Relay in place of Oil Light in Fuel Pump Circuit

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Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Tue Feb 07, 2017 5:14 pm

I am looking at an installation that uses LED's in place of the original incandescent warning lights. In this configuration I am thinking that using an LED as a direct replacement of the original Oil Light would act as a switch and power the fuel pump. So I am looking at using a mini relay to provide the voltage drop required to prevent the pump from running.

Does anyone know whether or not a relay would reduce the current to the pump enough to prevent it from running with a low oil pressure condition?
Steve
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby Monza Harry » Tue Feb 07, 2017 8:37 pm

Steve the light can't back feed to the pump as the power goes to the oil pressure switch and then to either the light or the fuel pump. Harry
I'm not a hoarder I'm a preservationist 78 Monza Spyder (Soon To Be 2+2 with Spoilers)
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby spencerforhire » Tue Feb 07, 2017 9:26 pm

Like Harry said. "Oil" light not an issue, but the "Alt" or "Charge" or Bat"(can never remember which one was used in an H-body) will not work properly with an LED. You could probably use a 5 or 10 ohm resistor in parallel with the LED to get the alternator to charge.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Wed Feb 08, 2017 11:48 am

According to the schematics, with no oil pressure, the Oil Light gets it's ground through the fuel pump. With oil pressure, power is applied to both sides of the Oil Light. This is what prevents the Oil Light from lighting while there is oil pressure.

My other thought is to place a resister between the LED and the fuel pump. Then the resistor would provide the voltage drop that the incandescent bulb normally would.
Steve
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby spencerforhire » Wed Feb 08, 2017 5:21 pm

You are correct in how the light operates, but unlike the Bat light and alternator, it's not necessary for operation of the pump. It will probably work anyway, with hopefully no flicker with the operation of the pump. All LED's need a resistor anyway to operate directly on 12v.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

Visit http://www.spencerforhire.ca
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Fri Feb 10, 2017 5:14 pm

This is actually for another application. I am building a wiring harness for my sailboat and want to incorporate the fuel shutdown function. So my only concern is the oil light and fuel pump wiring. Guess I will give it a try before I build the harness out.

After taking another look at the circuit, the oil light actually has two paths to ground. The fuel pump and the starter. But being they are in series with the light, I guess the light drops all the voltage therefore leaving nothing to activate the starter or the fuel pump.
Steve
1973 Vega GT
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby spencerforhire » Fri Feb 10, 2017 5:25 pm

I use these generic fuel pressure safety switches when installing aftermarket electric pumps in customers' vehicles-
https://www.summitracing.com/int/parts/ ... /overview/
Essentially the same as an H-body switch. Would work the same way if you wired in a light.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

Visit http://www.spencerforhire.ca
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby Monza Harry » Fri Feb 10, 2017 8:04 pm

Steve that "path to ground" through the starter doesn't exist that is the "Exciter" for the fuel pump, during starting that is hooked to the old "Ign" terminal for points type ignition that bypassed the the "resistor wire" used for normal running. Now from memory the light may back feed through the pump but I don't remeber it that way but age is a MEAN mistress! :lol: I do seem to recall that the wiring passes through the seat belt reminder buzzer [that does have a diode in it] so that if you unplug that noisy B :censored: d your pump won't run! Please don't ask how I know. :x :cry: [I do always and always have worn my belt, but I would start the car and do whatever was needed (change the tape, or radio station, down my "Big Gulp" etc.) before buckling up and moving, so in true to me character it annoyed the S :censored: T out of me, so I ended up diconecting the plug at the belt instead]. Harry
I'm not a hoarder I'm a preservationist 78 Monza Spyder (Soon To Be 2+2 with Spoilers)
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Sun Feb 12, 2017 11:39 am

The fuel pump is definitely the oil lights path to ground. It also has a parallel path to ground through the starter solenoid. So if you were to replace the bulb with a solid piece of wire the starter would run and the fuel pump would run. This is why you cannot jumper across the oil pressure switch while it is still plugged in to test the pump.

Now in looking at my circuit in the boat, I will actually need to use a relay in place of the bulb. The relay when activated will light the LED and provide power to an audible alarm on the control panel. I think I may need to use a resistor on the fuel pump side of the relay to provide the proper voltage drop to prevent the fuel pump and starter from running.
Steve
1973 Vega GT
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby spencerforhire » Sun Feb 12, 2017 3:51 pm

You're right to say you can't replace the bulb with a straight piece of wire, but you can indeed put a jumper into the oil pressure switch connector. 3 wires-
Pink= ignition
Purple= starter
blue= pump
Ignition to pump works fine(only recommended for testing purposes, not for regular operation)
Edit-0 Obviously NOT with it plugged in, because then with ignition on and not running, the pump is connected to start.
Guess i didn't read your post correctly.,
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

Visit http://www.spencerforhire.ca
User avatar
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Tue Feb 14, 2017 7:48 am

I did some research on the resistance of the original 194 light bulb used in the indicator, and it works out to be about 51 ohms. The relays I have been looking at are between 280 - 400 ohms. So, looks like I should have no problems substituting the relay in place of the light bulb in the circuit.

Didn't realize there would be that much resistance in the coil of wire in the relay.
Steve
1973 Vega GT
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Joined: Sun Dec 15, 2013 11:37 pm
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Re: Relay in place of Oil Light in Fuel Pump Circuit

Postby sgancarz » Wed Feb 15, 2017 10:37 pm

I know there are a few versions out there, but if anyone is interested, here is my version of a simplified drawing of the Fuel Pump circuit.

Image
Steve
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