On acceleration vent goes to defrost and when I back off on the accelerator, it returns to vent. Any ideas how to fix?

AH, 'cmon tennisshoes, that's at least 3 cents and ten points worth

It seems to me that there probably is a vacuum check valve somewhere between the HVAC system and the engine. If it was directly piped with no check valve, many cars would exhibit similar behavior. --- The first car I drove on public roads was a English Ford and it had vacuum powered windshield wipers. When you were accelerating from a stop when it was raining, the wipers would slow down to almost a stop. I had to ease up on the gas pedal to speed up the wipers to see where I was going...

I had same issue with 1996 Dodge. Look at your vacuum hose dia. find the hose that goes to heater controls and install another check valve. Also not a bad idea to check your intake vacuum with a gauge and see what your engine is producing for vacuum. You should have at least 17 in hg at idle. The needle should respond smoothly with accel and decel of the gas pedal. The following is a good ready reference for vacuum systems... .......... You should get between 17-22 in. hg. depending on your elevation above 2000 feet MSL or mean sea level. To perform this test hook vacuum gauge directly to manifold vacuum. 1. Low steady needle usually indicates leaky intake, a leaky vacuum hose, late timing, or incorrect camshaft timing. 2. If reading is 3-8 inches low and fluctuates at that low reading suspect an intake manifold gasket leak at intake port or faulty injector. 3. If the needle has regular drops of about 2-4 in hg. at a steady rate the valves are probably leaking. (Perform a compression check). 4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire. 5. A rapid vibration of about four in hg. indicates worn valve guides. 6. Slight fluctuation of one inch or so indicates ignition problems. 7. Large fluctuation perform a compression check. 8. If needle moves slowly through a wide range check for PCV issues.

Hey is there any check Valves in the air conditioning on a 03 dodge ram? And where are they.

buy a chevy, my 2000 ram has the doggy 360 engine and it always has vacum around 16 hg( which the dealer swore, until they went under, was normal) I have found the line for the cruise control goes bad and the vacum leak hurts the air direction in the cab. if you really jump on a 360 it will default to defrost because the engines are so poor, but check the entire line to your cruise control, and you may find it leaks. i also had the intake manifold gaskets leak, which you cant see. if not a line do the intake gasket to see if it helps, if over 75 thousand it will need them anyway.

I'm having the same problem with my 01 Dodge Ram but it didn't start until I changed the actuator for the 4 wheel drive and my cruise control wont hold speed going up hill, please help.

I have a 96 Ram 1500 and suddenly the Cruise Control quit working...light, but won't set. Also noticed heater went to defrost when accelerating. Was working on clip on battery box and noticed the vacuum hose to the control module is broken. Haven't got it fixed yet, but thinking that may be the cause of both problems.

Kevin, explain to me how the 5.9l is a poor engine. I've had 3 of them so far and put 265,000 on one of them before I sold it. I'm driving a 98 dodge ram with a 360 now and I've had it for 7 years. 0 problems except for the common vacuum problem. Stock the magnum 5.9 puts out 250hp and with a 300$ tuner you can get 300hp and close to 400ft lbs of torque. With just a tuner.

Absolutely. I've towed with every 360 I've owned and they've never let me down. If you really want to wake up the magnum get a superchips tuner. The dodge 360 gets some of the highest horsepower and torque gains from a tune. Very noticeable difference. For 300$ it's about the cheapest horsepower gains you can get. And you get 5 different settings. Performance, towing, stock, mileage plus, and 87 octane. The 87 tune will take any ping and spark knock out when you run 87. Most magnum 360s don't like 87.

And yeah the 440 is an awesome big block. Friend of mine had one in a 70 charger. Phenomenal torque

First, the 360 is a great engine. My 97 just turned over 206K and has only required 2 water pumps and one fuel pump beside normal maintenance. Still does not require oil between 4,000 mile changes. My cruise servo was leaking vacuum which caused two things. First the cruise would not hold speed on hills and second, my A.C. went to defrost on hills or acceleration. Replaced the servo and all is well. I love 360's!

I have a 2000 Dodge Ram VAN 2500 5.2 l. I have the same problem- blower switches from dash vents to defroster at part throttle, returns properly when you lift off the throttle, just like many others here. After much searching on the net, here's what I've figured out: First, on the 318, there's a vacuum port on the passenger side of the intake. Small molded 90 degree nipple, then a check valve, then a Y, one plastic tube goes to the passenger front side, behind/below the headlight, to a T, one drops to the Cruise servo, the other to the vacuum reservoir. Going back to the other side of the Y, that hose goes up and to about the center of the firewall. There it goes through via a rubber grommet, and straight to the control switch. 2 lines come off the control switch and pass back through the same firewall grommet. One tube goes to the air control door behind the left headlight area. The other goes to the heater hose water valve solenoid. Other vacuum lines coming off the control switch go to different controllers under the dash. I traced and checked all lines... all like new. Check valve- perfect. Under the dash- unmolested and like new. I've been told the number one culprit that causes this issue is... the control switch on the dash! So, I hope this helps lots of you guys that have been searching for the answers and keep coming up short... like I've been. Looks like $150-$225. It's the entire 3 switch rack in one unit. Dodge should've had a recall and a revised/ improved part to fix this common problem... but didn't. At least not that I'm aware of.

