1998 GMC Safari - Rear heat blowing cold

[BooYeahZ]

Hi:

Recently my rear heater starting blowing cold air. The motor blows fine on high and low settings, it just blows cold air instead of warm or hot air.

I followed the hoses up to the front of the vehicle but there are alot of hoses that interjoin along with vaccuum hoses etc. Here is where I got lost....

There must be some sort of switch or pump that the coolant is not flowing to the rear heater.

Can someone give me some direction as to what needs to be replaced?

Thanks in advance

[email protected]

[the blur]

there's a heater control valve somewhere in the line. probably vaccume operated.

[BooYeahZ]

Thanks for your reply....I just wish I knew what I was looking for. I have no clue. I am by no means a mechanic although I do perform minor maintenance myself. Big jobs are left for the garage. Guess I'll have to bring it in to the dealer.

[chaqueno]

3 yrs ago my 89 van did the same. My steel lines were also wet and were about to rust through so I got ( I think 1/2 inch and 5/8 ) heater hose and cut the steel pipe above the control and put the hose on from there right to the heater core. I tucked and wrapped the hose neatly underneath. A simple and cheap solution. Which reminds me that I have to check my 93 now before the winter really comes.

[BooYeahZ]

Hi and thanks for the reply....

I do not have any leaks whatsoever....It just blows cold air.
Where is this control located that you mentioned?

Thanks

[chaqueno]

Take the engine cover off inside the van and I do believe you will see the control valve there. Follow the 2 lines from the rear heater (Under neath the van from behind the driver's seat) I'm guessing it is a vacume controled switch and if so then you will need to put a plug on this hose if you take the switch out. Hope this helps! Also if you want ,go to the dealer and ask the parts counter person if he could give you a copy of the diagram showing the control valve. This will show exactlly where it is. I go and get printouts quite offten when I'm not certain on things.

[BooYeahZ]

Thank You chaqueno...
I have followed the lines and they go to the front of the engine on the passenger side just behind the air conditioner cylinder. They cross connect to other hoses, a small electrical unit with vaccuum hoses.
I will contact the dealer for a diagram. Hope it is free....lol

[BooYeahZ]

I think I have it.
The Heater Control Valve Solenoid is the problem. The actual Heater Control Valve itself is fine. I simply bypassed the valve with the vaccuum hoses and the rear heat works fine.

Thanks

[dkw]

Originally Posted by BooYeahZ

I think I have it.
The Heater Control Valve Solenoid is the problem. The actual Heater Control Valve itself is fine. I simply bypassed the valve with the vaccuum hoses and the rear heat works fine.

Thanks

I have the same problem. Can you tell me where the Heater Control Valve Solenoid is?

[BooYeahZ]

Heater Control Valve just behind the AC accumulator on the passenger side under the hood. Connected to the Heater Control Valve is a Heater Control Valve Solenoid. It just clips on the wall above the control valve. This solenoid has an electrical connection so that when the rear heater switch is turned on and off it switches on and off the solenoid which in turn switches on and off a vaccuum also connected from the solenoid to the valve. The solenoid has 2 vaccuum connections (top / bottom). Disconnect the vaccuum hose from the top of the solenoid and the vaccuum hose from the control valve as well. Plug the vaccuum hose which was disconnected from the top of the solenoid directly into the valve, therefore bypassing the solenoid. Note that this will give a constant flow of coolant the the rear heater core because the valve will always be open because this vaccuum hose has a constant suction. This vaccuum hose from the top of the solenoid is coming from a tee directly on the vaccuum line therefore it has a constant suction. That is why when hooked up directly to the valve, the valve will be open at all times. The solenoid is not working because the bottom hose has no suction when the rear heater is turned on therefore supposed to turn on the solenoid opening the solenoid vaccuum.