Fellas, Worry Not! The ECM most definitely *does have* protection built-in! The engineers ain't dumb when it comes to making the thing reliable. That statement doesn't mean that we shouldn't take every opportunity to add simple protection to our added components! Every little bit helps add reliability to our systems. Failures are dang inconvenient!
Chris, the ECM drives the lift pump directly. No external relay. I'm assuming that it is using protected transistor switching internally. The evidence is that the ECM's lift pump output voltage is 12 volts, even though the alternator voltage is around 14. This is a characteristic of transistor drivers and protection circuits, which have a minor voltage drop of their own. Relays are, to a degree, less reliable than transistors. More moving parts and a contact that can corrode, pit, and go intermittant!
In the unlikely event that the driver transistors in the ECM were to be blasted by back EMF, they could fail either shorted, in which case the pump would run continuously, or open, and the pump wouldn't run at all. I don't know if either of these conditions would set a code or not. Do codes get set with a failed lift pump?
Bob, thanks for your comments regarding EMF. I couldn't have said it better! You're not using a Fluke Scopemeter, are you? They're neat! What kind of filter mount did you use for your pre-filter? I'm sure that others are interested in your inexpensive pre-filter! At $1. 99, you could change the cartridge once a month, and still be money ahead. By the way, my only concern with the PF-2 filter would be with any 'rubber' components or adhesives that are used on and in the filter. Diesel fuel may affect the 'rubber' parts differently than oil. The media will probably be stable. 'Rubber' nowadays could be any kind of synthetic material, including rubber.
In the aviation industry, there is great concern about today's auto 'gas' swelling and deteriorating the various flexible seals and hoses in the fuel system, because of the wierd formulations and chemicals they are putting in car 'gas' today. Some guys have been using car gas in light aircraft for years, and only in the last 10 to 15 years has this become a problem.
One more comment: Adding a diode to a relay that already has a diode built-in will not do any harm. If you're not sure, add the diode! Just get the polarity right the first time! The banded end of the diode goes to + (positive). Radio Shack has billions of 'em.
Thanks you all for your kind comments, fellas. I know I've rambled a bit, but I think it's important stuff.
You're all very welcome!
My next post is gonna be "yup" or "nope".
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Tom
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Tom Glover - VE7DQ, C-GWTG
99 QC SLT, LWB 4x2, ETC/DGP, Wht/Agate, etc.