Diesel Place banner
1 - 17 of 17 Posts

·
Registered
Joined
·
318 Posts
Discussion Starter · #1 ·
I have 2 bad PMD's. I thought my harness was bad, but hooked directly to IP and no start, or start and die. I also tested by hooking directly to IP on THREE running trucks and having THREE no starts/or start and die. Pulled the PMD off my bucket truck and now my dump truck runs great.

Looking at these PMD's, I imagine they have a small circuit board inside. Then the two transistors are simply bolted on with the contacts poking into the body of the PMD. How could these things ever go bad. Too hot if in the original location on the IP, but mine is mounted in the hole on the front bumper. Maybe water gets in.

I figure the circuitry inside can take the heat. The transistors however can not. There are transistors all over ebay/amazon/internet for about 10 bucks each. I removed the transistors from my totally dead, useless PMD. I used a brake spring hook to pry them out. I am thinking of doing a $25 experiment. I'll buy 2 transistors and bolt them in to my trash PMD and see what it does (no big deal I always have a spare in the glove box anyway). I don't know how to seal against water intrusion where the contact pins push through, but you guys will have an idea. Maybe applie some high temp silicon with a toothpick tip to the sides of the contact prior to installation?

Transistor MJ15004 Mexico ----that's the transistor, and the bottom number BM0026 apparently is the batch number or something.

Has anyone tried this? Why not on some old farm use trucks?
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #2 ·
NEVERMIND, I kept digging and found all the info and the video put on by ROSWELL.
 
  • Like
Reactions: OkDually

·
Administrator
Joined
·
22,667 Posts
NEVERMIND, I kept digging and found all the info and the video put on by ROSWELL.
I believe if you look in the 6.5L FAQ, parts list section, you can find the part number for the transistors and other stuff.
(y)
 
  • Like
Reactions: Kgw51gmc

·
Registered
1995 GMC Suburban
Joined
·
18,532 Posts
The key is to not ruin the PMD circuit before getting to desoldering the transistor.
It is covered by potting materials.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #5 ·
The key is to not ruin the PMD circuit before getting to desoldering the transistor.
It is covered by potting materials.
Yeah, I noticed that after I RIPPED out the transistors out of my dead PMD. I had a sense of "F it" when yet another PMD failed. I took the front cover off the PMD later, and noticed all the potting epoxy. I doubt there is any way to property cut through that stuff without killing this particular PMD the rest of the way. I have others so it's not a big deal, but in the name of science I ordered $10 worth of transistors from ebay (from china). They are the MJ15004G (the G means lead from what I read, otherwise the same). They were a less than $2 each or so, free shipping.
I'd like to cut through the epoxy if there's a way, then solder on the new transistors and see if it works or not. The contacts on the old transistors pulled out clean so the guts are probably still good.
I wish I knew the best source for thermal paste, I just got some little syringes of it on amazon, probably only enough for one PMD and I have 3 trucks.
And wouldn't it be better if the transistors were in direct contact with the heat sink in order to transfer the heat by conduction in lieu of through air? Those thermalloy covers on the tranistors---Keep them or trash them? Do they create the contact to aid in transistor cooling? What an achilles heel these PMD transistors are.
 

·
Administrator
Joined
·
22,667 Posts
Yeah, I noticed that after I RIPPED out the transistors out of my dead PMD. I had a sense of "F it" when yet another PMD failed. I took the front cover off the PMD later, and noticed all the potting epoxy. I doubt there is any way to property cut through that stuff without killing this particular PMD the rest of the way. I have others so it's not a big deal, but in the name of science I ordered $10 worth of transistors from ebay (from china). They are the MJ15004G (the G means lead from what I read, otherwise the same). They were a less than $2 each or so, free shipping.
I'd like to cut through the epoxy if there's a way, then solder on the new transistors and see if it works or not. The contacts on the old transistors pulled out clean so the guts are probably still good.
I wish I knew the best source for thermal paste, I just got some little syringes of it on amazon, probably only enough for one PMD and I have 3 trucks.
And wouldn't it be better if the transistors were in direct contact with the heat sink in order to transfer the heat by conduction in lieu of through air? Those thermalloy covers on the tranistors---Keep them or trash them? Do they create the contact to aid in transistor cooling? What an achilles heel these PMD transistors are.
The best way to remove the potting material is with a Dremel tool. ( just be sure to wear a protective mask so you don't breath in any of the materials floating around in the air).
Any good computer parts supply like Tiger Direct or Radio Shack, etc.. should carry the thermal paste.
Those Transistors generate a LOT of heat so a small air gap would be preferred with the paste on the heat sink. I would trash the thermal covers

