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oil pressure switch delete?

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manbearpigisreal

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Hi all,
I am finally LS swapping my 1988 K5 and i need help figuring out how to delete the electric oil PSI switch by the distributor.
if you connect the wires together, the fuel pump just runs forever even with the key off...
how do you trick the truck into not looking for this switch anymore?

i appreciate any help. thanks.
 
manbearpigisreal said:
Hi all,
I am finally LS swapping my 1988 K5 and i need help figuring out how to delete the electric oil PSI switch by the distributor.
if you connect the wires together, the fuel pump just runs forever even with the key off...
how do you trick the truck into not looking for this switch anymore?

i appreciate any help. thanks.
Click to expand...
If you insist on deleting it, you need to supply power from a keyed source not direct power
 
On my 1990, the switch also feeds the instrument cluster I think. So that might be a consideration before you delete it entirely. But the switch is redundant power to the fuel pump in case the relay fails. It is open until a few PSI pressure closes it. The circuit is always hot. You could install a manual switch in its place, I suppose, and have a way to manually bypass the relay, I guess?
 
instruments still work
a switch wont work. i tried wiring it directly to power and the result was the same.
start, then immediately die.
 
manbearpigisreal said:
instruments still work
a switch wont work. i tried wiring it directly to power and the result was the same.
start, then immediately die.
Click to expand...

As mentioned, that switch is redundant to the fuel pump relay. It doesn't need connected if the system is working properly. Odd that it starts and dies, if the switch is the problem then the relay isn't working right. It's obviously still wired correctly if jumpering the two wires gets the pump to run. There is no tricking it needed though. Stock, the truck has no way of detecting if it's connected/working properly or not. Disconnected, it just won't close and power the pump in case of relay failure.

At some point (1990?) GM started using a combination oil pressure sensor/switch, but it's three wires. I don't believe that was used as early as '88.
 
either wire it to a similar oil pressure switch on the LS motor (i am not familiar) or just leave them disconnected. all it does is provide redundant power to the fuel pump anytime the psi is above 3 or so. there isnt anything magical about it.
 
But you have to give your LS PCM control of the fuel pump relay, no matter what you're doing with the oil pressure switch.
 
dyeager535 said:
As mentioned, that switch is redundant to the fuel pump relay. It doesn't need connected if the system is working properly. Odd that it starts and dies, if the switch is the problem then the relay isn't working right. It's obviously still wired correctly if jumpering the two wires gets the pump to run. There is no tricking it needed though. Stock, the truck has no way of detecting if it's connected/working properly or not. Disconnected, it just won't close and power the pump in case of relay failure.

At some point (1990?) GM started using a combination oil pressure sensor/switch, but it's three wires. I don't believe that was used as early as '88.
Click to expand...
not true
examine the wires
it needs this to stay running
need to know how to delete this from the pcm seeking it
 
manbearpigisreal said:
not true
examine the wires
it needs this to stay running
need to know how to delete this from the pcm seeking it
Click to expand...

it only needs it to stay running if the relay is bad. otherwise it is redundant. at least on TBI stock wiring. not sure about LS swap. the ECM for the TBI doesn't have any idea what the oil pressure switch is doing. I dont know about LS computer....
 
LS PCM reads oil pressure from a sensor. At least some will run without it. I had a sensor go bad in an '05. The cluster would indicate 0 pressure and a warning appeared on the dash, but you could keep driving.
 
manbearpigisreal said:
not true
examine the wires
it needs this to stay running
need to know how to delete this from the pcm seeking it
Click to expand...

Absolutely true in all stock GM TBI/TPI vehicles, clearly shown in every GM wiring manual. Also specifically explained in the GM repair manuals.

Look at it this way: it wouldn't be possible for the fuel pump to prime as it does with TBI (key on, engine off) if the oil pressure switch being closed was required for the pump to work.

Even the TBI fuel pump relay was controlled with the ECM, I'm sure Blue is correct with the LS PCM needing to control the relay as well.

