Defrost adjustment knob not working

[JWA]

If I'm understanding your issue correctly it would seem you have a significant vacuum leak (or complete loss, symptoms unclear) somewhere between the intake manifold and the air flow system. In most cases tracing the vacuum line off the intake will reveal a cracked portion.

There is an access lid center of the dash board near the windshield:

Typical schematic for the vacuum system:

Image of the vacuum connection block leading into the electro-vacuum switch that controls the air flow ie: Vent, Heat, A/C, etc etc:


Check for vacuum at the black line of that block--if its present then you'll need to trace the remaining lines to their servos or motors and determine whether they're all working properly.

More than likely though you have little or no vacuum at the connection block--a very common problem with E-Series.

Hope this helps!

[c7train]

Wo! I'll get on that ASAP! Thanks for such a precise post!

[JWA]

Glad to help!

[c7train]

I finally got around to taking a look at the vacuum issue. I have good vacuum at all the lines with the exception of the blue and whit lines, so I'm not sure where to go from there, I see the lines disappear under the dash and go into the engine ay, but upon looking under the hood i see nothing. Can anyone give an idea of where the lines go in order to save some searching time? Thanks for all the help thus far, I was hoping for a simple fix, which I think it still might be, but just a smidge more complicated.
Cheers,
C.

[Scalf77]

[c7train]

Hmmm, that's interesting. I will compare things according to that key, any idea where the selector module that all the lines goes to might be? I do have vacuum at the black line, so I'm sorta lost as to where to go next. I'll do some digging and see what happens. Thanks for all the help!

[JWA]

First off there is only one vacuum line in the engine bay---it leads from the intake manifold to the reservoir and check valve, listed as Item #'s 13, 14 & 15 in Scalf77's diagram.

From the reservoir the function selector is next in-line, Item #16---this is mounted on the dash, has Heat, Vent, Mix, A/C, Max A/C etc. I show the backside of that control along with its vacuum routing block which connects to the back of that control.

Familiarize yourself with the so far included diagrams and photos, compare them to your actual parts and you should be able to diagnose what's at issue.

Do keep in mind its very possible the reservoir and check valve assembly have failed causing this. Should that be the case the fix is more complicated but doable by the motivated DIY, assuming time isn't a huge issue.

[86Scotty]

JWA, is the reservoir the big black plastic canister on the frame just behind the front right wheel? Is the check valve in it or elsewhere? I'm not having problems but would love to know. Also, mine's a 95 so older dash and older other parts. Is it in a different location on the 97 up vans? Thanks very much.

[JWA]

Scotty I think your description of the reservoir is correct however I'll admit I've never ever seen one installed or even loose. From my understanding the check valve is either integral to it or mounted somewhere close to its installed location.

Van-wise its rumored to be nearly impossible to access without significant removal of parts near the heater core/evaporator case or in that general area. Even following the plastic line running off the intake manifold isn't helpful because eventually it disappears and still doesn't reveal where the reservoir is to be found.

Most '92 and later E-Series are similar in operation of the heating system regardless how the dashboards appear. (Beginning '08 or so the blend door seems to have gone back to a cable actuation operated by the temp control which is a nice improvement.)

There are a few threads on FTE about work arounds for that infernal *$&#@& reservoir which I'm happy to post here if its allowed or helpful.

[86Scotty]

Post away, forgiveness is always easier to get than permission.