97 Astro rear heater blows cold

[53degN]

My 97 Astro's auxillary rear heater suddenlly stopped giving heat. The blower and all other heater functions work fine. Rad is full and no leaks. Thanks for any help.

[old_master]

A vacuum operated water valve controls coolant flow for the aux heater. The valve may be stuck or there may be a problem with vacuum to the valve such as a kinked or broken vacuum line. Depending on the aux system, the valve could be located under the hood, under the body of the vehicle, or in the aux heater housing. It will be connected inline with a coolant line so if you follow the coolant lines from the engine to the aux unit, you'll find it. Make sure vacuum at the valve changes when the temperature control on the dash is changed, (the engine must be running). If vacuum does change, the valve itself is stuck and requires replacement.

[53degN]

Thanks for the help, I'll follow the vacuum lines and coolant lines. Somewhere there must be an electric component too because turning the thumbwheel fan control allows coolant to flow to the aux heater. I know this because I could see the temp fall on the gauge when I switch it on. On second thought I guess the thumbwheel switch could be a vacuum switch too. Doesn't feel like it but would explain a few things.

[old_master]

The thumbwheel for the fan is strictly electric and controls only the blower motor. The thumbwheel for the temperature operates an electric servo in the heater case and a vacuum operated water valve under the A/C accumulator. This is how the factory system operates. If you have an aftermarket unit, they are designed differently. Aftermarket units are usually, but not always, installed in conversion vans.

[drew300]

-Same problem with my '97
I traced the vacuum line back to an electrically operated valve that controls the vacuum.
The electric part of my valve quit. If I apply vacuum, the water valve opens, so I know it's this valve. The 12 volt supply to the the valve works as it should, on / off with the dash control.
But I haven't replaced this valve yet. Looking from the front of the engine, it's down by the rocker cover, on the passenger side. Mines' silver, with a power plug, and 2 say 1/4 " black vacuum lines, about 2" maybe 2.5 " long, if memory serves.