climate control,126

[larry perkins]

ok guys and gals i need an assist.
i did read some of the threads on climate control but didnt find my situation.
i have a 1990 350sdl and the problem is when the ac is running and the temp is set at 72 deg f,all is fine,air is flowing out the center vent,once the temp dial is lowered then the fan picks up to faster speed but after about 5-10 seconds all the cool air is directed to the defroster vents. i tapped into the vac line that supplies the switch over valve block with a vacuum gauge and sure enough i can see the vac reduce from 15-20 down to about 5 in and then the flaps move the air to the defroster vent.
drawings dont show any control valve from vac pump to switchover block.
any insight is apperiaced.
larry perkins lou ky

[stevebfl]

I would guess that as the change is made a servo element that has a large leak is switched on. I can't think of any that would switch under those settings but I would either check them all or watch with a gauge which one opens at the point in question.

[larry perkins]

thanks for the reply.
i did forget to mention that as soon as i return the temp selector back up to 70's f that the center vent opens right back up.
i will keep at it but was trying to trouble shoot as much as i can remote because this 91 350sdl has the passenger side air bag and to have full access to the switch over block it looks like i have to remove the metal rod that sorta covers the sw block and the hardware(metal extends on over under the air bag).
steve is the system designed to have a dump valve(1-7?)be operated when defrost is demanded?
thanks again
larry perkins lou ky

[stevebfl]

No there are no dump valves. A switchover valve that switches off will vent the circuit previously held to vaccum, but there is no intended dumping of system vacuum. Each element that is functioning will have vacuum to it. The center vent and defrost are spring loaded to a default position. That position occurs with low or no vaccum. The default is to have air flow through defrost and none through the center. Ths is for safety reasons.

[larry perkins]

ok steve you hit the nail on the head.
if i read the manual right,the amount of fresh air is adjusted with the inside air temp required,u want cooler air u get less fresh air.
niether of the diverter flap pods will hold vaccum(am assuming that means frish air flap)pictorial shows these pods located behind the glove box(pictorial shows #42 to 1 of y7 and #43 to 2 of y7) vac diagram list them both as #42. i did use a golf tee to plug the #1 and#2 switch over valves and i never lose the center vent flow as temp dial is reduced to max cooling,coures i did lose the recirculate function.
as i read the instructions for removing and installing the air bag i might think i am lining up the new jersey battle ship for a shot with the long gun,lol
last question,instead of removing the air bag,would you think i could have any suscess going after the bad connection-bad vac pods from the blower area,sorta take the backside approach?
larry perkins lou ky

[stevebfl]

I was going to suggest the recirc door elements but they are two stage and my idea was that the first stage was engaged as soon as the AC was on (not EC). The second stage is usually switched with a dash switch or the "Min" position detent.

I don't do any disassembly anymore, just diagnostics/consultations (I have ten techs who do actual disassembly) so I can't say off hand what it takes to get in there, but I don't think we remove the airbag.

[emmydotnet]

Larry:
have you resolved your recirc pod issues? I am trying to resolve similar issues I have with my 91 560 sel. I believe we have the same setup. I have the (1) piece switchover valve (Y7) with 7 valves. The symptom I am most concerned about is intermittent no-center vent air. at about 70-80F ambient, the center vent would work fine, but when outside temp goes up to mid 80s or higher, center vent closes and air is diverted to the dash.

I have tested all six pods at each hose going to the switchover. Here are the results:
Pod #42 going to Y7/1(dark red) - No vacuum
Pod #42 going to Y7/2(green/blue) - No vacuum
Pod #39 going to Y7/3(red/yellow) - Vacuum ok
Pod #42 going to Y7/4(Red/Black) - Vacuum ok ( intermittent, but holds after few pumps)
Pod #38 going to Y7/5(Red/White) - Vacuum ok
Pod #41 going to V7/6(green) - vacuum ok
Pod #38 going to V7/7(Red/Blue) - No vacuum

The recirc pods are both labeled #42 and all (4) diaphragms appear to be bad assuming the hoses are ok (which I cannot reach at the moment - it looks like I may need to remove the pass air bag).

The lower diaphragm of POD#38 also appears to be bad.

On the switchover valve assembly, with the inlet plugged(with finger), push button assembly disconnected, and no power, all output valves do not hold vacuum. Is this normal? Do I need to apply power in order to test the switchover valve assembly?

Larry, Steve, any inputs would be much appreciated. I would like to make sure I diagnose properly before ordering and replacing parts.

[larry perkins]

ok folks, i dove in, removed the air bag(pass side)and frame that supports the bag, replaced #41 and 42 pods, now all is well.
thanks for the help
larry perkins lou ky