1999 GMC Jimmy. Cannot shut heat off.


Eriedipper
05-26-2009, 08:11 AM
I've read a lot of posts on this site, about uncontrolled heat from the Jimmy heater. Seems like it is quite a common problem.
On my model, (1999,) it looks like I may have to remove the dash to access the heater actuator which may be the culprit !

As a quick fix....has anyone ever spliced into the heater hoses near the fire wall under the hood, a couple of shut off valves?

I'm seriously thinking of doing it. Two brass valves from Canadian Tire $17 each, & 15 mins to install?

It's not fancy but it should work.

Eriedipper

old_master
05-27-2009, 09:08 PM
Manual AC or Automatic temperature control? In most cases it's more difficult to cobble it than it is to fix it right. Have you checked the vacuum supply to the HVAC mode switch in the dash? The system defaults to full heat if it loses vacuum.

MagicRat
05-27-2009, 11:06 PM
I've read a lot of posts on this site, about uncontrolled heat from the Jimmy heater. Seems like it is quite a common problem.
On my model, (1999,) it looks like I may have to remove the dash to access the heater actuator which may be the culprit !

As a quick fix....has anyone ever spliced into the heater hoses near the fire wall under the hood, a couple of shut off valves?

I'm seriously thinking of doing it. Two brass valves from Canadian Tire $17 each, & 15 mins to install?


I did this on 2 of my cars (and I even got the valve from Canadian Tire, too!!) ('87 Mustang and '79 Chevy van, no A/C on either one)

Both vehicles had slight air leaks around the air duct blender door. In the summer time there was always a slight flow of hot air through the vents, even though the heat lever was all the way to 'cool'.

The valves worked very well.

You do not need two valves .... one will suffice. There is no coolant flow through the heater core when just one hose is blocked.

I used the ordinary $6 valve, the type that is intended to be soldered on to 1/2 inch copper pipe, for in-home use.

I soldered on an inch of 1/2-inch copper pipe on to both ends. Most rubber heater hose slips over this very nicely and can be attached with ordinary hose clamps

FWIW a couple of my heavy trucks from the 1970's (international S2600 and Ford L9000) used these shut-off valves as original factory equipment.

Eriedipper
05-28-2009, 05:44 AM
Thanks for your replies guys, I agree that it is best to fix the problem properly. But I am running out of ideas and time.

My Jimmy has automatic climate control, and all the controls work fine except the temp control. (The a/c is Kaput! compressor seized)

On previous posts, there is great coverage on the temp actuator wheel cracking etc, but on my 1999 Jimmy, it seems to be different!!

They describe about moving the "Vac Unit" to one side, and behind it you can access the actuator. I cannot see the Vac Unit on my Jimmy.

Does anyone know where this "Vac Unit" is on the 1999 Model please?

On a previous post of mine I have attached a photo?
In the foreground of the photo is a loose wire with a plug, which I am suspicious about!! I don't know where it is supposed to go??

Good to hear that the one shut off valve will do the job!

old_master
05-28-2009, 12:55 PM
You really don't need any valves at all. Remove both hoses from the heater core and connect them together. Coolant will flow right through them and keep circulating.

Eriedipper
06-01-2009, 08:40 AM
Success! I have just installed a shut off valve to the intake hose going to the heater core, & it works. (See attached photo)

I decided on a valve (rather than just joining the hoses together as suggested) because I wanted to make it easy for my Wife to turn the heat off & on if she was driving the vehicle!

After installing the valve, (10 mins) there was instant control on the heat coming out of the heater. Not too fancy but it works!

Thanks again for all the advice.

A (cool) Eriedipper :cool:

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