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TBI fuel pump shenanigans

CK5

CK5

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So, long story short, went to tighten down the oil pressure line at the back of the motor and out of habit pushed in on the two wires that plug into it, just to make sure they were on all the way. When I did, my fuel pump kicked on and would not stop until I disconnected the battery.
This is a 1989 TBI 350 in my 72 with no key in the ignition.
I think this is a safety feature to kill the fuel if it doesnt sense enough oil pressure?
Anyway, when I pushed in on the wire connectors one or both of then went in farther and the pump started right away. I reconnected the battery and the pump now turns on, any ideas?
I haven't tried to back the wires off how they were before yet, I wonder if previous owner wired something incorrectly???

20260105_150704.jpg
 
Factory bypass switch

Relay on the fire wall primes the unit
Once pressure is reached that switch closes and drive the pump. You may have collapsed the switch leaving it normally closed

Should be available at vato zone et al
 
I don't know what the terminals are like on an '89 switch, but my '90 takes a different style, Metri-pack(?) with smaller pins.

I have a thought that the '87 may be similar with the 1/4" blade terminal.
 
You guys are talking about just the silver screw in part with the terminals correct?
 
Bent77 said:
Factory bypass switch

Relay on the fire wall primes the unit
Once pressure is reached that switch closes and drive the pump. You may have collapsed the switch leaving it normally closed

Should be available at vato zone et al
Click to expand...
Not sure if related but I do have the coil uninstalled as I am replacing that.
 
Before I tightened the line and cleaned it with brake cleaner...

1767663011641.jpeg
 
That switch is wired in parallel with the relay.
It will power the pump when the relay does not.

It could be the switch is close, probably the spades pushed in when wire connectors were checked. Wires shorted together will run pump too.

That circuit should be key on power source, sounds like it might be wired battery hot atm.
 
Bent77 said:
Wonder if your flood job with the brake cleaner made it love you long time
Click to expand...
Very possible, all these parts are like 35-40 years old, just looking at them can break them.
 
Interestingly (or not) I got a replacement pressure switch and it would randomly stick in the closed position after shutdown. Just need to disconnect the switch, but frustrating for it to "fail" out of the box like that. If you were deaf or ambient noise was high, when you parked you'd come back to a very dead battery.

I can see why people just remove the thing, you can already bypass the relay with one wire, don't know as the risk is worth the redundancy when a trail fix is so easy.
 
dyeager535 said:
Interestingly (or not) I got a replacement pressure switch and it would randomly stick in the closed position after shutdown. Just need to disconnect the switch, but frustrating for it to "fail" out of the box like that. If you were deaf or ambient noise was high, when you parked you'd come back to a very dead battery.

I can see why people just remove the thing, you can already bypass the relay with one wire, don't know as the risk is worth the redundancy when a trail fix is so easy.
Click to expand...
When working correctly, it does provide some safety if you lose oil pressure. Would have saved one of my 1st built engines if I kept the original TBI/wiring my K5 came with. I lost oil pressure from the oil pickup tube falling out since I was too lazy to weld it and my factory oil pressure gauge was not working at the time. Also provides protection wheeling if you're too much at an angle and it no longer pickup up oil in the pan.
 
Shawn said:
When working correctly, it does provide some safety if you lose oil pressure. Would have saved one of my 1st built engines if I kept the original TBI/wiring my K5 came with. I lost oil pressure from the oil pickup tube falling out since I was too lazy to weld it and my factory oil pressure gauge was not working at the time. Also provides protection wheeling if you're too much at an angle and it no longer pickup up oil in the pan.
Click to expand...

The oil pressure switch runs in parallel with the fuel pump power. If the relay fails, the engine will keep running as long as there is oil pressure. It will only start if you crank it long enough to build up some oil pressure if you don’t replace the relay. The only time the switch would save an engine from low oil pressure is if the relay failed first and then the engine lost oil.
 
Yeah, a TBI engine can kill itself because it doesn't know the oil pressure. Newer cars are even more insidious because they know the pressure and will just keep going. Apparently, it's too dangerous to lose power steering and brakes if you're in traffic. I have news for them - those things are going away sooner or later when the oil pressure goes away. The difference is whether you need a tow truck or a tow truck plus an engine swap.
 
Blue85 said:
Yeah, a TBI engine can kill itself because it doesn't know the oil pressure. Newer cars are even more insidious because they know the pressure and will just keep going. Apparently, it's too dangerous to lose power steering and brakes if you're in traffic. I have news for them - those things are going away sooner or later when the oil pressure goes away. The difference is whether you need a tow truck or a tow truck plus an engine swap.
Click to expand...
I like how my freightliner with a Detroit engine has the computer that shuts it down to protect the engine but gives you a warning for a few seconds and if you need a little more time to get to safety you can press and hold a button that keeps it running.
 
It's so refreshing here to read how so many of you folks actually know what the oil pressure switch does on a TBI fuel pump circuit.

That "engine safety feature" has been a wide spread old wive's tale for so long. And then it spread like wildfire with the popularity of the internet.
 
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