Tractorman

Since you are the origin of this post, I am giving you all the credit for making me think outside the box. I did a test the other morning - a 15° morning. I started the truck and put the selector to Max AC (recirc) and set the fan to a high setting. The cab definitely warmed faster, but was slow to melt frost on the windshield. I can already see another improvement on the horizon - to be able to use any mode (such as defrost) while recirc is selected. It would seem that this combination would clear the windshield quickly while warming the cab faster. I am going get a vacuum control switch to operate the recirc door independently and make the modification soon. This morning ( a 25° morning) I warmed up my wife's ice covered TDI Passat. I selected recirc and defrost (because you can!) and both the windshield cleared and the cab warmed quickly. So thanks again @Doubletrouble! - John

That sounds like a good solution. I probably would have gone to a mechanical pump for reliability, but since my original VP44 and lift pump were replaced under warranty way back in 2005, I stayed with the in-tank lift pump and fuel pump relay provided by the Dodge dealer. I have since removed the in-tank lift pump and mounted a used frame-rail FASS DRP-2 lift pump with a coarse screen on the suction side. I still use the stock fuel filtration downstream of the lift pump. I did this mod in the Spring of 2016, almost 100,000 miles ago. I carry another used FASS lift pump with me for an emergency roadside replacement if necessary. - John

What really makes no sense to me is why did Cummins drop the OEM mechanical pump when the 24 valve engine came out? Not only was it a very reliable lift pump, but there was also a hand operated plunger as part of that pump to use for priming when necessary. It would seem that the pump could have easily been modified by Cummins for the flow requirements of the VP44. Then the wheel would not have had to bee re-invented and all of the lift pump issues for the 24 valve engines would have never happened. - John

After traveling freeway speeds for a awhile I would just stop at a rest area and put my hand on the carrier bearing to feel for temperature. Normal condition would be for the bearing to run just warmer than ambient temperature. This bearing does not receive much of a load - just keeps the driveshaft centered. When I had around 290,000 miles on the original bearing, I started noticing that the bearing began to run warmer than usual. I changed it out then. Also, this is a good time for you to take apart the slip yoke and thoroughly clean out the old grease and put in new grease. A sticking slip yoke can cause launch shudder and vibrations. Many times launch shudder is misdiagnosed and the real culprit is a sticking slip yoke. This has happened twice with my truck over approximately 347,000 miles. - John

What you are reporting is what I would expect. Good to know that it warms your cab faster. I had the perfect opportunity to experiment with the recirc setting the other morning as it was down to 15°. But..., I forgot to park the truck outside. I should get another opportunity soon. Thanks for reporting back. - John

Intermission Please take 10 minutes and refresh your popcorn! - John

@Dieselfuture asked you if that fogs your windows up more. You said, Ok, in mind I see a conflict here. How are you getting cold air from outside if it is in "full time recirculate"? - John

I commend you for being so considerate to your neighbors. Using the block heater is a great solution. You are a good neighbor! - John

I don't think moving the lift pump to the frame should make a difference, either. In fact, I installed a used frame-mounted FASS lift pump a few years ago. It is run by the fuel pump relay that a Dealer installed in 2005 along with my VP44 replacement and in-tank lift pump conversion that was done under warranty at that time. I use a fuel pressure switch with an LED indicator lamp in the dash for my low fuel pressure warning system. Even with only a 1/4 second bump my fuel pressure warning lamp goes out briefly when I turn the ignition switch to "run". I know I am not much help here, I just wanted you to have something similar to compare to. - John

My truck is an '02 and I am the original owner. Back around 2005, warranty covered VP44 replacement and a retro-fitted in-tank lift pump. At this time the dealer relayed the lift pump using the control signal from the ECM, so I do not have to make this modification. Hats off to you for recognizing that the factory should have never wired the lift pump directly from the ECM in the first place. Great detail in showing how to make the conversion! - John

From the FSM for my '02 truck... The transfer pump is self-priming: When the key is first turned on (without cranking engine), the pump will operate for approximately 1/4 second and then shut off. The pump will also operate for up to 25 seconds after the starter is engaged, - John

The reason it works for them is because of that "Well, everybody does it!" theory. - John

The one in the in the link is 1/4" pipe thread, but still the same concept. It should be a tapered thread and it should snug up if the tapped hole hasn't been enlarged. - John

When I go to the character map, all I get is a list of the guys on the Board of Directors for this site! So it looks like you are using your phone to reply to posts. I am still using my laptop. I did go on line and found out how to do it on a windows based computer. So, with some ° of difficulty I can do it now. I have to hold the ALT key and select 0176 on the numeric keyboard. Someone with a ° must have thought this up! - John

I think that just might work. It look like the nights are going to be dropping to around 12 degrees here in about five or six days from now. I think I will park my truck outside one of those and test the theory when I start the engine the following morning. How did you get the degree symbol in the 45 degrees? - John