The first thing to usually go in the PMD is the amplification portion of the signal to the ECM which will cause the stalling, no start condition
 

·
Registered
1995 GMC Suburban
Joined
·
18,532 Posts
No Radio Shack anymore, at least they are not selling the same stuff. LOL!!!
For thermal paste.
I will order from Amazon or Newegg.
The brand is Arctic Silver or Noctua or Thermaltake.
You only need a veeeeerrrrryyyy thin layer.
In a CPU, you need only a small pea size.

The size of the heatsink is also important.
You don't need anything fancy just aluminum heatsink.
Make sure both surface are touching each other.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #8 ·
The best way to remove the potting material is with a Dremel tool. ( just be sure to wear a protective mask so you don't breath in any of the materials floating around in the air).
Any good computer parts supply like Tiger Direct or Radio Shack, etc.. should carry the thermal paste.
Those Transistors generate a LOT of heat so a small air gap would be preferred with the paste on the heat sink. I would trash the thermal covers

The first thing to usually go in the PMD is the amplification portion of the signal to the ECM which will cause the stalling, no start condition
No Radio Shack anymore, at least they are not selling the same stuff. LOL!!!
For thermal paste.
I will order from Amazon or Newegg.
The brand is Arctic Silver or Noctua or Thermaltake.
You only need a veeeeerrrrryyyy thin layer.
In a CPU, you need only a small pea size.

The size of the heatsink is also important.
You don't need anything fancy just aluminum heatsink.
Make sure both surface are touching each other.
Thin layer= good to know. I wanted to gob it on there. And our radio shacks are all gone here, killed by internet. Thanks.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #9 ·
The best way to remove the potting material is with a Dremel tool. ( just be sure to wear a protective mask so you don't breath in any of the materials floating around in the air).
Any good computer parts supply like Tiger Direct or Radio Shack, etc.. should carry the thermal paste.
Those Transistors generate a LOT of heat so a small air gap would be preferred with the paste on the heat sink. I would trash the thermal covers

The first thing to usually go in the PMD is the amplification portion of the signal to the ECM which will cause the stalling, no start condition
Very good information. I appreciate it.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #10 ·
What happens if I don't put a resistor in the eBay PMD, will it run at all? I don't want to pick the number 5 resistor out and break it when it may only be in truck couple farm weeks.
 

·
Administrator
Joined
·
22,667 Posts
Thin layer= good to know. I wanted to gob it on there. And our radio shacks are all gone here, killed by internet. Thanks.
Actually, they are online (y)
 
  • Like
Reactions: Kgw51gmc

·
Administrator
Joined
·
22,667 Posts
What happens if I don't put a resistor in the eBay PMD, will it run at all? I don't want to pick the number 5 resistor out and break it when it may only be in truck couple farm weeks.
The way the ECM is supposed to work is that IF a resistor is not detected the ECM will set a PMD code and will default to a predetermined fuel setting.

However, there have been a few reports here in the forum of a no start, no code set condition if the resistor is not installed.
So to be on the safe side install the resistor
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #13 ·
The way the ECM is supposed to work is that IF a resistor is not detected the ECM will set a PMD code and will default to a predetermined fuel setting.

However, there have been a few reports here in the forum of a no start, no code set condition if the resistor is not installed.
So to be on the safe side install the resistor
Sounds like the best bet, only takes few seconds.
 
