/16" I.D.) hard plastic line that is black in color returning back from vacuum reservoir. This line is factory zip-tied to the primary vacuum tap line all along the right inner fender area. The hole in the fitting on that line is so small that when I unplugged it to test for vacuum I could get no conclusive feel.

This afternoon I will remove the splash guard at the bottom of the fender liner and inspect the reservoir from the bottom to check for leaks (doubtful due to idle checks by unplugging working hoses) and any blockage in that small hard plastic line.
 

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Sorry, i don't mean to sound like I'm beating a dead horse by adamantly insisting on a vacuum canister issue, but from what you're saying, I'm 98% sure that's the culprit.

Unless someone has been physically tampering with the vacuum lines in the dash, i honestly can't see this being the problem.

Again, the vacuum that helps runs the vents is hardly noticeable, hence the need to "store" the needed vacuum.

I suggest blowing cigar smoke through the lines to find the breech. Smoke will clearly flow out from the problem area... Most likely from a broken or cracked nipple on the vacuum ball where the line plugs in.

Just another thought
;)
 

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I am still trying to figure out what the lower left thing is. :dunno:

The vacuum lines that go to the axle are for the disconnect on your front axle and to the t-case. They engage and disengage the front axle, most guys put a solid axle on the passenger side and do away with the vacuum disconnect because of all the problems. but your MPG's will go down.
I just had the heater control out of my 85. The "item" in the lower left in the diagram is the control mounted on the back of the heater control. The dash slide operates this control turning vacuum to what ever setting is selected. However, I am having the same problem of not getting vacuum to the heater control. Also, I am not getting the blower motor to operate. So, I guess it will mean pulling the dash apart, again. Mine sat for about ten years. Nothing vacuum operated works.
 

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Discussion Starter #16
jfxj67, I'll focus in on the vacuum reservoir and that small tubing run that looks like it 'might' be the feed to the slide switch.


Yesterday afternoon was warm and I blew off wrenching to take a 36-mile bike ride. I should have wrenched.

When I got home from the ride I had the symptoms of either a severe case of Bonk...or Ebola. It's 9:30 AM and I just got out of bed still weak, sore and feeling like the Cherokee when it was running a tooth retarded!
 

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maybe you feeling the effects of a 36 mile bike ride? :rolleyes:
 

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FWIW, i recently grabbed a fresh set of heater controls for my '95 at the junkyard.


ForumRunner_20141018_092603.jpg

You probably already know that the vacuum harness easily unplugs from the back of the controls. It's what the harness from the controls is what idk about...



ForumRunner_20141018_093053.jpg

Considering i don't have a bicycle, my weekend project is tearing out my dash so i have more than just the MAX and NORM settings.
:rofl:
 

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Discussion Starter #19
The bike ride was actually a short after work ride. Yeah the steep climbing hurt but that flu had me down for the 10-count right thru Sunday evening.

I think I'm back in the game now.

I bought one of the Dorman plastic ball type vacuum reservoirs for $11 at Advance (they wanted $22, but price matched Summit Racing's price).

Wasn't worth a trip in the rain to the local bone yards (only to get raped out of $10 for an age hardened or cracked unit) at that price.

http://www.summitracing.com/parts/rnb-47076


If the CDC isn't here tomorrow to monitor my temperature I'll try plumbing the new res into the main vacuum line off the intake manifold and then try feeding every plastic line that looks like it might end up somewhere near a heater control.

Either that or join jfxj67 in a dash pulling party. Charging up my drill-drivers now might be a good idea because...

It suddenly started blowing only warm air at the defroster vent. 50° outside air temp, motor showing about 190° on the dash gauge, plenty of coolant and the flow rate looked good into the expansion tank. All the hoses under the hood felt hot enough...yup, probably going to end up tearing the entire dash out to put in a new heater core unless I find something else that suddenly made it stop blowing 'hot'.
 

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if the blend doors are working, and both your heater hoses at the firewall are hot, try just flushing out the core.
 
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