Vacumn cuts off A/C fans

[rtcraver]

So, I'm sure this happens to others...
Is there a way around the issue of the A/C fan/duct closing when accelerating? When I start out, the A/C blower stops until I let up on the Gas... Any comments on getting around this?

Thanks,

TC

[DMCMW Dave]

So, I'm sure this happens to others...
Is there a way around the issue of the A/C fan/duct closing when accelerating? When I start out, the A/C blower stops until I let up on the Gas... Any comments on getting around this?

Thanks,

TC

Bad check valve (or missing if someone took it out and forgot about it.

http://store.delorean.com/c-365-7-3-0-electricalmechanical-controls.aspx item #28 on this page. Should be right behind the ignition distributor, at least in that general area.

[TTait]

My check valve is fine, any other likely causes?

Tom

[Ron]

Vacuum leak.
Bad mode switch.
Vacuum line or port for air tank blocked.

[Bitsyncmaster]

Vacuum leak.
Bad mode switch.
Vacuum line or port for air tank blocked.

I just thought of a way to test the tank and the check valve. After shutting the engine off, see how many times you can switch from one mode to another and stop hearing the actuators and flaps work. One of us will have to test that to see how many times they work on a good car. Probably selecting something like floor heat and vent may work.

[Rich]

Interesting experiment, Dave. I'm volunteering. Results here:

Returned from a drive an hour ago. Restarted it just now, then ran your test immed. after shutdown beginning with mode switch at VENT.

Toggled the mode switch back and forth between VENT-HEAT positions. A toggle in either direction counts as 1 cycle for now.


Result: 7 such mode switch cycles with what sounded like full flap movement each cycle followed by 1 slower flap cycle with the sound of partial movement, then nada.

A/C condition: Mode switch rebuilt and silicone lubed ~10 years ago. Does not hiss. A/C works fine including under full accel.


I just thought of a way to test the tank and the check valve. After shutting the engine off, see how many times you can switch from one mode to another and stop hearing the actuators and flaps work. One of us will have to test that to see how many times they work on a good car. Probably selecting something like floor heat and vent may work.

[Ron]

I just thought of a way to test the tank and the check valve. After shutting the engine off, see how many times you can switch from one mode to another and stop hearing the actuators and flaps work. One of us will have to test that to see how many times they work on a good car. Probably selecting something like floor heat and vent may work.:) Good idea!

[TTait]

After a prolonged absence from driving the car which included a new fuel pump installation and brakes, I'm able to come back to this thread today with a notable update.

While the problem persists, I now know more.

I did the test suggested above (great idea!). My actuators are quiet but I got 9ish full cycles, and could still hear something happening until 11, so I think I have plenty of vacuum stored and no major leak.

What's more telling happened on my test drive. The heat of summer has passed in Los Angeles and I noticed no problems at all running in Vent, so I tried AC, again no problem. Hoping it was solved, I switched back to Max AC and the problem immediately returned (and I was really cold).

So what I am REALLY seeing in my car is that in MAX AC only, on hard acceleration something is closing, or failing to stay open, and choking off the return air being drawn in from the cabin. This is happening after I pulled the Plenum box this spring and rebuilt it with lots of new foam insulation and neoprene etc. so its very well sealed and insulated.

I'll go over the diagrams to refresh my memory as to what opens to let air into the box in MAX and see if I can figure out what's going on there, but perhaps some of you have seen this particular thing happen before?

By the way, aside from this, I'm pretty happy with how the car is driving for the first time in a long time (or maybe ever with this one - I'd have to think about it).

[dmcnc]

I spoke with Rob Grady about this same issue (mine has it as well), and apparently automatics have had this behavior since they came out of the factory. I don't think anyone has figured out why.

[TTait]

Well that's worth thinking about... Yes, I have the auto.

It's not because of the high G forces when accelerating with the Auto. I doubt the cross gate cable is holding something something open, so that still seems to leave vacuum.

[Ron]

Maybe try putting a check valve on the line feeding the mode switch (black)....

[TTait]

It can't hurt, but I don't think I'm losing vacuum.

Does the Automatic transmission consume vacuum in any way?

I don't think I'm hitting the throttle hard enough to be activating the WOT switch for the AT, but I suppose I could unplug both WOT connectors and see if the problem goes away. It is worth noting that this is a new problem, and I did just repair my WOT switches... HMMM But I don't think I'm hitting
the throttle that hard. Hard enough for the kick down sensor on the transmission, maybe, but I don't think I'm at wide open throttle for real.

Is there an alternate set of electrical diagrams for the Automatic Transmission, and we are just all used to looking at the diagrams for the MT and we are missing something here? Maybe when the AT kick down triggers they have a circuit to lift power to AC including the Compressor and Cabin fans but because we are all looking at the MT wiring diagram we just aren't seeing it? Could it be that this is a "feature"?

Tom

[Ron]

If the flap is closing, the actuator must be loosing vacuum.

Place a check valve there to divide and conquer. If it then works correctly, the problem is the stock check valve, supply (leak), etc.
Otherwise it is a leak in the mode switch, the actuator diaphragm, connection there, etc.

No, the auto trans does not consume vacuum, I.E., it does not vent the vacuum supplied to it.

Transmission wiring (and more) can be found HERE (http://dmctalk.org/showthread.php?350-Automatic-Transmission-Governor).

======

...when you open the throttle, engine vacuum will drop.
...turbo and super chargers kill vacuum.

I'm betting on a weak check valve and/or leak(s).

[TTait]

I'm betting on a weak check valve and/or leak(s).

I hear you. I have a kit with another check valve and a supplemental vacuum accumulator I can install, but I pulled my check valve and it holds pretty good vacuum, and I'm getting more cycles out of the heat/vent test suggested above than the 1 published baseline. Maybe I can get the supplemental vacuum boost installed this weekend.

Somewhat unfortunately I was having some electrical issues as well that I though was going to force me to pull the center console again, but sorted them without having to pull it so my access won't be quite as good.

It may but I wonder if the problem is confined to automatics? Perhaps its because in a MT I would tend to downshift more than the AT does?

Any suggestions for a good slot for a grapefruit sized additional vacuum accumulator? I'm currently eyeing space near the brake master but other suggestions are welcome.

Tom

[Ron]

It may but I wonder if the problem is confined to automatics? Perhaps its because in a MT I would tend to downshift more than the AT does? That makes sense to me, if there is a leak... When under heavy load, say, starting up a long hill, you would shift a manual to 2nd before an AT would. When you let off of the accelerator to shift the MT, the vacuum will increase (replenishing the supply if there are leaks). Meanwhile, the AT's pedal is on/near the floor, so engine vacuum remains low. But that shouldn't matter because the flap is already in position. Once its actuator is fully engaged, the only way the flap should move is if the mode switch is changed or the vacuum at the mode switch input port drops. (Theoretically, you should be able to shut the engine off and the flap should remain in position, because of the check valve...) So it seems to me that it is not a "supply-reserve" problem (although a larger reservoir may help), but a leak. This is why I suggested putting a a check valve immediately before the mode switch input port as a test...