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.

14 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.

9 of 9 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...

3 of 3 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.

10 of 10 people found this helpful.

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

7 of 7 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.

1 of 1 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.

3 of 3 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.

1 of 1 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.


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.

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!

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