  • Like
Reactions: OkDually

·
Registered
Joined
·
24 Posts
Yeah, I noticed that after I RIPPED out the transistors out of my dead PMD. I had a sense of "F it" when yet another PMD failed. I took the front cover off the PMD later, and noticed all the potting epoxy. I doubt there is any way to property cut through that stuff without killing this particular PMD the rest of the way.
The way to remove potting epoxy is to heat it with a hot air gun (preferably a hot air tool used for surface mount soldering that has good temperature control, not yee-old heat gun meant for stripping paint). It gets kindof "grainy" and brittle, and then you can dig through it with a wooden stick, or a plastic tool of some kind so as to not damage the PCB beneath.

I have others so it's not a big deal, but in the name of science I ordered $10 worth of transistors from ebay (from china). They are the MJ15004G (the G means lead from what I read, otherwise the same). They were a less than $2 each or so, free shipping.
Ordering transistors from China on Ebay is NOT a good idea. There's a 99% chance they are fakes - probably just any old random transistor from e-waste that they were "recycling" that week. What they do is remove transistors from old garbage, and then clean the old labeling off and re-label them with part numbers that match desirable but hard to find transistors that they know people are looking for.

I'd like to cut through the epoxy if there's a way, then solder on the new transistors and see if it works or not. The contacts on the old transistors pulled out clean so the guts are probably still good.
I wish I knew the best source for thermal paste, I just got some little syringes of it on amazon, probably only enough for one PMD and I have 3 trucks.
You don't need the special "arctic silver" or any other fancy thermal paste that modders use for CPUs. Just regular old white thermal paste. You can get it in little jars for probably $10 - enough thermal paste to last a lifetime. Another alternative is Penaten diaper cream - it's basically the same thing marketed in a different tin for a different purpose - they're both just zinc oxide paste. I have computers that have diaper cream "thermal paste" on the CPUs for years when I couldn't find my little jar of the actual thermal paste when I needed it.

Here's 100g of thermal paste for $15 https://www.amazon.com/Halnziye-HY400-Performance-Compound-Silicone/dp/B00XSXQN5W but that stuff looks to be silicon oil based paste. Here's a good datasheet that compares the silicon oil based paste to the synthetic oil based paste, and also shows that the main ingredient in both of them is zinc oxide. I have the 25ml jar of the MG Chemicals stuff, and use it for a lot of electronics repair, and I've barely put a dent in the jar in probably 15 years now.

Notice how the silicone oil based one has a thermal conductivity of 0.7W / m*K and the synthetic oil based one has a thermal conductivity of 2W / m*K - more than double!

And wouldn't it be better if the transistors were in direct contact with the heat sink in order to transfer the heat by conduction in lieu of through air? Those thermalloy covers on the tranistors---Keep them or trash them? Do they create the contact to aid in transistor cooling? What an achilles heel these PMD transistors are.
Yes, you want direct thermal contact between the transistors and the heatsink. I would keep those thermalloy covers - that's what they're there for. Air is a downright terrible thermal conductor - you do NOT want any air gaps. Also note that in TO-3 transistors, the actual silicon die that's in there is mounted to the BOTTOM of the transistor - the little dome that sticks up to the top is just a cap, and there's a huge air gap between the underside of the dome to the bottom plate where the heat is actually being generated. I think that's the purpose of those thermalloy covers - they make contact from the bottom plate of the transistor up to the heatsink. The way these PMDs were designed, with the hottest part of the transistor being the furthest away from the heatsink is just downright garbage electronics / thermal design.

I'm doing headgaskets on mine, and still have an old PMD mounted directly to the side of the pump. While the intake is off, I might remove it, so I have a dead one to experiment on as well. I take it removing the PMD from the pump doesn't leave any openings in the side of the pump, does it?
 

