12v fuel pump in 24v vehicle?

[ZGjethro]

I purchased my M37 about a year ago, and put two new matching batteries in it. It ran great for that year after some work, but now I have charging issues. I get 28.5 volts when the truck is running and about 25v when it is off, shortly after running, but it is not holding a charge. The problem is the fire dept I bought the truck from tapped 12v from one battery to power the 12v fuel pump. I think this is a bad idea in a series battery system as it leads to an unbalanced system. I would like to replace the 12v electric fuel pump with a 24v model. Do you guys have a pump model which works well?

I would also like to run the fuel pump power through a relay which closes when the engine is running. I have heard of using the oil pressure sender, but am unsure how to wire it. Currently the pump runs as long as the ignition is on.

[snowdad]

ZGjethro,
If you go to the website m37.crwdesigns.com/index.htm, he has an excellent wiring diagram and explaination about what you want to do by wiring it to the oil sending unit. Click on the technique section and scroll down till you see it. It even has a photo of the setup.
Phillip

[Mark@Sea]

Running a tap killed one of my batteries in pretty short order. Lesson learned.

[mikkelborg]

My truck had the same kind of setup on it, also installed by a fire department, it eventually killed both batteries. I learned that push starting 6000 lbs of truck, by yourself, with a stone cold motor, sucks! When I replaced the batteries I replaced the fuel pump too. Instead of buying a 24volt electric, which was special order at my local Napa, I asked for a mechanical fuel pump for a Chrysler Industrial 230, basically the same as our T245. It bolted right up and has worked flawlessly for me, it even came with a nice little glass sediment bowl. My only complaint is that there is not a primer handle on it so it takes a lot of cranking to get started if the fuel lines are dry, which only happens when I change my diesel style spin on fuel filter (another fire department addition). All other times it fills the carb bowl and the truck fires up just as quickly as it did with the electric pump. I opted to go mechanical because it eliminated the extra wiring, reduced the load on my charging system, and I did not have to run a separate fuel pressure regulator any more. Also, a new mechanical pump from Napa is compatible with modern gas so it won't fail right away like a NOS original style fuel pump.

Collin

[ZGjethro]

Snowdad, thanks for the great link. That was exactly what I was looking for. I was wondering how to use the oil pressure sending unit, when an added pressure switch needed to be added. I am not sure if I will go electric, or put on a new mechanical unit, like Mikkelborg did. I like the simplicity of the mechanical unit, as long as it works as well as the electric pump. I currently have a negative disconnect, the ignition switch, and a 12v switch for the fuel pump. I need to turn all three on or off. Are there modern rebuild kits for the original fuel pumps which work with modern gas? I would like to keep the vacuum boost of the original

[cuz]

Yes there are modern kits in the $80 range.

Here's a wiring layout for use with a relay.

Wiring fuel pump.JPG (81.5 KiB) Viewed 3799 times

[Master Yota]

Another option would be use both the electric and mechanical - just not at the same time. Keep the electric for priming the lines and pump, and then shut it off. Some electrics have a flow through design that allows fuel to flow through them when they are off. A primer pump is very handy, and a good spare if the mechanical one fails unexpectedly. The best part is without the electric pump running constantly, there "should" be no harm done to the batterys if its used intermitantly.

Two birds - one stone...

[8543bob]

GOOD MORNING ALL,
I went with a 24V electric fuel pump. The system works great. I did not wire thru a oil pressure switch, but direct thru the igniotion. I got the pump from Sam Wierner in Ohio. he deals lots of surplus equipment. Its a 24V bendix pump that does not need a pressure regulator. Its worked great for me for close to two years.
I have used the oil pressure switches on various marine applications. I have found that some have developed leaks around the plastic. Go with a top notch switch. good luck, BOB

[vargaspj]

I used a FACET electric fuel pump part number 480564E- 24V Negative Ground, 4-5 psi, 32 gph. Includes anti-siphoning valve (positive shut-off). 1/4-18 internal fitting. Available from AIRCRAFT SPRUCE for $97.50
12-24 inches dry lift, Transient Protection, Sealed Electronics, Built in check valve, Built in Anti-Siphon Valve, UL Certified, Complies to Mil-std-461D, Replaceable 74 micron filter, Built in magnet to collect metal particles for
additional filtration, 1.6 amp average, Self Priming and Self Regulating, 32 ounces and 5 inches, Correction Resistant over 100 hours of salt spray, Built in Pressure Relief.
Mounted it on the frame back near the fuel tank. It has a wire coming out of the case; I put a Packard connector on the end and connected it to the existing wire from the switch on my dashboard.
It had the right PSI as the rating on the original mechanical fuel pump (4 to 5-1/4 PSI) so it doesn't overwhelm the float needle on the carburetor and cause problems.
I replaced another electical pump in the same location that had a 6 to 8 PSI rating and it was causing me problems at the carburetor.
There was a switch on the dash (MIL-type positive-locking type) that was there when I bought the truck; you have to throw the switch to ON to activate the fuel pump before starting.

[ZGjethro]

Thanks vargaspj. I just ordered the Facet pump this morning. Replacing the 12v pump with a 24v electric one is easier for me than re-plumbing in the mechanical one.

[N1VSM]

FYI, marine (sailing kind, not beach storming kind) applications are all 24V. I got a 24V marine diesel fuel pump with 4PSI out for my truck - works like a charm. Just remember to mount it near the tank, as they push better than they suck.

Along with a 5PSI oil pressure switch, I hooked a 60A zeener diode inbetween the starter switch and the + in on my pump. That way, the pump gets power while my foot is pressing the starter. Gets rid of the need for an override switch. And, yes. It is fused in all the right places.