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Electric Fuel Pump Operation

J

jon_allen

Regular Contributor
"Hi Gang,
I'm trying to d

"Hi Gang,
I'm trying to diagnose a hard cold start condition on a pair of new small block carbed Crusaders. Some times when I turn the key to the on position you can hear the engine mounted fuel pumps run and sometimes not. Could some one please explain the logic behind this system?
Thanks, Jon Allen"
 
"Here's the deal: To preve

"Here's the deal: To prevent a HUGE fire if a fuel line parted, the fuel pump is wired to shut down if the motor does. This is accomplished through an oil pressure switch (similar to an idiot light switch). To make it easier to fire the motors when cold--and gas tends to evaporate from over a week or so--Crusader wisely built in a by pass system (since there's initially no oil pressure to run the pumps, to fill the empty carbs).

Now, if you can't hear those pumps running on by pass, then you have two options: Repair what's keeping that from happening or--the best solution--install a momentary push button to energize the pumps yourself. Not hard to do, and it'll never screw up again!

Jeff

PS: One strong caution: Before firing up your motors, you should always wheel them over a few revolutions, listening carefully, to prevent damage. (You should hear RRRRRRR, not RRR--hesitate-RRR-hesitate.) If water leaks into the cylinders. There's lots of ways that can happen, and the result is massive, expensive engine damage. If I had fuel injection, I would immediately install a momentary switch to energize the starter(s) without allowing the motor to start."
 
"Hi Jeff,
Thanks for the repl

"Hi Jeff,
Thanks for the reply. It makes sense now. I hate to push my luck here, because in reality I'll probally never be able to return the favor, but could you please explain how the bypass system is wired from Crusader so I can check it out and also do you have a suggestion as to how I can create my own if need be.
Thank You Again. Jon Allen"
 
"Not sure how it's wired.

"Not sure how it's wired. Possibly they use a timer that runs the pumps for "X" seconds every time you turn on the ignition. If so, I'd hard wire a momentary switch (use a horn button switch) and eliminate that sucker. Do it once and you're good to go from now on.

Jeff"
 
"Jon:

look at the wiring di

"Jon:

look at the wiring diagram in your owner's manual (or here if it isn't handy - http://www.crusaderengines.com/Owners%20Info/manuals.html
)

The basic setup provides two paths to energize the fuel pump. During cranking, B+ is fed thru the starter solenoid to the pump. (This is pretty standard in the industry and not unique to Crusader.) This is how the bowl fills initially; if its dry, more time cranking will be required to fill the bowl. Once the engine starts, oil pressure builds. This closes a switch which provides B+ to the fuel pump for 'normal' operation.

If you want to eliminate the excess cranking to fill those float bowls before turning the key, wire that momentary switch to provide a fused B+ to the fuel pump. (It also serves as a backup if that oil pressure switch fails open.)"
 
"Thanks, Mark. I wondered how

"Thanks, Mark. I wondered how they do it. I;ll bet that circuit isn't working right during cranking.


Jeff"
 
I found another variation earl

I found another variation earlier this evening. Sierra has an oil pressure operated SPDT switch to feed the B+ to a fuel pump. Wired thru the same scheme described above. I knew AC made them for auto but didn't know there was a marine variant....shows even old dogs learn new tricks.
 
"Hi Jeff and Mark,
Thank You

"Hi Jeff and Mark,
Thank You both for your help. I am now very confident that I can address my starting concerns. The help from this website is second to none. I'm an Audi mechanic in the Boston area and let me tell you ; The level of professionalism and decency from the members here is not usually found in my industry.
Thank You, Jon Allen"
 
"..."I found another varia

"..."I found another variation earlier this evening. Sierra has an oil pressure operated SPDT switch to feed the B+ to a fuel pump. Wired thru the same scheme described above. "

How can that work? No oil pressure,the fuelpump runs, which it is NOT supposed to do.

Jeff (dumb mechanical guy)"
 
"Common side goes to fuel pump

"Common side goes to fuel pump.

NC side goes to terminal on solenoid. Provides B+ to pump during cranking.

NO side goes to B+ switched thru ignition.

Once oil pressure is established, path to pump switches to the NO terminal and pump runs via ignition circuit. Loss of oil pressure opens switch and shuts off pump."
 
"Gotcha (I think). The

"Gotcha (I think). The only trouble with that way of doing it (vs. a dedicated momentary button) is that you're still cranking AS the fuel pump starts filling the carbs. That takes, what, 3 to 5 seconds, during which time the starter is grinding itself to death.

Jeff"
 
"Yes, but it is simpler from t

"Yes, but it is simpler from the OEM's view.

Your momentary switch is great, mostly from being simple.

The lazy man's way would be to exploit what they do on the EFI (ECU equipped) models. Turn the pump on for a second or two when the key is switch on. Easy to do with a one-shot and a relay."
 
"Hi Guys,
I will probally go

"Hi Guys,
I will probally go with a momentary on/off switch even if everything checks out. I'll wire it from the battery with a 5a fuse installed and tie the other end into the wiring at the fuel pump.This way if I ever want to drain fuel from the tanks I won't have to have the ignition switched on. That could be dangerous. I'm also thinking about adding a one way check valve between the pump and carb to prevent drainback.
Thanks Again. Jon"
 
Jon:

Good conclusion....I&#

Jon:

Good conclusion....I'd be inclined to skip the valve (search this board for "anit-siphon valve" and make that decision after reading).
 
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