HVAC problems

[Tom]

Tom,

Thank you for providing such detailed information.

We will start with your suggestions.

Where is the solenoid located?

You're welcome. The solenoid is on the side of the heater core console under the dash, as shown in the factory diagram below. Keep in mind, however, that the sole job of the solenoid is to actuate the plunger in the heater valve under the hood. So the first things I'd do is set the cabin to max heat with the car warmed up, and then go look to see if the plunger is in (cold) or out (hot). Assuming the plunger is sucked into the heater valve (in/cold), I'd remove the vacuum hose and see if the plunger pushes out to allow hot coolant into the cabin, and confirm the vent temps are then hot. If so, then you just need to figure out why the solenoid isn't shutting off vacuum to the valve -- due to a bad solenoid, control unit, vacuum plumbing, temp sensors, etc. That's where the tests in my chart will come in handy. If the heater valve is in the open position (plunger out) when the cabin is set for max heat, then you have to figure out why the vent temps are not hot even though hot coolant should be flowing to the heater core -- due to a bad servo or linkage, air in system, etc. Another possibility is that you remove the vacuum line from the heater valve, and the plunger remains in and the valve remains closed. That would mean the heater valve is stuck closed.

Are you doing the work yourself, or is it at a shop?

solenoid.jpg (1.22 MiB) Viewed 753 times

[Fejjj951]

I am doing the work with a long time friend who is in the process of opening a shop.

Thank you again for all of this information Tom!

[dr bob]

Bob,

Thank you for this!

What does POLAPS stand for?

Plain Old Local Auto Parts Store

The check valves are used in almost any car that relies on engine vacuum for controls. With the reservoir ***, you can keep those vacuum-dependent control functions working for a while even when there's no vacuum in the engine manifold.

*** For some reason, vacuum systems components get tagged with names that defy their actual duty. One might think that an 'accumulator' or 'reservoir' might be something that accumulates something. But nooooo... In this case, it 'accumulates' the absence of something. The mysteries and idiosyncrasies of technical English.

[dr bob]

I'll compound the discussion on those solenoids --

If you find that the valve closes correctly on vacuum but fails to open when the solenoid is de-energized, try reversing the hoses connected to that solenoid. The solenoid should vent the heater valve side to atmosphere when de-energized. If the hoses are reversed on the solenoid, vacuum is applied and the heater valve closes when the solenoid is energized. However, de-energizing the solenoid will seal the vacuum in the hose to the heater valve, and the vent will bleed air into the vacuum system.

[Tom]

I'll compound the discussion on those solenoids --

If you find that the valve closes correctly on vacuum but fails to open when the solenoid is de-energized, try reversing the hoses connected to that solenoid. The solenoid should vent the heater valve side to atmosphere when de-energized. If the hoses are reversed on the solenoid, vacuum is applied and the heater valve closes when the solenoid is energized. However, de-energizing the solenoid will seal the vacuum in the hose to the heater valve, and the vent will bleed air into the vacuum system.

It tool me 23 minutes, and many brain cells I'll never get back, to sketch that out and see your logic. Well done. The vacuum hoses are tightly routed and organized on the side of the heater console, so highly unlikely they are mixed up there (see picture above), but certainly possible the hoses are switched at the firewall (see picture below). If so, and if my remaining brain cells are still functioning, I think that would mean the recirculation solenoid would only work right when the heater is on (which may go unnoticed).

The more I think about the oddball symptom @Fejjj951 describes -- making no heat under vacuum but full heat under boost -- that's what you would expect if the heater valve were connected directly to manifold pressure. The solenoids are not designed to deal with boost, since they should never see boost, so hard to know if it would allow boost to pass through when de-energized. (I might test one later.) If everything else checks out, I'd be inclined to pull the vacuum line off the firewall (see picture below) and hook it up to a vacuum/boost gauge to see if it just follows manifold pressure due to some fault with check valves, vacuum reservoir, etc.

All that said, I'd still start with the very basics as outlined above -- i.e., see if the heater valve opens and closes as designed under vacuum/atmosphere; see if you get heat when the valve is manually opened; then run the diagnostics in the chart if the heater valve is working right. If all that checks out, then start jumping into these rabbit holes.

heater valve plumbing.jpg (480.39 KiB) Viewed 712 times

[Tom]

Since rabbit holes are kind of my thing, I went out and tested a solenoid. See picture below. When there is no power applied to the solenoid, air flows freely between port A and C (and B is blocked off). As dr, bob said, this means the heater valve gets atmospheric pressure when the solenoid is de-energized, which opens the valve to let hot coolant into the cabin. Although B is blocked off, I did confirm enough boost will overpower the solenoid and allow boost to flow from B to A, meaning if there is boost on the vacuum line, it will open the heater valve whether the solenoid has power or not. But that would not explain this situation completely unless there's more than one fault. When power is applied to the solenoid, air flows freely between A and B (and C is blocked off), which sends vacuum to the heater valve to close it.

Just riff'n here, but if the solenoid were internally leaking such that A, B, and C all flowed freely between each other, one could imagine a scenario where there is still enough vacuum to pull the heater valve shut any time there is full engine vacuum (where the unlimited supply of vacuum overcomes the 'leak' at C), yet can't sustain vacuum for long under boost when relying on the oxymoronically named vacuum reservoir alone (given the leak at C), causing the valve to always be closed under vacuum and always be open under boost. No idea if the solenoids ever fail that way, however. Just thinking out loud. I'll stop now.

heater-solenoid.jpg (877.86 KiB) Viewed 704 times

[ROB III]

Tom (and dr. bob), don't ever stop. I thoroughly enjoy this type of discussion (this whole thread) as I often learn more about diagnostics for our older platform units and diagnosing in general than I care to admit.
Thank you and Fejjj, thanks for asking in the first place.

[Fejjj951]

You guys are amazing! I appreciate all of the brain work here and your willingness to help. Maybe you could add this info to your detailed HVAC testing guide.

We will probably start our testing next weekend as my wife and I are going to Niagara Falls for a Peter Frampton concert.

I will update this thread with our findings.

[Fejjj951]

Sadly my family has had some life events happen and I have not been able to apply all of the excellent provided knowledge to the problem. My 94 year old Dad had a bad fall, smashed his face and punctured his left eye so I have been spending all of my free time in the hospital with him.

[Tom]

So sorry to hear that. Caring for aging parents can be so hard sometimes. Wishing your dad and your whole family well. Cars projects can always wait.