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Old 07-31-2005, 09:06 AM   #1 (permalink)
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Reference Material: PMD

Here is yet another edition in the never ending saga of “As your Truck Runs” - or doesn’t…..

As we, read, and re-read the same questions, we all would like to just get the T-shirt that says “It’s the PMD” This little gem has been more cause of heartbreaks, truck sales, warranty claims, and maybe even global warming. (well, indirectly anyway)

The PMD, it stands for Pump Mounted Driver, or FSD - Fuel Solenoid Driver. These are one in the same. Different vocabulary for the same article. Why two names? Two reasons: 1. Different location and 2. To confuse the heck of out people who thought they knew what they were talking about. Different location I say in jest, only because sometimes the PMD (Pump Mounted Driver) isn’t pump mounted anymore, it maybe somewhere else, like maybe in the bottom of the wastebasket.

This engineering masterpiece has acquired a few other names along the way. Most of which were screamed at the top of one’s lungs with their rump hanging out from under the hood on a busy roadside. These names I don’t think are permitted with this forum’s software. Colorful metaphors, and words to make a sailor blush. Things that only get said when in the middle set of lanes on a busy interstate going 85 mph+ to watch the tachometer go to zero. These are the situations that cause one’s rear to chew the seat padding out right down to the lift pump. Yes, it’s our friend the PMD.

What does the PMD do? Well, you’ll know when it doesn’t do it anymore, let’s just say that. The PMD, sits on the side of IP. (Injection Pump)* If it’s in it’s factory original position. It may have been moved. The PMD is the boss telling the IP what to do. Open the fuel solenoid, close the solenoid. Read a few posts down in this thread for more information. Maybe you know this feeling? So, the pump is relieved of it’s duties of delivering fuel to the engine when the PMD quits.

How do I know if my PMD is bad? That is the question of the century. Some believe that if they drive their truck into the dealership with a bad tire, somehow the PMD is to blame for it. Not exactly so, but it is problematic. The only known way of diagnosis is to, well, replace it. If the symptoms stop, you’ll know it was the PMD. There is not PMD test stand, the computer doesn’t have a diagnostic code for it per se. It just doesn’t do what it’s supposed to do, and no way of knowing either. Sad, but true.

Is the PMD the only thing that will cause a stalling issue? No it isn't. There are multiple problems that will appear to be a PMD issue. Problems with battery cables, grounds, ignition switch (or relay better termed, not the key switch) optical sensor filter harness problems and fuel supply issues. It is best to verify as best as you can which component is failing before just throwing parts at it.

Why isn’t there a test for the PMD? It has to do with the way that if fails. There is no software running inside of it to test it’s operation. It just stops working intermittently, or altogether. Most often is just quits long enough to stall the engine. It doesn’t have to miss very many combustion events before that happens. Now, read a few posts down further to see extra information on this question.

Can I fix my PMD? Well, yes, and no. You can’t really take the PMD apart to fix the component inside that has failed. However, there is something that you can do to prolong it’s life. Re-torque the nuts on the backside of the PMD. When the PMD is removed from the pump, on the back side you’ll see to plastic covers. These cover the transistors. These transistors drive the fuel solenoid. They carry a fair amount of current too, with a rating at 500 watts a piece. I don’t know if you could play any tunes on your stereo equipment with a PMD, but hey. . . These transistors put off a lot heat while running. Since they are also in the engine valley, they are exposed to high temperatures, then they cool. Repeat the cycle over and over again, and you will see that this will loosen things up.

There are a set of nuts on these transistors that can be carefully loosened and re-tightened. Did I mention carefully? If you think that a little tight is good, so a bunch is better, a bunch of money just came out of your pocket for a new PMD. You can break the connections on the backside of the transistors when they connect to the rest of the circuitry. When you gently, (notice gently) loosen and retightened them you do two things. First is you break free any oxides that build up and hinder the connection to these transistors. Secondly your snug them back down again to insure a good connection. Not too tight, but snug enough to insure good contact.

What kills these PMD’s? The main *theory* going is heat. Millions of dollars are spent across the United States to keep precious computing equipment in specialized rooms at specific temperatures. Not these babies. This little hunk of computing power is kept in the broiler at all times. Located in the engine valley and generating it’s own heat doesn’t help matters any. Now, as mentioned, this is our best theory.

Why didn’t they create some way of cooling them? Well, they did. The side of the injection pump where the PMD is mounted has the fuel supply cool the side of the pump. This removes the heat from PMD while the engine is running. Many people think that this is the best designed location to keep them in, because the fuel cools the PMD the best. Liquid cooling is very efficient. Far more efficient than just air cooling, because it moves heat up to 15x faster.

Doesn’t this design keep them cool enough? For the most part, yes it does. Especially while running, fuel is constantly being circulated through the pump, removing heat constantly. Problem arises, as the theory goes, that during the off-cycle, the heat build up from the engine damages the PMD. There is an amazing amount of heat held under the hood during off cycle. Just imaging snuggling up to that engine after being on the open road for a while.

Are there any other options for cooling the PMD? Well, yes there is. Some are good ideas, and some aren’t. Mainly, it’s called remoting the PMD. Moving the PMD off the pump, away from the engine heat. This is accomplished a few different ways.