Almost true. When the selector is placed in the Max AC position, two separate things happen. 1. the AC compressor circuit is activated. (but the ac compressor does not necessarily turn on). 2. the vacuum controlled door that controls whether fresh air enters the cabin or whether cabin air is recirculated within the cabin shifts to cabin recirculation mode and stays there regardless of whether the ac compressor is running or not. My point in my earlier posts is that once the outside air temperature falls well below freezing for several hours (and the engine has been shut down overnight) the static refrigerant pressure will be so low that the low pressure switch likely will not allow the ac compressor to be turned on. Even if it does turn on, the cycle will be so brief that it will have very little cooling effect. So, @Dieselfuture, even if you turned on your AC compressor toggle switch under those conditions, the compressor still would not turn on and the air would still be recirculating in the cabin. I am thinking that you may already know this - I am just trying to clarify. Thanks for you patience. @Dieselfuture, I am basing all my posts on above statement in the original post. Unfortunately, the description "really cold days" can have different meanings for different people. For me a 45 degree day does not fit that description - a 25 degree day does. On a 25 degree day the compressor is not going to run, so warming up the cab in the recirc mode could work well. No arrow on mine, just a snowflake symbol. - John

You're on the Board of Directors - can't you make him wait? In my opinion the "Max AC" label is a poor choice of words to describe the function that is being performed. How about calling it "Recirc"? - since that is what it does. After all, that is what most other auto manufacturers call it. John

Not quite. Set the top selector to Max AC. This puts it into the recirculation mode. All other selections bring in outside air. I see @dripleyposted while I was writing. - John

If you are considering that an exceptionally cold is a below freezing day, then I would think that the interior of the truck could be warmed faster in the recirc mode without fogging windows as long as the interior of the cab was dry. Of course the heat would be coming out of the dash vents, but who cares if you are not in the truck. Then, like you say, you can take it out of the recirc mode when you get in the truck to drive away. I will have to try that sometime in the near future. I think you might be on to something. - John

This works fine if the outside air temperature is well above freezing. Once the outside temperature falls to freezing or below, the AC compressor cycles less and less, finally to the point it does not cycle at all if the outside temperature falls well below freezing. This is due to the low saturation pressure of the refrigerant. The recirculating heated air will warm raise the saturation pressure of the refrigerant somewhat, but the exposure of the condenser being first in line to the cold outside air will cool the refrigerant so rapidly that the saturation pressure will likely be too low to cycle the compressor. Note the saturation pressure of the refrigerant at 30 degrees in the chart below: A few years ago when my heater core was leaking, my side windows would begin fogging when it dropped to around 40 degrees outside. The fogging would creep around to the edges of the windshield and slowly worsen. With defrost selected he compressor was cycling, but the on cycles were short and the off cycle were long - not giving enough time on to dry the outside air further. With the added moisture from the coolant leak the windows would fog. I even tried setting the controls to AC recirc, but it was worse as I expected it would be. (Note, I have a refrigerant recovery system, so I had the correct weight of refrigerant in the system). At that time I needed to go on a long trip over the Cascade Mountains during the winter. I knew that I would have window fogging problems. I rigged a momentary switch that I could hold in my hand inside the cab to bypass the low pressure switch so I could run the AC compressor for as long as I chose. Once I started climbing the pass the outside temperature fell to below 40 degrees (raining and foggy) and my windows started to fog. I depressed the switch for 2 or 3 minutes followed by a release time of one minute or so and the windows cleared right up. If I were to have done this for a long period of time I am sure that the evaporator core would have started to freeze up, but it got me over the pass with great visibility. The reason I mention this trip is because it shows the limitations of the AC systems in our trucks. @Doubletrouble, your idea may work for you, but you could have window fogging under certain conditions for the reasons mentioned above. A lot will depend on the climate where you live and what the outside conditions are when you use the recirc function for heating. I know that years ago a friend of mine had a Toyota that would allow air recirculation in the cab with heat and no AC. The friend complained that sometimes the windows would fog during the cold season. He told me that he used the recirc all of the time. I suggested the he turn the recirc function off and go to defrost mode or defrost / floor mode. He did and his fogging problems disappeared (literally - ha! ha!). - John

It does seem like a good idea, but there is one thing you are overlooking. That nice warm air will never exit the cabin, consequently the air will continue to pick up moisture (from wet clothes, wet shoes, your own breath, etc.) and that moisture will begin to condense on your cold windshield and side windows. You may be warm and toasty - you just won't be able to see. By always bringing in outside air into the cabin during the heat or defrost modes, the added moisture to the air will always be exiting the cabin while fresh dry air is entering. - John

Then how do we know if it's true? We need references! - John

I don't think you can hijack a thread you started. - John

I Installed these cabin filters from Geno's after I replaced my heater core about 75,000 miles ago. I wished I would have done it when the truck was new. - John

I put an adjustable resistor in mine a few years ago. I set it at 143 degrees. I check the resister every couple of months or so to make sure it hasn't changed - so far it has remained steady. It has worked fine under all driving conditions and I don't get the heavy timing rattle on a cold engine. My morning lows have been averaging 18 degrees for the past few days. I experimented with the setting last year during the coldest part of the winter. I set it at 180 degrees - no side effects. - John