Hey you electrical experts, can you give this a quick look and tell me if I'm on the right path. I know the drawing is a little childish but I just threw it together real quick before I had to leave for work. Just want to make sure I'm not missing something before I start into this part of the project. BTW, It's a EZ Wire loom that already in the car (ugh!). Someday in the distant future I'll yank it and put a full cloth loom in it. For now, This'll have to do. Thanks in advance guys!
No guru, I'll have to do until one comes along. Is that a magneto? I guess not but have to ask. I'd get rid of the pressure switch, you do have it wired so it'd work except the fuel pump won't do anything until the starter is engaged, you'll need to bypass that to primed the carb.
Not a mag. Just the Hunt look-a-like distributor. Just wanted the extra safety of the oil pressure activated fuel pump safety switch. Here's a super quickie of how I'm thinking for the fuel pump relay.
the fuel pump will run while you're cranking the engine, which is what you want. It gets power from the starter.
I can't see an issue on the starting unless the car has been sitting for an extended length of time and the bowls go dry.
Start terminal should supply full voltage directly to your distributor, once released off of "start" position then current should flow thru the ballast resistor to prolong points lifespan. Easier to get the engine fired up with full voltage going to the distributor. Oldmics
Wiring looks ok if the pressure switch connects to the NO terminal when oil pressure exists. Does not matter if the ignition switch ignition terminal is open while cranking the engine. Oil pressure will (should) build up during the cranking of the engine, however if the engine starts and does not have enough oil pressure, you could have a problem.
Don't take the power to the pump through the oil pressure switch as I seriously doubt it will take the starting current load long-term. Instead, use the power out of the pressure switch to energize the relay coil and use a separate fused power supply to the relay and from there to the pump. I would check the amp draw on the pump; remember that motors/solenoids have 'current inrush' where actual current can be up to 1200% of the running current for an instant. Using about 300% of the running current when figuring switch/relay sizes will insure long contact life. Your ignition switch is rated at most for 30 amps (some are only good for 20 amps) total connected load, so add up all continuous loads (loads that are 'on' for more than moments when the vehicle is being used; ignition, fuel pump, any of these type loads connected to either the ignition or accessory terminals) to see what you have. You also need to de-rate switches/relays from their 'rated maximum' by 20% to prevent overload and protect them from current spikes; running switches/relays at 100% of their rating will significantly reduce their service life. A 30 amp switch should not have more than 24 amps connected to it. They lose power to the 'acc' terminal when turned to 'start', not ignition.
Is that a race only application? Makes sense to have some oil pressure before the third part of combustion enters the scene.
Crazy Steve-You're absolutely correct. I have it drawn incorrectly. That what happens when I try and think on the way out the door. I used Hunts diagram for the ign circuit hence the reason it's wired to the Ign terminal. Same goes for the Oil switch. Used the Holley drawing. Should have picked up on the high load for pump power going through the switch. Funny that they have it drawn that way.
El Cab-No, just a street driver. I had seen something on the HAMB in regards to the safety switch. Seems like a good idea if you crumple it up and the pump keeps running. Would like to be able to salvage something if it gets munched. I think they call that being pro-active.
If you need a fuel pump relay, then that must be some honking sized pump. I have bought 12 volt electric pumps for carbs that draw maybe 1-2 amps. If that oil pressure switch can't handle that current it shouldn't be used at all.
For what you're trying to do, I think I'd use an inertial fuel pump switch off a late-model rather than a oil pressure switch. That will cut power to the fuel pump in the event of an accident whether the motor stays running or not, plus you don't have to wait for oil pressure to drop before it operates. An oil pressure switch would be more useful as protection IMO on the ignition circuit; lose oil pressure, the motor stops instantly rather than waiting for the motor to run out of fuel.
Remember current inrush. If the switch is rated for say, 5 amps, even a 1 or 2 amp 'running current' pump will likely expose the switch contacts to currents above it's rating. And small contacts tolerate overcurrent less well compared to larger ones.
The PS-64 oil pressure switch will last 10 years in this application...no relay...holley blue pump. I guess it's better to do what the guys say, who've never tried it?
Check your ignition switch, some of them do not energize the run circuit when in the start position. It will crank all day but not fire up! The older cars had a wire coming off the starter solenoid to fire the ignition with full voltage while cranking, once started, the ignition switch went to the run circuit which sent power out through the resistor wire to the coil. Use heavy wire to the fuel pump, the longer the wire, the bigger it should be, the pump will last longer.
As far as a fuel pump relay, better safe than sorry. It's cheap insurance. Joe H-Good point on the starter switch. I think I'm good on this one but I'll double check now.
That switch is designed to operate an oil indicator light... which draws probably 3 watts, or about .4 amp...
That'll work, although I'd make the alternator and fuel pump connections at the battery side of the solenoid rather than at the battery to prevent corrosion issues...
[QUOTE="Crazy Steve, They lose power to the 'acc' terminal when turned to 'start', not ignition.[/QUOTE] Lots of GM ignition switch looses power to the 'ign' when in 'Start' and the yellow wire backfeeding from the starter solenoid supplys power to the coil. Once the engine fires the key releases from the 'start' and returns to 'ign' continuing to supply the coil.
Steve-Good point. That would be the way to go for this car too as the battery is going in the trunk. Speaking of which, I'm trying to figure out what gauge cable to use. Batt in trunk and sol on firewall. It's a 30 Chevy coupe so figure Model A length.
I just used #1 on a longer car and I am starting to think it should have been heavier, it starts without authority. I ran 2 #1 cables from the trunk to the enginebay/frame, 1 cable to the starter and the ground going to the cylinder head and frame. It took a little over 20feet of cable, I had 25' pieces and there was leftovers at each end. It starts and does ok but I might pull it all out and go to 1/0.
Anyone got info on the Oil Pressure Switch being used? So the ignition and fuel pump will not turn on until the starter turns the engine long enough to make oil pressure?
No, the ignition is ran right from the ignition switch. The fuel pump runs either from the starter relay or the ignition switch.
Look at the schematic relay R1. The Oil Pres SW operates R1 and turns the fuel pump Off/On. How does the Oil Pres SW affect the IGN circuit? On the industrial side we use a VDO 0-100 Ohm Oil Press sender. That way you can play with Relay Logic and timers.
The switch closes on oil pressure. Rated at 5psi so as soon as the motor starts to crank it'll close. It's protection so the pump doesn't continue to run in the event of an accident or other issue that shuts the motor down.
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