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Max A/C mode vent to defrost under acceleration
#304233 June 26th 2009 10:20 pm
Joined: Jun 2009
Posts: 67
journeyman
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journeyman
Joined: Jun 2009
Posts: 67
Alright, this will be my first "request help" post, so here goes.
I have a '94 Dodge 250 van, and I have just a small problem.
when i put the select on Max A/C, at idle, it works just fine.
when accelerating, usally over 1,500 RPMs, the vents shut off and it goes to defrost mode, let totaly off the gas, and it comes back down. it always does this, and its really annoying on the highway, when cruising at 70 MPH and 2,000 Rpms, it will only stay on the defrost vents. when i put it on Normal A/C, it works fine. i like to use Max as it blows harder and doesn't wear out the air compressor as fast to cool outside air when on Normal.
I am thinking its got a vaccuum leak in the dash somewhere, i have already checked the lines that go from the intake manifold, all the way to the firewall, and they are good.
or is this just something these vans do?

any suggestions?
thanks in advance


Dodge Ram B250 (3/4 ton)
Stock 5.2 Magnum engine (All fixed now, no more clacking valvetrain)
46RH Transmission w/ dual coolers
Chrysler 9.25 rear axle w/ 3.55 geared Dana Trac-Lok Posi setup
Magnaflow high flow catalytic converter, with high flow Dynoflow exhaust system
Rear wheel ABS brakes



Re: Max A/C mode vent to defrost under acceleration
DodgeRamVanMan #304261 June 27th 2009 12:19 am
Joined: Mar 2007
Posts: 4,463
Likes: 12
pooh-bah
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pooh-bah
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It does sound like a leak to me... the defrost mode door is spring loaded, so by default you get defrost... you need vacuum to pull it down.

Have you checked to see if there's also air coming out of the heater vents at the same time as the defrost vents? There are separate actuators for the Heat-Defrost door and for the Heat-A/C (Hi-Lo) door...

On my vintage of van, ('81) and probably yours too, there is a restrictor valve (looks like a tiny fuel filter) in line with the defrost door vacuum actuator... if you have a leak in that line, the restrictor, or in the vacuum actuator itself, the restrictor would slow the "flow" of vacuum, (plus at higher speeds you have less manifold vacuum anyway) so you'd have the symptom of losing control of the actuator without screwing up any of the rest of your vacuum system.

I've seen the little rubber boots which couple the vacuum line to the actuators rot and crack or get pinholes... The mutli-hose plastic connector at the back of the climate control switch assy. is notorious for literally disintegrating. (BTW, you can get the little rubber boots at many car parts stores)

If anyone has worked on your stereo, since the Heat-A/C (Hi-Lo) door actuator and the Heat-Defrost door actuator and all their hoses are right behind the stereo, they're likely to get bumped, wires tangled in their linkage, or their hoses (which are so small that they look like wires...) can end up rerouted at odd angles... which can kink or damage the rubber boots.

Do you have a hand-held vacuum pump? I used one of those to wring out my entire heater & A/C vacuum control system. I found a few leaks, and even an unused hose that wasn't plugged off, causing a large leak.

If you'd like, I could send you the vacuum diagrams, control chart and mode descriptions I have from my service manual, which should be almost identical to yours. I pulled the entire control system out of a '94 Dodge like yours and it could drop straight into my '81.

As an aside, straight from the Dodge manual - MAX A/C "This mode of operation should only be used to rapidly cool down the vehicle interior and in exceptionally hot and humid weather. The temperature control lever should be placed in the full cool position for best results."

It appears they intended for the owner to operate the A/C in normal mode the majority of the time.

Clear skies!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
Re: Max A/C mode vent to defrost under acceleration
Ram4ever #304364 June 27th 2009 3:38 pm
Joined: Jun 2009
Posts: 67
journeyman
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journeyman
Joined: Jun 2009
Posts: 67
That vaccumm diagram would be nice, if it would not be too much trouble, then i could get an idea of where the vaccumm servo motors and hoses are. as for the radio question, yes i put a radio from a 2001 dodge ram truck in there (i love that CD/Tape/AM/FM) but i didn't tamper with any wiring for the radio, but i may have knocked something loose back there. i am off today, so i will check it out behind the radio area.

thanks


Dodge Ram B250 (3/4 ton)
Stock 5.2 Magnum engine (All fixed now, no more clacking valvetrain)
46RH Transmission w/ dual coolers
Chrysler 9.25 rear axle w/ 3.55 geared Dana Trac-Lok Posi setup
Magnaflow high flow catalytic converter, with high flow Dynoflow exhaust system
Rear wheel ABS brakes



Re: Max A/C mode vent to defrost under acceleration
DodgeRamVanMan #304396 June 27th 2009 8:05 pm
Joined: Mar 2007
Posts: 4,463
Likes: 12
pooh-bah
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pooh-bah
Joined: Mar 2007
Posts: 4,463
Likes: 12
Hey DodgeRamvanMan, here you go:

BTW, this might be interesting for anyone with a Dodge van who wondered just how those Bi-level climate controls are supposed to work...

[Linked Image]

[Linked Image]

[Linked Image]

[Linked Image]

[Linked Image]

If you have any difficulty reading these, let me know and I'll PM you larger resolution copies... I reduced and sharpened these so as to not go too far out of site recommendations. My originals are 2451x3206...

Hope that helps!

Clear skies!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
Re: Max A/C mode vent to defrost under acceleration
Ram4ever #304647 June 29th 2009 1:40 am
Joined: Mar 2007
Posts: 4,463
Likes: 12
pooh-bah
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pooh-bah
Joined: Mar 2007
Posts: 4,463
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BTW, unless you looked carefully at the Control Chart you might have missed it - the "water valve" is the heater shutoff valve, which is in the heater hose out in the engine compartment.

