HVAC Troubleshooting

[Dropped992door]

I have a 99 two-door Tahoe. I recently noticed about a month ago the AC would start blowing hot under heavy acceleration, or a change in vacuum manifold pressure. There is a solenoid on the firewall that runs from the manifold, and then down to the heater control valve that is between the engine and the heater core. Once the heater control valve, loses vacuum, then it opens and allows heat transfer from the heater core even if the climate control is on cold. If I run a straight hose and bypass the solenoid, you can see the vacuum valve on the heater control valve close. But under heavy acceleration, it will still open back up due to the drop in vacuum pressure in the intake manifold. Is there anybody out there that can tell me how the Vacuum solenoid is wired and or how it works. It is just a two wire plug, I feel that it requires a 12 V signal in ground to open and close the port within itself. I bought a new doorman part and used my Mityvac to create a vacuum in the porch and use the multimeter to check Continuity and there was no change.

 

[east302]

how the Vacuum solenoid is wired and or how it works. It is just a two wire plug, I feel that it requires a 12 V signal in ground to open and close the port within itself.

Not sure if this helps, but the solenoid valve energizes when the “max” detent is selected on the a/c panel temperature dial. If the water valve still comes open when the solenoid is bypassed, then that should rule out the solenoid.

How much of a temperature difference are you seeing at the vents under this condition? The damper at the heater core is bypassing the core entirely so no air flow is passing through it regardless of water valve position. Temperature swing for open versus closed should be minimal - less than 5 degrees would be my guess.

Have you checked your refrigerant charge? Is the damper itself moving when it’s not supposed to - check the position indicator dial on the door actuator for movement.

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[1998_K1500_Sub]

I recently noticed about a month ago the AC would start blowing hot under heavy acceleration, or a change in vacuum manifold pressure. There is a solenoid on the firewall that runs from the manifold, and then down to the heater control valve

The heater water control valve may have a vacuum leak, or the may be a leak elsewhere in the vacuum supply to that valve, that leaves it susceptible to low manifold vacuum.

Or, there may be a restriction in the solenoid-controlled vacuum valve on the firewall that limits the amount of vacuum available to the water control valve and, perhaps, makes it susceptible, particularly if there was a vacuum leak downstream from said valve.

I've had that solenoid-controlled vacuum valve plug before; I cleaned mine by removing it and spraying Brakleen through the vacuum path. I might have had to energize it while doing so, to open the path completely; I honestly don't remember.

There's no vacuum reservoir can on that system AFAIK to supply vacuum while the engine is temporarily under heavy load, so you needn't look there.

There might be a check valve somewhere in that vacuum circuit to mitigate the problem; I've never checked. If there is one, it would be nearer to the engine.

 

[Dropped992door]

Not sure if this helps, but the solenoid valve energizes when the “max” detent is selected on the a/c panel temperature dial. If the water valve still comes open when the solenoid is bypassed, then that should rule out the solenoid.

How much of a temperature difference are you seeing at the vents under this condition? The damper at the heater core is bypassing the core entirely so no air flow is passing through it regardless of water valve position. Temperature swing for open versus closed should be minimal - less than 5 degrees would be my guess.

Have you checked your refrigerant charge? Is the damper itself moving when it’s not supposed to - check the position indicator dial on the door actuator for movement.

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So by the information you’re telling me, the solenoid should have a 12 V and ground signal correct. Ground probably chassis and 12 V when the max button on the AC panel is pressed? I don’t think I have a vacuum leak, I replace the preformed factory hose from the manifold to the solenoid with a solid rubber line just to eliminate that issue. I did think that the water control valve may have a leaky diaphragm. I also tried putting a one-way flow valve in line, so I would not lose vacuum, and that did not help.

 

[east302]

Yep, the panel sends 12V to the solenoid…

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[Dropped992door]

I used a variable voltage power supply and a meter. The new solenoid activated at 6v. This would lead me to believe it would have to be actuated off of the “blend” circuit off off of the control panel inside the vehicle.

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[east302]

It should be a switched circuit (either energized or not) based on the temperature dial being in the “max” detent position. If the panel is energizing the solenoid at other dial positions then I could see your air temp fluctuating.

I had a cheap dorman a/c panel that did that just before it tried to catch on fire. Never again.

 

[Dropped992door]

Thanks. My next Diag was going to test the voltage at the solenoid connector, then swap to my other ac control panel. It is a OEM panel but had to replace it due to a bad and melted connector on the fan speed circuit. I had soldered in a new connector and put in a cheap control panel. Thats before i found out then fan speed switch could be swapped by itself.