The final solution to my 1997 Ram 1500 - 5.9L air conditioning going to defrost on partial acceration - and - the cruise not performing well, was finally traced to a break in the hard plastic line going to the cruise head. Noting to do with my dash controls. The line had a half moon crack where it plugged into a 90 degree rubber elbow that in turn plugs onto the cruise head vacuum nipple. The crack was even with the end of the rubber fitting and could not be viewed or felt until I removed the cruise head. I simply cut off 3/4" of the hard line and inserted the hard line back into the rubber elbow and all is perfect, both A/C and cruise. As a side benefit my 207K miles truck runs better and just got 17.3 mpg on a 250 mile city/highway trip, up from 14.5 before finding the vacuum leak. 5.9/360's forever. I love the new hemi's also, don't get me wrong.

I found the same thing on my 96 Ram 1500-5.9/ Hard plastic vacuum line to the cruise vacuum module. That fixed my cruise control, but upon hard acceleration, I still get some AC going to defrost, so probably another leak somewhere, but not nearly as bad as it was. I found mine when I replaced the batter and the clip nut broke and I had to take the battery box completely out to put on a new nut...and saw the vacuum hose broken.

As for the comment that the 360 is a doggy engine with all the problems this guy mentioned.....he must have gotten a dog! I have run my 96 Ram 220,000 trouble-free miles except for the common vacuum problem with the hard plastic vacuum hose breaking as it turns the corner going into the vacuum control. New it got 19MPG and still gets around 16 with this many miles. It has had hard use, but been well maintained. I pull many trailers full of hay weighing 8500 pounds and pull a stock trailer with 4 horses all over the state and not one problem! Also went clear to PA to pick up a tractor weighing 9500 pounds on a flatbed trailer weighing 2 ton. Ran 70 plus most of the way, never heated up, and ran like a top At this mileage, I am still running at 40psi oil pressure. Would never own a Chevy since the morons went to that DexCool crap that plugs up your radiator and looks like you poured cement in it.

I am experiencing the problem with vent switching as well. Q; Could the windshield fog outside while the AC is coming from the defrost vents, on a 90* day?

Possible, but unlikely. Fix the core problem by fixing your vacuum leak. It is almost without exception the rigid plastic line to the cruise servo, right at the servo. Trust me. As for fogging, sounds like heater core trouble.

I've looked at that line at the servo and the connection. Seems fine, I did pull a vacuum test on the intake, on the right side of the engine, where the heater control / 4X4 lines originate. I pulled 20 psi. the elbow rubber fitting seems loose before the Check valve. Could this be where my leak origin is? Still may remove the heater / Ac control panel and pull a test there. Thanks Mark helpful

I'm having the same problem, air switching to defrost upon acceleration. Only difference from my truck to you guys is I do not have cruise control on my truck so where should I look for my vacuum leak I've looked everywhere?

Gentlemen: I am not sure who is asking exactly what, but here is my actual experience with my 97 Dodge SLT Laramie, 5.9L automatic. Over the course of 2-3 years, my cruise slowly stopped working and my A/C would vent through the defroster ducts on acceleration. I looked and looked and looked and finally discovered that the hard vacuum line to the cruise head was cracked with a quarter moon opening right at the rubber 90 elbow that plugged onto the cruise head, impossible to feel and see with the elbow on the cruise unit. Repairing this break fixed my cruise and A/C problems. An added benefit was increased engine power and almost 2 mpg improvement. With this said, I can assure you that your A/C problem is a vacuum leak, cruise or not. You just have to find it and as I pointed out, it can be impossible to see. Actually, as part of my search, I disconnected and plugged various vacuum lines or vacuum using items until I discovered that cutting and plugging the line to the cruise head fixed my A/C problem. That lead me to go the difficult route and get to the cruise head under the battery of course. Hope this helps. Paul Boyd

Gentlemen: I am not sure who is asking exactly what, but here is my actual experience with my 97 Dodge SLT Laramie, 5.9L automatic. Over the course of 2-3 years, my cruise slowly stopped working and my A/C would vent through the defroster ducts on acceleration. I looked and looked and looked and finally discovered that the hard vacuum line to the cruise head was cracked with a quarter moon opening right at the rubber 90 elbow that plugged onto the cruise head, impossible to feel and see with the elbow on the cruise unit. Repairing this break fixed my cruise and A/C problems. An added benefit was increased engine power and almost 2 mpg improvement. With this said, I can assure you that your A/C problem is a vacuum leak, cruise or not. You just have to find it and as I pointed out, it can be impossible to see. Actually, as part of my search, I disconnected and plugged various vacuum lines or vacuum using items until I discovered that cutting and plugging the line to the cruise head fixed my A/C problem. That lead me to go the difficult route and get to the cruise head under the battery of course. Hope this helps.