·
Registered
Joined
·
280 Posts
fwiw and this was a few years ago when I messed around with this.... I thought it would be worth trying to remote locate (a few inches) the transistors as I had a pmd that was failing when hot... well, I got some new transistors but when it came to cleaning the board and desoldering the old ones I think I may have applied too much heat at some point (maybe when soldering in heavy wire to relocate) but when I got it all together I had a no start for the setup. Shelved the project for when my current PMD goes funky and can try again. Next time instead of desoldering I will just nip the leads and drill them out. I already have a nice Al heat sink setup with new transistors waiting. Thankfully my current PMD is holding well and fuel prices have me driving as sparingly as possible.

For those that remember my engine rebuild thread it is at about 12k and holding great but that one weird clank noise persists with no ill side effect.
 
  • Like
Reactions: JMJNet and OkDually

·
Registered
Joined
·
318 Posts
Discussion Starter · #16 ·
The way to remove potting epoxy is to heat it with a hot air gun (preferably a hot air tool used for surface mount soldering that has good temperature control, not yee-old heat gun meant for stripping paint). It gets kindof "grainy" and brittle, and then you can dig through it with a wooden stick, or a plastic tool of some kind so as to not damage the PCB beneath.



Ordering transistors from China on Ebay is NOT a good idea. There's a 99% chance they are fakes - probably just any old random transistor from e-waste that they were "recycling" that week. What they do is remove transistors from old garbage, and then clean the old labeling off and re-label them with part numbers that match desirable but hard to find transistors that they know people are looking for.



You don't need the special "arctic silver" or any other fancy thermal paste that modders use for CPUs. Just regular old white thermal paste. You can get it in little jars for probably $10 - enough thermal paste to last a lifetime. Another alternative is Penaten diaper cream - it's basically the same thing marketed in a different tin for a different purpose - they're both just zinc oxide paste. I have computers that have diaper cream "thermal paste" on the CPUs for years when I couldn't find my little jar of the actual thermal paste when I needed it.

Here's 100g of thermal paste for $15 https://www.amazon.com/Halnziye-HY400-Performance-Compound-Silicone/dp/B00XSXQN5W but that stuff looks to be silicon oil based paste. Here's a good datasheet that compares the silicon oil based paste to the synthetic oil based paste, and also shows that the main ingredient in both of them is zinc oxide. I have the 25ml jar of the MG Chemicals stuff, and use it for a lot of electronics repair, and I've barely put a dent in the jar in probably 15 years now.

Notice how the silicone oil based one has a thermal conductivity of 0.7W / m*K and the synthetic oil based one has a thermal conductivity of 2W / m*K - more than double!



Yes, you want direct thermal contact between the transistors and the heatsink. I would keep those thermalloy covers - that's what they're there for. Air is a downright terrible thermal conductor - you do NOT want any air gaps. Also note that in TO-3 transistors, the actual silicon die that's in there is mounted to the BOTTOM of the transistor - the little dome that sticks up to the top is just a cap, and there's a huge air gap between the underside of the dome to the bottom plate where the heat is actually being generated. I think that's the purpose of those thermalloy covers - they make contact from the bottom plate of the transistor up to the heatsink. The way these PMDs were designed, with the hottest part of the transistor being the furthest away from the heatsink is just downright garbage electronics / thermal design.

I'm doing headgaskets on mine, and still have an old PMD mounted directly to the side of the pump. While the intake is off, I might remove it, so I have a dead one to experiment on as well. I take it removing the PMD from the pump doesn't leave any openings in the side of the pump, does it?
I am going to print out your reply, that's a lot of good information! Thanks! And, no, no holes with PMD removed from ip.
 

·
Registered
Joined
·
24 Posts
I am going to print out your reply, that's a lot of good information! Thanks!
Just for reference, while the intake was off as I was reassembling my engine, I did remove the old PMD from the side of the injection pump, but I dind't do anything with it yet other than dump it in the top part of the toolbox in the garage. I did notice that it has the thermaloy covers on the transistors, but there was no thermal paste between it and the pump.

The good (working) PMD is mounted behind the bumper on a large piece of aluminium plate as a heatsink. I know that one has thermal paste between the PMD and the aluminium (and it's in a MUCH cooler spot being behind the bumper) and it's never had an issue.
 
1 - 17 of 17 Posts
Top