Heres one i have been trying to figure out for a while. I have long suspected that there is a fault by design in our PMDs. I have been trying to figure out the curcuit and there are some complications and findings i would like to share.
1.All faulty units i have tested have intact MJ15004g power transistors leading me to believe heat sinking of the transistor is not the problem.
2. All faulty units showed some minor signs of water intrusion on the edges of the PCB.
3. Sensitve logic electronics and load resistor curcuits are encased in epoxy and will overheat enevitably.This process i believe is to protect intellectual rights on the curcuit.
4.There appear to be several logic curcuits that tie into the turbo wastgate, optical sensor and fuel metering control through the PCM. These are a pain in the to figure out.
5.Switching resolution on the 15004gs can potentialy be as high as 7200 rpm (of couse youd be picking up your heads several blocks back)
What i need to do is figure out all resistor values in this curcuit and after busting open a bunch i realize that there must be a better way to get these things out of the epoxy. All the componets tend to break off with the epoxy. Some i can read with a microscope and i have managed to retrieve the info on about 50% of the curcuit. The logic chips are impossible to read.
Does anyone have any ideas on how to remove epoxy via some chemical or acid?
The back side of the board handles the high current switching of the main transistors that sends trigger voltage to your fuel solinoid driver. Its very interesting to see how the #1-#9 resistors that we stick in the end of the plug affect the curcuit.(maybe some depleted uranium head bolts are in order here). You can see all the transfer pins that connect to the top side of the board.
- PMD Backa.jpg (31.19 KiB) Viewed 5874 times
The top side shows the several logic curcuits involved in calculating the math on pulse widths and so forth. One of the OP amps is almost intact and you can plainly see the 14 pin if, then, else curcuit. Several resistor networks are shown as well. It makes me wonder. Do the military turbo trucks use the same PMD we do? Clearly there are some ways to fuel up these things allthough the motor would need to get the heads buttoned up better with new pistons and some way of cooling and the list goes on.
- PMD Fronta.jpg (39.61 KiB) Viewed 5871 times
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Here are the first traces of the curcuit board for those of you who might find this interesting. I have figured out about 30% of the curcuit which is the simple stuff. A lot of the resistor values were readable under magnification. I have a idea how this thing works sort of. To know more i will have to take a detailed look at a PCM under operational condition in the truck while monitoring saw tooth variables on an scope. I will also need a Tech 2 programmer to disable sensors so i can trace the logic funtions down in relation to the PMDs operation. I guess my biggest problem is I am an idiot. I had the opportunity to learn this stuff in high school and i didnt so now im getting stuck on ohms law equations and having to look stuff up. What i iniatally thought would take a couple weeks will take much longer i think but i will do it.
On the front board there are still several areas i have not been able to map so if you look close under Op amp and chips you can see the incomplete traces. traces are in yellow, board jumpers are the red dots( a few mistakes on photo) chips are blue, grey is major solder connections. The items in white boxes are resistors, capacitors and diodes about 30%determined. There are also a few small transistors whether PNP or NPN i dont know yet.
- PMD front plot-1.JPG (163.01 KiB) Viewed 5871 times
The rear of the board is basic. Only 4 componets attach on this side and they would be 2-1ohm 5 watt resistors and 2- MJ15004g power transistors. All traces are in red.
- PMD rear plot-1.JPG (103.94 KiB) Viewed 5872 times
My initial summary on all this is vague because i just dont know enoegh yet to blap my mouth off. Clearly to recreate one of these is some work but more important is how to go about making some improvements in the design and function. That is the battle. First off i think its gotta be bigger in size so people can work, fix or modify them to fit there needs. I invision somthing half the size of a laptop in side the truck. TD offset and resistor change should be able to happen on the fly. I would seriously beef up the high current side (solinoid voltage) with bigger wire and isolate it from the logic side perhaps even 2 curcuit boards. Also a 20 second warp switch (5400rpm) on a timer to bypass the govenor curcuit( with propane inj) so you can pass on the highway and last but not least a simple computer fan to cool the transistors and the 2- 5 watt resitors curcuit board. As far as doing an epoxy casing---- i will save stuff that for the body work, Its strong stuff!!!!
Greenmeanie
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Vehicle: 1997 HMC4