I have a broken hard plastic vacuum line in my '99 3500 van. Does anyone know where to buy replacement line? Dealership says it's discontinued, and auto parts stores don't carry it.

Thanks for the advice. I'm assuming that flexible rubber hose would work just as well as the hard plastic hose? Just wondering about pulling a vacuum on something that was flexible.

Someone mentioned installing a 12v vacuum pump. How difficult is this? Would it be possible to install a pump ONLY on the line going to the aur controls? Would this screw anything up? I have wild vents too, supposedly no vacuum leaks, and when I let off the gas I hear the hissing of the vacuum pressure return back tot he control switch inside. Please help!

Your problem is a vacuum leak plain and simple. A vacuum pump is just a large band aid. Look very closely at the line and connections at your cruise servo or the servo diaphragm itself may be leaking.

I have posted a thread about this. This has been the most incorrectly answered question and problem about the ram 1500 that i have found. First I am a Master mechanic Retired of 30 years from a Dodge Dealer. The problem is most often and 99% of the time cause by the check valve that controls the Vacuum line manifold system that is hooked up to all of those vacuum modules . Look on the passenger side of the engine compartment and locate the multiple hookups that all come from one line. it looks like a mess of multi sized plastic lines that all hook together. This vacuum set up is what is losing vacuum. There is normally 1 cause for that and it is the check valve that is located at the manifold 90 deg connector for that line. there is a check valve at the manifold hooked up to that line system. Most models have 3 connection points there coming off the manifold. It is the Middle connector or the 3 , ( if someone has not changed it) that if you look closely at the rubber 90 connection that has the check valve right there at the manifold. simply remove that check valve and replace it. A new check valve is about 5$... solves that vent problem and cruse control problem 99.9% of the time.. some replacement check valves have the wrong sized hose hookups on them , just get a vacuum line connector reducer that will go into your hose and reduces it down to the mini line and then buy a short section of line in the correct size and hook it up... it does not matter if the line is smaller......vacuum all works the same... also when replacing the check valve make sure it is going the right direction, they are a one way valve....they must be able to open if sucked on from the engine side ... thats what creates the vacuum.....good luck...also when you are checking for vacuum leaks a can of starting fluid comes in handy. LIGHTLY spray it around lines, if there is a leak at a location, it will increase the rpms when a leaky line is sprayed.... remember Starting fluid is flammable , so LIGHTLY spray the lines...you are best to use that method of location with a COLD engine. Its a heck of a lot safer that way.

Cw95525 thanks alot!! And thanks everyone who was involved in this thread!! It ks much allreciated!!

What does the check valve look like that's on the passenger side can anyone tell me please....

This was like mine on my Dakota.

CW95525 I have done a vacuum check on all the connectors I can find and they all hold vacuum. I changed the Check valve on the manifold and the one going to the vacuum can but still have the same issue... hard throttle air goes to heater to get it back to air I have to turn it off. medium throttle the air goes to defrost till I back off.Can you give me any ideas on where to go from here... Thanks for your time

any place the hard plastic lines are close to a heat source like a manifold they can become brittle and crack ... sometimes there can be a crack in the line at the vacuum can..check the fittings there if that does not work a replacement of the vacuum can may be needed. depending on what model you have , the can is located under the battery box , you have to remove it for access . Good luck..

Thanks... I have done a vacuum check on the can but I'll check all the kines and see what I find Thanks again

Anyone have vacuum issues on giving throttle its the lines not leaking they are weak and folding inward find the weak one by pressing in fingers if not hard then replace even behind the dash track them all

I have this same issue but when my air vents switch with acceleration the engine also Boggs way Down to where it will die if I don’t let off. Do you think theses are two separate issues? From the research I have done it seems like my issue maybe the intake manifold gasket. Would a leak in the gasket also cause the symptoms described in this thread?

Mine had large hissing at idle then engine would die air idle control was sticking cheap part to reaplace however I pulled it an cleaned it fixed problem later found online $10. & replaced (saved old one)

David, same issue with 98 b1500 van when idle or would sometimes kill when switching from reverse to drive. Replaced idle air control valve (very inexpensive), which helped with idle issues, but still has killed a few times when switching from reverse to drive. Definitely have an AC vacuum leak that hasn't been repaired yet, so I'm thinking the issues must be related. Hope this helps!

Can anyone help me I have a 96 dodge with the 5.9 magnum in it I replaces the fual pump and the idle air control and the map senser but when I come to a stop it stalls when I push the break real hard and it only does it on warm up after its warm it idles just fine and takes off it get to about 130 140 and when I do the first stop thays when it stalls if anyone could help that would be great

Some times juat easing the break it stalls as well