Edit: Here it is out of the 1991 Light Duty Truck Fuel and Emissions Service Manual

"As a backup system to the fuel pump relay, the fuel pump is also turned on by an oil pressure switch. When the engine oil pressure reaches about 28 kPa (4psi), through cranking the oil pressure switch will close to complete the circuit to the fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn on the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi)."

and

"The oil pressure switch serves as a backup for the fuel pump relay to help prevent a "no start” situation. If the fuel pump relay was found to be inoperative, the oil pressure switch circuit should also be tested to determine why it did not operate the fuel pump."

and out of the troubleshooting flow chart:

"OIL PRESSURE NORMAL. START ENGINE. DISCONNECT FUEL PUMP RELAY. ENGINE SHOULD CONTINUE TO RUN. DOES IT?"
 
Last edited:
well the wiring in the pic shows how it is now with the wires that would have gone to the sensor going to nothing
it starts and runs for a few seconds, then dies.
 
Well whether two wire or three wire, it's not a sensor for the fuel pump, it's a switch. It doesn't do anything magical or complex, it just closes when it sees ~4psi of oil pressure.

I presume you've already put a high pressure pump in the tank?

Since the pump runs with the wires jumpered, does it still start then die that way? What kind of pressure does the pump put out while the pump is running vs what the LS wants to see?

I don't know LS well, others are going to have to chime in if things like crank sensors and all that can cause an issue like this.
 
manbearpigisreal said:
well the wiring in the pic
Click to expand...
Pic?
dyeager535 said:
I don't know LS well, others are going to have to chime in if things like crank sensors and all that can cause an issue like this.
Click to expand...
If it starts, the crank sensor is connected. I don't know if it will start on TBI pressure (like 13psi), but it's doubtful. The normal approach is to drop a TPI-style pump into the TBI sending unit and it's good for the 50+ psi.
manbearpigisreal said:
no all wiring is stock
Click to expand...
When the truck was stock, it didn't have an LS in it, so obviously this is isn't 100% true. However! If I take that statement as literal truth (that you've only added wiring), that means you still have the TBI ECM in place and it's providing the priming pulse. When it doesn't see any RPM, it won't keep the pump on. I mean, how else can the pump ever turn on without connection to the LS controller? If it was connected to the LS controller and the engine was running, why would it ever turn off? (But if the latter is true, we have more questions to answer.)

The help we can provide is very limited until you tell us what you're working with. Is this a stock GM PCM or an aftermarket controller? Have you installed a fuse/relay box? Can you link to a diagram showing any information about the wiring? What information are you using to get this engine installed?

The changeover appears pretty simple (but again, how can we know what you have going on?). Logically, you would run a wire from pin 9 of the red/green PCM connector (not blue) to the DK GRN/WHT wire at the relay. Both setups are using a switched +12V to control the relay. If you're using a Holley, FI Tech, etc, refer to their documentation.

1758024326931.png
 
Blue85 said:
When the truck was stock, it didn't have an LS in it, so obviously this is isn't 100% true. However! If I take that statement as literal truth (that you've only added wiring), that means you still have the TBI ECM in place and it's providing the priming pulse. When it doesn't see any RPM, it won't keep the pump on. I mean, how else can the pump ever turn on without connection to the LS controller? If it was connected to the LS controller and the engine was running, why would it ever turn off? (But if the latter is true, we have more questions to answer.)
Click to expand...
Tis was my first thpought when you wrote "the wiring is stock". Impossible with an LS swap.
 
i guess you cant post pics in this forum...... so... let me paint the picture.....

stock 1988 tbi k5 blazer.
stock wiring harness is all taped up to the firewall.
I CONNECTED THE MAIN BUS WIRE TO THE BATT/ALT/JUNC just like it would have been stock.... i did this to maintain all stock lights, fuses, ignition, and yes; fuel pump.
the stock fuel pump housing, has a late 90s high psi fuel pump installed in it now.

new stand alone wiring harness and stock PCM installed for 5.5 ls. VATS cleared. it does start and idle, and respond to throttle.... for 5 seconds...

oil PSI sensor switch has a orange (hot) and a brown (fuel pump?) wire.
the brown and orange run to the fuel relay.
connecting them, the fuel pump runs non stop.
disconnecting them, fuel pump primes and starts.
 
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