i did the search and read all of the failures and symptoms of the fpdm going bad. it is obvious that it supplies power to the fuel pump. is it just a relay? what other function does it have? how does it work? if i had a schematic i guess it would be obvious.

 

Imo I think it works much like the ETAC fan controller. It receives a digital request based on what the fuel rail pressure sensor is seeing and then uses pulse width modulation (varies the voltage supplied and ultimately the speed of the pump) to control the fuel pumps output to maintain the desired fuel rail pressure.

 

that would make sense and explain the different types of failures. if it were a pure relay then it could give reduced voltage but more than likely just fail.

sounds like another weak spot in the truck, similar to the solenoid for the 4 wheel drive iwe.

what a place to put something like that and what a way to mount it. would have been much better to install inside on the back cab wall. would be harder to get at but it would be protected and probably last forever.

thanks for the reply

 

The main purpose of this highly complex device appears to be to leave you completely stranded at the worst possible time and place. That said, it does one heck of a job of it.

 

I think the electronics must put off some heat that needs to be dispersed during operation... why else would they have bolted it to that big chunk of aluminum? Aluminum is expensive so they didn't do it for S&Gs.

One of the other indications that it gets warm is that its not integrated into the fuel pump. The lowest cost solution would probably be to put these controls into the pump (fuel sending unit) itself. But if it puts off heat then you don't want it inside a tank of gasoline.

This would also be the reason its out in the elements, instead of in the cab. If it was enclosed with no air movement then it could overheat.

Just my educated guess as an automotive engineer. My realm is brakes, not fuel systems.

 

I very much agree with that, the etac fan controller get hot as hell! I used to have a blister to prove it

 

According to the FSM:


The FPDM receives a duty cycle signal from the PCM and controls the fuel pump operation in relation to this duty cycle. This results in variable speed fuel pump operation. The FPDM sends diagnostic information to the PCM on the fuel pump monitor circuit. For additional information, refer to PCM Outputs, Fuel Pump and PCM Inputs, Fuel Pump Monitor in this section.

 

could a bad fpdm cause random misfires and no crank issues randomly

 

Misfires - yes. No crank - no. No start - yes.

 

I keep getting a random misfire in the number one and number 6 cylinder I have replaced every part that I can think of all 8 coil packs all eight spark plugs still got the misfire replaced with known good spark plugs and known good coils crank sensor cam sensor....... Just keeps coming up pcm is new fule filter new air filter new there has to be something im missing

 

Injectors. Wiring to injectors and coils. Compression.

 

I would think that if it was a fuel pressure problem that you would be getting misfires on all cylinders. It sounds to me like you might have a couple of bad fuel injectors or a bad electrical connector or damaged wiring. I used to have a Nissan Maxima that would misfire occasionally and I finally chased the problem down to a bad connector where it plugged onto one of the fuel injectors. One of the Goobers that worked for the local Nissan stealership had pulled it off and lost the seal and moisture got into it and corroded the contacts. I finally found an auto parts store that could order a complete replacement connector with all the seals, etc. I cut the old one off and installed the new one and never had any more problems with it.