Oil pressure sending unit bypassed... no fuel power

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Oil pressure sending unit bypassed... no fuel power

Postby ColinOpseth » Sun Mar 28, 2010 4:36 am

Hey,
OK, so I probed my fuel pump connector at the tank (blue wire). No power. I know that the oil pressure sending unit was bypassed (as in, it's gone). Here's what was done. I think there's a factory splice in the harness where the two purple wires join together. One goes to the solenoid switch and the other to the oil pressure switch. Basically, someone cut that wire that went to the switch and connected it to the dark blue that comes out of the firewall plug. I suppose you'd consider that bypassed.

Even still, I should have power at the fuel pump, right? I have my dash completely out right now so I can figure this out. My headlights weren't working and I fixed that (bad ground). About half of my dash lights aren't working, either. Funny thing is, I can't see where the harness could be unplugged. Am I missing a ground under the dash somewhere? I don't see any wires grounded anywhere.

Damn, I hate wiring. At least the car should run. I had my wife turn the engine over and I got spark. If I can get this fuel pump running I'll at least be able to take it down for exhaust. Technically, I can drive it but it wouldn't be pretty.
'72 Vega with '93 Camaro LT1/M29 T56/12 bolt 3.31. 16" IROCs all around. Sanderson headers into duals with an H-pipe and Flowmasters. It's loud but at least it's faster than your grandma's Buick. pwned.
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Re: Oil pressure sending unit bypassed... no fuel power

Postby spencerforhire » Sun Mar 28, 2010 7:46 am

If you can find an original oil pressure switch and plug( and a hole to screw it into), the way GM had them wired is still just about the best way to power a fuel pump. The original plug had pink(ignition), violet(starter) and blue(pump+) wires. at rest(0psi.) the blue is connected to the start wire so it will power the pump while cranking for initial start up. When there is oil pressure(minimum 10lbs., I think) the pink is connected to the blue taking over from the starter. This is a fail-safe method so that in the event of a crash(or stalled engine), the pump will be off. Most fuel injected vehicles are wired like his now, showing once again the Vega was ahead of it's time! This is much safer than just running ignition power to the pump- which is what you'll have to do to get this thing running. For now, find the pink in the harness and splice it into the blue.
The "fleet"-
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
06 Silverado 2WD ex.cab daily
03 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
06 GMC Canyon Shop truck
07 Colorado project( 5.3 4L60e swap)
99 Saturn SL1- wife's car
01 Saturn SC2- son's project
07 Saturn Ion Redline project
and 4 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: Oil pressure sending unit bypassed... no fuel power

Postby ColinOpseth » Sun Apr 04, 2010 3:02 am

I'll get it sorted out after it's back from the exhaust shop. I have a new other things that will keep it in the garage until they're sorted out so this is on the list.

Until then, I want to run my fuel pump from the correct place (in-line, near the tank). Can I run the pump using the spade terminals on the fuse panel? There are three empty spade terminals (BAT, ACC, IGN). I probed with my voltmeter and only get power on the ACC when the key is in the run/accessory position, and only on the IGN terminal when I crank the engine. I should be able to join those two terminals into one wire using a butt connector for now? I won't be driving it on the street much, just to sort out a few things. Or would I have to run two leads to the back of the car and join them directly on the pump?

Thanks again.
'72 Vega with '93 Camaro LT1/M29 T56/12 bolt 3.31. 16" IROCs all around. Sanderson headers into duals with an H-pipe and Flowmasters. It's loud but at least it's faster than your grandma's Buick. pwned.
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Re: Oil pressure sending unit bypassed... no fuel power

Postby spencerforhire » Sun Apr 04, 2010 7:09 am

Colin- That "ign" terminal in the fusebox should have power during ignition AND start, not start only. Whatever you do, don't jump 2 circuits together; as a temporary measure, hook the fuel pump power to whatever pink wire is powering your ignition coil. The original wires from the oil pressure sender must be under the hood somewhere.
The "fleet"-
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
06 Silverado 2WD ex.cab daily
03 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
06 GMC Canyon Shop truck
07 Colorado project( 5.3 4L60e swap)
99 Saturn SL1- wife's car
01 Saturn SC2- son's project
07 Saturn Ion Redline project
and 4 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: Oil pressure sending unit bypassed... no fuel power

Postby 72 SS PNL » Mon Apr 05, 2010 12:34 pm

id use the signal(ign) from the fuse box to trigger a relay , and run the pump power direct(through a fuse or circuit breaker) from the battery or off the alt output terminal

i wouldnt power a pump direct from the fuse box , unless it was the stock vega pump , anything else is gonna draw alot of power , and is best direct from the alt or batt
first and best car owned 72 GT(changed to SS) panel wagon

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Re: Oil pressure sending unit bypassed... no fuel power

Postby sayporsha » Mon May 31, 2010 10:40 pm

Have you checked the fuel pump fuse? Back in 1978 I had a 71 Vega that kept blowing that fuse. I took it to the dealer & they replaced a faulty oil pressure sender. They said if the car thinks it has no oil pressure it intentionally blows the fuel pump fuse to kill the engine.
Jeff
73 V8 Vega
78 Porsche 911SC
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Re: Oil pressure sending unit bypassed... no fuel power

Postby cjbiagi » Mon May 31, 2010 11:27 pm

The oil pressure sender is nothing more than a pressure sensitive switch. It's not going to blow the fuse with low oil pressure, it's just going to open the contact to cut power to the pump. Now, you could of course still have a blown fuse so you do want to check that. This is one of the simplest circuits, so if you can get a volt meter just start checking for voltage at that blue wire. If you do not have it there then start back tracking. If you do then go the other way and start checking it at the fuel pump connectors. You also have to have a ground that is picked up at the rear of the car. No ground, no worky..........
Clyde.........75 Monza 2+2
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Re: Oil pressure sending unit bypassed... no fuel power

Postby stage169 » Sun Jun 06, 2010 12:08 am

Fuel relay circuit is taken from Car Craft. Besides the part numbers shown for the oil pressure switch I think Federal Mogul/Carter PN A68301 is the same. The relay is a typical late model relay used in tons of cars. Good for 20amps on the N.O. contact for a fuel pump.

I looked over my wire harness and shop manual for the other crude drawing. Hope it makes sense. PPL = PURPLE, PNK = PINK & BLUE DK = DARK BLUE :bored:

This is just like Spence said but from a Service Manual.

The pump operates under the following two conditions:
1. Ignition in START position, no oil pressure.
2. Ignition in RUN position, oil pressure (engine running)
The two postion oil pressure oil switch controls pump operation. With no oil pressure, during cranking, current from the starter solenoid PURPLE wire feed, goes through the oil pressure switch, to the DK BLUE wire to the ELECTRIC FUEL fuse to the DK BLUE wire to the fuel pump. Once the engine starts, and there is oil pressure, the oil pressure switch directs current from the PINK wire (from the ignition switch) to the DK BLUE wire, through the fuse, and to the pump.

The manual also says a FULL fuel tank will have max resistance and an EMPTY tank will have low to zero resistance.

Good Luck, Brian
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