Does anyone happen to know at what PSI the Ford AC system begins to self-vent from having too much pressure in the system?

Here's what I'm working on, so you guys will understand why I'm trying to find out. Perhaps the creative minds here will come up with an alternative solution too. I'm trying to figure out how to make the AC work with my Cummins swap. What I already know is to take the Ford lines and splice the Dodge compressor manifold to them so it will hook up to the Dodge compressor, but still use Ford everything else -- the condensor, accumulator, evaporator, etc... The Ford wiring will be spliced to the Dodge compressor to control clutch lockup, and the Ford pressure sensor will also screw right into the Dodge end of the line so it will be working too.

The problem I'm running into is how to get the fan clutch to lockup when the AC pressure gets high enough, such as when I'm driving through town or stuck in traffic, so it will draw air across the condensor and keep the system from venting through the pressure relief valve. The Dodge fan has an electronically controlled lockup, that locks when it receives a ground signal. I was thinking I could have a bung put in the high pressure line and put a Hobbs switch in it that could send a ground signal to the fan.

What I need to know is the pressure at which that Hobbs switch should close. I figure it needs to be less than the what the blow off valve is set at, but not so low that the fan is on all the time, causing a parasitic drag on the engine.

The easy option is to set the fan to lock up when the AC is turned on, but then again, I'm dealing with a parasitic drag on the engine, and in a situation where I don't need the fan on, like driving down the highway, having the fan locked would just cause me to burn more fuel. That's what I'm trying to avoid...having the fan locked up when it's not needed.

I've got a couple other thoughts on this though, to know whether it's even needed or not. The Powerstroke fan is not electronically controlled, it's a thermal clutch, so it would never know that the AC was on. If the system didn't need control of fan lockup with the 7.3, and I'm not changing anything but the compressor, do I even need to worry about the fan with the 5.9? The 5.9 fan is still a direct drive fan, BTW.

My other thought is that if the factory Ford pressure switch is still in the line and functioning correctly, is it even possible for the system to over pressurize and start to self vent? I would think that it would sense the pressure rise and shut off the compressor in a low speed, low airflow situation.

I've been thinking about this a lot lately, along with a few other things about the swap, while I'm waitin on the injectors, which should be here this week. I'm trying to iron out some of the details and problems I know I'm going to encounter once the engine is back in the truck and I'm trying to get everything working.

So, what do you guys think about the AC and fan? Thanks in advance for your input.

 

Jeremy,

I believe that what you say, quoted above, is correct.

If all is working properly, you should never have dangerously-excessive high-side pressures.

If you're really concerned about this, there are other ways to monitor the parameters to determine if the fan clutch should be activated. Could it be something as simple as when the high-side goes off, the fan starts?

Pop

 

Thanks Pop. That's what I was thinking too, but another guy I know who has done the conversion did have his begin to self vent once. However, I do not know whether or not he has the factory pressure switch in place. I sent him an email to try and find out though.

What you suggest to activate the fan, like when the high side senses pressure to activate the fan would probably work quite well. That's one reason I'm glad to have this forum and guys like you around...that's something that I hadn't even thought of.

So, if I do this, I'd need to know what the signals are for the two wires coming out of the factory sensor. With only two wires, I'm sure it's a simple pressure switch, but does it complete a live circuit or close a ground? The Dodge fan uses a ground signal to lock the clutch, so if the sensor closes a ground, that would be quite simple to tap into one of the wires (but which one) and run it to the fan. Or if it is a powered wire, assuming it's 12V, I could wire in a relay that would close a ground and send that to the fan clutch.

 

Can someone with a DVD set of wiring diagrams help out here? Mine are on CD and, can you say, "suck"?

Pop

 

Wiring diagrams would most definately help. Hopefully someone will be able to provide some. This may need a bump to the top tomorrow afternoon/evening to get some more exposure.

 

On the 2000 it should be a standard n.o. pressure switch on the accumulator. It completes a power circuit to the compressor clutch from the a/c control panel. The blow off operates at 450-500 psi head pressure. However, I would have the fan lockup at a much lower pressure than the blow off. 300 psi is what I would say to have it kick in if I could set it up that way. On most of the electrically operated fans from chrysler cars, the fans begin increase speed around 250 and are at full speed by 300. Kickout occurs at 400. On vehicles with dual evaporators, typical high side pressures will be around 300 constantly at full load.They use a pressure sensor in most newer applications and the PCM controls the compressor.

I wouldn't necessarily have the main engine fan come on with the compressor cycling. If it is a viscous fan clutch with electronic lockup like on the 6.0 (i've never worked on one) it should provide enough air flow for most idle a/c conditions. Is it possible to have the Cummins PCM control a/c operation? It should have an input requesting a/c operation and if you install the pressure sensor it can control the rest as well.

 

Great info! I see what appears to be a pressure swith on the accumulator, but there's also one on the high pressure (I think it's high pressure -- the smaller line) side, just a few inches off the compressor. I don't know which one does what.

I believe the fan is a viscous fan...the fan does not rotate freely when attached to the engine. There's definite resistance to it when I try to spin it, so it sounds like what the 6.0 has. If that's the case, I may be worrying about nothing, if the fan will pull enough air and the compressor will kick off before blow off.

I don't know for sure if it's possible to have the Cummins PCM control A/C or not, but I don't beileve so. The Dodge trucks have two computers, one for the engine that's bolted to the block, and another for the truck, which you may already know. The only one I have is the engine computer on the block. I don't know which controls what, but I'm doubting that the engine computer would control the A/C. I do know that the fan lockup is on the chassis harness though, and not the engine harness. However, that's another avenue that I could pursue.

 

not sure if its what your looking for...this is for 2000 truck A/C wire's

 

No, actually I didn't know that about the computers. I've never worked on any dodge or cummins setup. Guess it makes it easier for you to some extent if you can keep the truck and engine separate.

The first diagram in that set is the important one. It is interesting what ford has called the HPCO (high pressure cut out) switch. "refrigerant containment" ha ha. I couldn't remember if the SDs had one or not, so they do. It won't do you any good though, it's wired in series with the clutch cycling switch. Apparently there is a monitor to the PCM as well, but it looks like it is just to see if the clutch is actually engaged or not, despite the fact that it is referred to as "head pressure switch". So you are already covered Jeremy.

Having the engine fan engaged with the a/c on or under high demand can improve it's performance but you will have to see if it is even necessary. At least you have the option down the road. IIRC, the 6.0 only uses the electric fan clutch for high temp situations, not necessarily high a/c head press, but I could be wrong. I just don't remember seeing one kick the fan on as the head pressure rose above what I would have liked to see.

 

Thank you Ron. Those diagrams will come in handy for sure.

aldridgec, I appreciate your input. I think I'm just going to hook it up and see what happens. If the system starts to vent or not cool very well, I'll start looking into options of hooking up a fan lockup.

I'm all worried about losing refigerant, but I forget I've got a friend who works at the Ford dealership that can refill the system for me with their big $7K machine...the one that puts everything under 30" of vaccum for a half hour or so to suck all the moisture and such out, then fill it from there.