sprfrkr

Hi, I am getting all hot air all the time in my 1984 US 928S.

- I replaced heater coolant cutoff valve under the hood and have confirmed the new unit actuates when vacuum is applied manually. It does not actuate from setting the HVAC head unit to cooling/ac.

- I referenced the following guide for my analysis even though it is for an S4: http://members.rennlist.com/pirtle/svc_act.html

Under the hood:

- I applied vacuum to the blue side of the check valve coming off of the brake booster. Holds vacuum.

- I went past the four way fitting down to the two way split to test cruise control and reservoir lines. Both held vacuum.

- I then tested the black line which I followed that went through the firewall. I believe this is the feed for the HVAC. Applied vacuum and it held.


Under the Console:
- I removed the HVAC valve block and applied vacuum to the black feed line. This would back feed up to the reservoir, check valve, and cruise lines. It held vacuum after pumping for a while.

- I then pulled each line and applied vacuum. They each held fine. The white line is the line that runs to the heater cut off valve I believe. That worked fine too.

Conclusion and Questions:
My conclusion is that the block itself is bad. Agree or disagree?

The help article for the S4 has a fourth line coming from the fitting, my fourth side was blocked off. Is this normal?

What is the vacuum block called? A quick search turned up no results. If cost effective, I will just replace it.




Three way connector (should be 4?)

bureau13

You can test the individual solenoid valve on the bench to make sure. One of mine had a slow leak so I got a replacement. Make sure you test them both powered and unpowered. The procedure is documented around here somewhere in one of the guides. Failing that, I described what I did in one of my recent HVAC threads.

sprfrkr

If I find a bad solenoid, where can I find a part no/replacement? I looked under HVAC at 928intl and did not see anything that jumped out at me related to this.

sprfrkr

I bench tested the solenoid block and found that three had slow leaks. I'd like to replace them, but can't find the part numbers.

Unfortunately, the leaking solenoids are not the heater cutoff valve, so I am still at a loss as to why that is not working. Could it be the head unit or a relay someplace?

Chalkboss

That line at the four way is supposed to be blocked, I believe. Have you tried Roger for the solenoid block? I seem to recall reading recently that he had them.

sprfrkr

Thanks. 928intl had them for $25 each when I inquired via email.

Now, the solenoids are not causing my heater coolant cutoff valve to not operate, it is something else. What should be my next step???

dr bob

Reminder that a vacuum leak in any of the circuits causes low vacuum for all the circuits. The definitive test is to watch the heater control valve with engine running, and see that the heater valve is fully closing. You can also put a T in the white line at the solenoid rack and look at actual vacuum there with the engine running.

-----

My AC was under-perfoming when I took the car out of winter hibernation last April. The refrigeration side was fine per the gauges. So I went looking for a vacuum problem, and eventually found that the vacuum actuator for the heater valve itself was leaking. Prior to that discovery, I'd tied the valve closed to shut off the hot water flow, but the vent temps were still into the high 50's so I needed to go further. So even with no hot water going to the heater, the leak at the actuator allowed other doors to operate poorly, including the outside air inlet by the passenger's feet. So I was getting a mix of hot outside air, aided and abetted by the comb flap not closing off the flow from the heater and outside air bypassing the AC evaporator. One dissociated leak caused all the problems. Disconnected the white hose, put a cap on it, and suddenly had cold AC again.

Yesterday, a well-traveled heater valve arrived here and was installed. This morning a short "tour" to the local ski area parking lot proved that the heater is fine even in mid-30's temps and the sunroof open. Everything is functioning correctly again.

Moral: Fix All The Vacuum Leaks if you want the system to work correctly.


The solenoid valves may have bits of decaying actuator diaphragm in them. Pull the whole little solenoid console (bracket) out with them mounted, and gently blow compressed air through them in both directions, energized and de-energized. That should get out any crud that may keep the little plungers from fully sealing.

kcfong2

I need some help!

Hi, I have a similar problem with my 1981 US 928 4.5L Manual. I performed the same vacuum tests as sprfrkr above, but have the following observation and question:

All the solenoids hold vacuum, even energized, and the system holds vacuum from the black line in the engine compartment to the solenoid block.

When I apply vacuum via my Mityvac to the black line, and turn the key on, energizing the system, I observe this:

Slider at OFF: Heater valve closes (no hot coolant). This is what I would expect.
Slider at ECO: Heater valve opens (default). All hot coolant to heater.
Slider at AUTO: Heater valve opens (default). All hot coolant to heater.
Slider at BI: Heater valve opens (default). All hot coolant to heater.
Slider at Defrost: Heater valve opens (default). All hot coolant to heater. Fan blower goes to HIGH.

My problem is I get HOT air through all the vents, and it appears the above is why. It appears my culprit is the AC Head unit, but is there anything else I can do to confirm this before plopping down $$$ for a rebuilt?

I should note, the high pressure of the AC system was 150PSI at idle with an ambient temperature of around 60-68F, so there is sufficient R12, and the compressor is engaging, so I'm pretty confident I've ruled out the "engine bay" portion of this problem.

I was thinking if I should "energize" each vacuum solenoid one-by-one, and also seeing if voltage is being sent to the solenoids from the AC head. Can anyone give a recommendation on next step(s)?

Thanks!

BIMMERMIKE

Check if setting motor operates ?

kcfong2

Bimmermike, I owe you big time!

It turned out to be exactly the Setting Motor, which I had no idea about. Once I looked that up in the WSM and walked through the diagnostics, I determined it was defective. A quick run to my local 928 recycler (PartsHeaven.com in Hayward, CA), and I was home with a replacement unit. 10 minutes later, I had cold air back!

For reference, the WSM that covers testing the setting motor is on pages 87-113 through 87-114, under the heading "Testing Setting Motor".

Amazing... this forum has proven invaluable.

Now, I noticed the center vent wasn't opening, and I'm not sure what the normal behavior is for the solenoids, but I'll post that question in a new thread.

Again, THANK YOU!