Can anyone tell me with any certainty, if the pmd changes the duration that the fuel solenoid is on or if it changes how often it cycles in a given time period. I am going to try and scope the signal to the Driver and then to the solenoid.

The FSD/PMD is slaved to the ECM/PCM: Its duty cycle timing and duration will be as commanded by the PCM, and will vary with load, temp, etc. The FSD itself makes no decisions or adjustments. I'd think you would need to graph and compare actual FSD parameters with PCM commanded figures to get an accurate estimation of what the fuel solenoid should be doing.

Oh, I understand all that. What I need to know is what changes? Length of solenoid opening time or the frequency of the opening events.

Both will vary; duration based on load and commanded fuel rate; frequency with RPM. There will only be one cycle per cylinder firing event; obviously there is no pilot injection, etc.

This is the way I understand it but I am no pro when it comes to the operation of the fuel system on these trucks.

There is no pulse width modulation like a gasoline fuel injector. They have an on/off digital style signal. No voltage moderation. Frequency is only affected by engine rpm and signaled by computer. Dwell is affected by computer calculations derived from sensors and conditions.

The PMD is simply an oversized transistor, or maybe a better word would be driver. We all know that a transistor really works very similar to a Bosch style relay and is often used for the same purposes. All it is there for is to convert a computer generated signal into a controlled power source that will be large enough to handle the load.

So in theory then, i can scope the signal to the driver and from the driver on known good pumps to get a corrolation, then when the c-time is out of spec, I can monitor and verify if the driver is the problem or not without just swapping parts.

So in theory then, i can scope the signal to the driver and from the driver on known good pumps to get a corrolation, then when the c-time is out of spec, I can monitor and verify if the driver is the problem or not without just swapping parts.
In theory, yes

I agree.

You would need to record a pretty decent amount of data to positively verify what you are seeing on the scope. PMD failures are often spontanious and inconsistant. It may be tough coming up with a large enough volume of data on "as they fail" PMD units.

Give it a try, it would be very interesting to see the information.

I would bet that the 'failing' or 'intermittent' PMDs are experiencing electromigration (spontaneous internal connectivity , or 'power leakage'). That is just SO hard to find, even with a 'scope, and even harder to do anything about.

We get that happening in switches and motherboards when we have ventilation failures... usually, it's too late to do anything about and we just put in new parts. The only solution is to prevent it from happening in the first place... cooling and contaminant barriers (which work against each other).

There is a fairly good reason why Heath's design is the only one that's warrantied for 7 yrs. With the exception of some home-built units I've seen on here, nobody else has addressed those conditions.

I am seeing a lot of issues with the c-time coming out of spec on these at higher rpms. This is with a new PMD, fuel control solenoid and armature. Just trying to pinpoint exactly where my culprit is. On the calibration stand there isn't alot data since everything is controlled by the computer. Just hoping that I can see that c-time difference with the scope.

Ahhh... now that might be worth digging for...