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

Asked by May 24, 2013 at 03:52 PM about the 1997 Dodge Ram 1500 2 Dr Laramie SLT Extended Cab LB

Question type: Maintenance & Repair

I have replaced the vacuum hoses and connectors, the check valve and inspected the vacuum reservoir.

21 Answers


That is a common problem when the vacuum supply is to low, you have already done the most common trouble spots, so now it looks like you may have a vacuum servo leaking or a control valve leaking, it can get rather involved tracing it all down so you might need to get a service manual that has all of the specs needed on what vacuum line is what and where it goes. A possible temporary fix would be to install another larger vacuum reservoir into the system or install a good 12 volt vacuum pump into the existing system, that will supply the system with a steady vacuum supply no matter what kind of hill you are trying to climb or acceleration. Just my 2 cents worth.

12 of 12 people found this helpful.

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

2 of 2 people found this helpful.

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...

4 of 4 people found this helpful.

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.

14 of 14 people found this helpful.

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

10 of 10 people found this helpful.

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.

3 of 3 people found this helpful.

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.

6 of 6 people found this helpful.

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.

2 of 2 people found this helpful.

Could very well be the problem parkergip


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.

3 of 3 people found this helpful.

I don't know if he got a lemon or not but I've always liked the the 360 Motor, good power and torque, only thing better was the 440.

1 of 1 people found this helpful.

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!

1 of 1 people found this helpful.

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.

2 of 2 people found this helpful.

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.

2 of 2 people found this helpful.

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.

1 of 1 people found this helpful.

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.

1 of 1 people found this helpful.

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

Your Answer

Ram 1500

Looking for a Used Ram 1500 in your area?

CarGurus has 10,872 nationwide Ram 1500 listings starting at $1,500.


Dodge Ram 1500 Experts

  • #1
    Dan Connell
  • #2
  • #3
View All

Related Models For Sale

Used Ford F-150
2,278 Great Deals out of 181,528 listings starting at $1,000
Used Dodge Ram 2500
158 Great Deals out of 4,939 listings starting at $2,999
Used GMC Sierra 1500
883 Great Deals out of 65,387 listings starting at $1,995
Used Toyota Tacoma
586 Great Deals out of 42,369 listings starting at $1,995
Used Dodge Dakota
108 Great Deals out of 3,690 listings starting at $1,700
Used Dodge Ram 3500
80 Great Deals out of 2,216 listings starting at $5,995
Used Ford Mustang
689 Great Deals out of 38,300 listings starting at $895

Used Cars For Sale

2010 Dodge Ram 1500 For Sale
67 Great Deals out of 1,414 listings starting at $5,995
2009 Dodge Ram 1500 For Sale
40 Great Deals out of 1,053 listings starting at $5,995
2008 Dodge Ram 1500 For Sale
69 Great Deals out of 1,553 listings starting at $3,000
2007 Dodge Ram 1500 For Sale
41 Great Deals out of 1,360 listings starting at $3,588
2006 Dodge Ram 1500 For Sale
51 Great Deals out of 1,254 listings starting at $2,994

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.