A big issue with running the A/C on max all the time is that normally the heater coolant flow circuit is always on so coolant is flowing through the heater core. Yep, even when the A/C is on. The only time the heater coolant flow is shut off is when in Max A/C or Off modes.

So, if you leave it on Max all summer, the coolant will stagnate in your heater core, which could lead to excessive corrosion, etc.

I had a heater core fail once... it was no fun. Atomized slippery Glycol mist filled up the interior until I realized what had happened. It took ages to clean up.

Clear skies!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
Re: Max A/C mode vent to defrost under acceleration
Ram4ever #304935 June 29th 2009 6:19 pm
Joined: Jun 2009
Posts: 67
journeyman
OP Offline
journeyman
Joined: Jun 2009
Posts: 67
ok, had a look at some of those vaccum lines, the check valve filter thing you were talking about is good and not leaking, as well the lines at the A/C Heater control panel are all good too. i didn't get a chance to check behind the radio on Saturday as family came over to visit so i had didn't get a chance. i will give it a shot here in about an hour and see whats back there.


Dodge Ram B250 (3/4 ton)
Stock 5.2 Magnum engine (All fixed now, no more clacking valvetrain)
46RH Transmission w/ dual coolers
Chrysler 9.25 rear axle w/ 3.55 geared Dana Trac-Lok Posi setup
Magnaflow high flow catalytic converter, with high flow Dynoflow exhaust system
Rear wheel ABS brakes



Re: Max A/C mode vent to defrost under acceleration
DodgeRamVanMan #305048 June 30th 2009 5:19 am
Joined: Mar 2007
Posts: 4,463
Likes: 12
pooh-bah
Offline
pooh-bah
Joined: Mar 2007
Posts: 4,463
Likes: 12
Do you have one of the hand-held vacuum pumps? That's about the only way to verify that there isn't actually a leak... remember, vacuum lines will leak *inwards*, so you won't feel any air current.

Clear skies!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
Re: Max A/C mode vent to defrost under acceleration
Ram4ever #305385 June 30th 2009 10:27 pm
Joined: Jun 2009
Posts: 67
journeyman
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journeyman
Joined: Jun 2009
Posts: 67
i have a vacuum pump somewhere, but it would take days to look thru my storages to find it. i just used a piece of vaccuum hose and sucked on it (yes it was clean, no bacteria) with my mouth on all the servo motors and held them there for about 15 seconds. all held up good as did their lines when i started at the control panel. as for the heater valve, i pulled off the vacuum hose going to it, and capped it off with a screw.
so now the valve is always closed. sometime this week i'll go to Ace hardware and just get a manual hand operated valve and put it there. then just open it for winter, close it for summer.
i'm now starting to think the control panel has had it, maybe worn out and leaking on the push-button area. because Normal A/C works fine all the time, Max A/C goes to defrost when i take it over 1,500 RPMs, while Norm does not.
could be the control panel, what do you think?


Dodge Ram B250 (3/4 ton)
Stock 5.2 Magnum engine (All fixed now, no more clacking valvetrain)
46RH Transmission w/ dual coolers
Chrysler 9.25 rear axle w/ 3.55 geared Dana Trac-Lok Posi setup
Magnaflow high flow catalytic converter, with high flow Dynoflow exhaust system
Rear wheel ABS brakes



Re: Max A/C mode vent to defrost under acceleration
DodgeRamVanMan #305401 July 01st 2009 12:45 am
Joined: Mar 2007
Posts: 4,463
Likes: 12
pooh-bah
Offline
pooh-bah
Joined: Mar 2007
Posts: 4,463
Likes: 12
It's certainly not impossible; having less manifold vacuum at higher RPMs might contribute to not enough vacuum to accomplish the pull down if there were a large internal leak, but all things considered, I'd be more suspicious of the plastic hose connector at the back of the control assy, than of the control assy itself.

Gotta love your determination at testing the vacuum motors... that job must have "sucked"...

;0)

(It simply *had* to be done...)

If you locate your vacuum pump, you can attach a small coupler to it and pump each line down and see whether there are any slow leaks. This is especially good for debugging the rubber and plastic connectors, and the diaphragms in the vacuum motors... for instance the fresh air/recirculate vacuum motor on my van leaks, but it's slow enough that it still functions... I have a spare on hand now.

Since the control assemblies are fairly expensive, (around $80) I'd sure try that first, following the flow in the vacuum diagram as to which lines should be active in each switch position.

BTW, if you get into a super-detailed analysis, I determined that there are 2 errors in the control assy's internal wiring diagram in figure #4... one of the poles isn't actually connected to the others the way they show it. The 2nd contact down from the top on the bottom most switch element of the "push button vacuum electric switch" (aka the control assy...) isn't connected to anything. This shouldn't effect your vacuum troubleshooting, but it's confusing when the A/C compressor isn't on and the diagram indicates it should be...
Also, the diagram shows the ignition switch connected to a ground... duuuuhhh. It must have been a Friday, or the Tech writer was a Vanner and it was the Monday afer an eVant...

;0)

It should actually be the 1-12RD large red wire running from the ignition switch to the positive 12 volt car battery terminal...

Clear skies!


-It's been such a LONG TIME... BlueShift>> 1981 Dodge Ram B250 Custom Sportsman Maxi Van


[Linked Image]

It's what you learn after you know it all, that counts...

Are you living to work, or working to live?

[Linked Image]

Learning from my own mistakes is good, learning from yours would be much better! [Linked Image]
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