First method is just merely moving the PMD above the intake manifold and placing a heat sink on it to dissipate heat. Some find that this is a good idea because all you have to do is merely move your PMD up and onto a heat sink. No muss, no fuss, fine with us. The draw back is that the heat sink is only as effective as the surrounding air temperature. If that temperature is 70°F, great. If it’s 170°, then it’s not as effective. Consider the location of the heat sink when remote mounting it. All this to say, if you have mounted your PMD on a heat sink over the intake manifold, you've just condemned your driver to an early death. Period.

Second method, is really remote mounting the PMD. This is taking it away from the engine heat altogether and allowing it to dissipate it’s heat under more tolerable conditions. This involves a heat sink, and extending the wiring harness so that you can move the PMD out farther. You may wish to fabericate your own wiring harness, or purchase one already made up.

To sum up cooling your PMD (since it makes plently of it's own heat) AWAY from all engine heat remote mounted, or mount it on the pump would be the second best. Mounting it over the intake on a heat sink will NOT cool the driver properly.

Where can I purchase a heat sink? There a variety of different places you can purchase them: site supporting vendors and some fuel shops may have them. Doubtful that you’ll find a dealer with one. Auto parts stores, sometimes you get a funny look after mentioning that word “D-Da-Diesel.”

Can I make my own? Why yes you can. A word of caution, you need enough heat sink to dissipate the heat. This is could be an expensive experiment if you don’t get enough cooling. If you are not comfortable with this, you may wish to purchase one already made up. Remember, location, location, location. The cooler, the better.

I’m going to replace mine, what do I need to do? In replacing your PMD, there is a calibration resistor built upside the plug. When you unplug the PMD look up inside and you will see a number. This number indicates what resistor number value you have. They range from 1-9.

What is the calibration resistor for? It is for setting up the pump when on the test stand. It calibrates the amount of fuel delivered. 1 being the least, and 9 being the most. All the pumps are set within a specific spec. The resistor dials in the last of the calibration to the last little cubic millimeters of fuel. see attached calibration value chart below from TD's files

Can I get more get more power out of my engine if I put a #9 resistor in? Why, yes, but not much. You’ll never notice it one bit. Each number is equivilent to 0.5 cubic millimeters. Note: that is read point 5. So what is point 5 of a cubic millimeter - taint much. That is a whole difference of 0.7% increase. That’ll make you dust that ole grocery getter next time at the light.

There is some theory behind the fact that if you have a higher number resistor, it will hold the solenoid open longer, which means the transistor is on longer. The longer the transistor is on, the more heat it makes. More heat, the less good it is on the PMD. Theory, as said, but worth noting.

What are some symptoms of my PMD going bad? Here’s a list of a few things to expect:
°Hard Starting
°Loss of cruise control (this may be other issues too)
°Hair loss, (just kidding)

Is there anything better out there to replace it with? Well, only one product has come on the scene so far. The SOL-D has arrived. It is a new product with not much history yet, but promises to be more reliable. It also is a more expensive product.

How do I remove my PMD? There are four screws that hold it to the side of the pump. It is most difficult to get all the screws. Some have made their own “tool” to reach the bottom ones. You may have to remove the intake to get to them. Since these are so problematic, some will leave the PMD on the pump as a “spare” and remote a new one. Then just unplug the pump mounted one and plug into the new remote mounted one.

Can I build my own remote harness? Yes. You should make sure that good connections are made with solder, so not to have any connection get loose. Otherwise, you’ll have the same problem as a failing PMD. Please note that should not create a harness that will stretch to the back bumper. There is limitation on the length of the wiring, and it will effect the pump operation once it reaches a certain length.

Is there a good location to remote my PMD to? There are a bunch of different places that you can put it. Remember, location, location, location. The cooler the better. If you scan through enough posts, you will see that some use the front bumper, the air filter box, some still in the engine compartment area. You should leave it in an accessible place still.
Is remote mounting the “cure-all?” There can still be a failure with remote mounting, but occurrences seem to be less. Remote mounting also creates a more serviceable location.

Locations best to worst:
1. Remote mount out of engine bay heat
2. Leave it on the pump
3. Mount over the intake on a heat sink.

Notice that #3 is last place, with #2 ahead of it. Pump mounting gives you liquid cooling. The point of failure there, is off-cycle, no liquid is cooling it.

What about my vehicle’s warranty? If your truck is still under warranty, they don’t like to see a remote mounted PMD. Actually here is a good note, because some still fall under these guidelines:

Hope this helps. If you have any further questions, feel free to begin a new thread about your own vehicles problems. Remember, help us out by putting your year and truck info in your signature line. After a few thousand posts, we have a hard time remembering if you are driving a Hummer, van, bus, or truck. Thanks!

Attached Files
File Type: xls PMD resistor chart(2).xls (21.5 KB, 1657 views)
Look here-> GM 6.5 Reference Material & FAQ's <-Look here

Diagnostic Links & Common Failures: Turbo System * Lift Pump/OPS * PMD * Stalling

°Please include your vehicle details in your signature line. Did you read the FAQ's and search before posting?
*Bus 1, rest in peace. Bone stock, now just bones. 4.10 rear 10/13 mpg, 182,000 miles with a set of MCI motor coach seats.
*Currently driving an "Canadian Camo" Snow White 2001 5.7L Chevy Express.
*Bus 2, 99 7.3L PSD.

Last edited by Turbine Doc; 01-17-2008 at 04:28 AM.
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