ninetysixZ28

Noticed that I'm losing vacuum to my A/C vent actuators (vents r redirecting to upper/lower instead of front) when engine lugs such as cruise control on during steep grade...I have T56...or when attempting to accelerate in high gear at low speeds.

Followed shbox instructions and:

1) replaced vacuum check valve (wasn't broken and didn't appear damaged but part was only $4)
2) actuator line holds vacuum
3) manifold line holds vacuum when manifold end plugged
4) vacuum tank line holds vacuum, however, this requires many pumps of my handheld vacuum pump...i'm assuming this is purging the tank?

Since several posts referred to degradation in the vacuum tank line; i replaced it with vacuum hose and a test drive revealed no difference. Actuators close when engine lugs.

BTW local dealer indicated that vacuum tank line p/n 12124134 is discountinued and that even Vintage was out of stock...told me to try salvage. Picked up 7ft. of vacuum hose from O'Reilly's for $.80/foot.

Is there a problem here or is this system operating normally? In other words, does lugging the engine create such a drop in manifold pressure that there is insufficient 'stored' vacuum in the vacuum tank to compensate? I'm asking since I'm thinking this is an 'extreme' condition because the system operates fine under all other conditions during which i'm assuming the system could be using the 'stored' vacuum from time to time.

ninetysixZ28

RPMs r usually under 2k.

All stock.

Have not measured engine vacuum. Only verified vacuum present in manifold line.

ninetysixZ28

Measure the vacuum at the manifold line.

20 @ idle with A/C off
18 @ idle with A/C on

cusz28

please don't mind my curiosity, but i don't think we have a vacuum problem on our hands, we have a vacuum control problem. it would be normal for vacuum to almost disappear at wot and extreme lugging at low rpm's, but what ever check valve in the dash control is supposed to retain vacuum to the switch actuators isn't working, and is allowing the vacuum to escape back to the system. does this logic make sense?

shoebox

The purpose of the tank and check valve (he already replaced) is supposed to negate vacuum control problems during those periods of low engine vacuum. It is possible under some conditions to deplete the vacuum supply.

cusz28

I know, but i'm asking if something in the dash control itself may be letting vacuum back.

shoebox

He also said the actuator line (which goes to the controls) held vacuum and I am assuming he had it in the mode he is having trouble with. If tested in every mode, then all the in-dash lines and actuators would be good.

Angelis83LT

If it only happens during the lugging I would guess that the vacuum is going away while it lugs and it depletes the vacuum enough to switch the vents to default. I would find out why the car is lugging down, fix that problem and I bet it will not have any problems with the vents anymore.

cusz28

injun, he never said he tested the dash switch, and i never said that IS the problem. i'm not saying i'm right, i'm just asking if somebody knows if the dash switch itself COULD possibly be the problem. the check valve i'm thinking about might not even be present. I was wondering if the dash control itself had a check valve built in. lets not argue just for the sake of arguing, it was just a misunderstood question.