eliminating sensors

[NEK5]

It seems that this seems to be the way people set up their TBI harness when putting it into an older vehicle.

KEEP:

Fuel Pump Relay
ESC Module
Knock Sensor
Ingition Module
Ignition Coil
Alternator
VSS - Vehicle Speed Sensor
MAP Sensor
TPS - Throttle Position Sensor
O2 - Oxygen Sensor
Idle Air Control Motor
ALDL Connector
EVRV Solenoid
Coolant Temp Sensor

ELIMINATE:
Fuel Pump/Oil Pressure Switch
Digital Ratio Adapter
Hot Fuel Module

OPTIONAL (depending on application)
Auto Trans Circuit
Downshift Relay
Why do you eliminate the three items they mention?

The optional ones I believe I have to keep as I`m running a 700r4. TIA!

 

[dyeager535]

I'd say the only reason for removing the oil pressure switch is laziness/lack of understanding.

I'll be the first to admit that I have no idea how the 12V test lead on the fuel pump relay works...I *think* it triggers the relay on, if that is all the test lead does, you want the oil pressure switch.

DRAC is eliminated because most setups use speedometer cables. DRAC is far superior IMO than speedo cables, but you know that. But if you don't have the transfer case for it, obviously not usually worth getting one to make it work.

Hot fuel module was only used on the 454's as I understand it, and I know why GM used it, but not sure why they NEEDED to use it. It circulates fuel after the vehicle shuts off, to avoid the fuel up near the engine boiling. Why that was only needed on the 454 trucks, no clue.

 

[NEK5]

I'd say the only reason for removing the oil pressure switch is laziness/lack of understanding.

I'll be the first to admit that I have no idea how the 12V test lead on the fuel pump relay works...I *think* it triggers the relay on, if that is all the test lead does, you want the oil pressure switch.

DRAC is eliminated because most setups use speedometer cables. DRAC is far superior IMO than speedo cables, but you know that. But if you don't have the transfer case for it, obviously not usually worth getting one to make it work.

Hot fuel module was only used on the 454's as I understand it, and I know why GM used it, but not sure why they NEEDED to use it. It circulates fuel after the vehicle shuts off, to avoid the fuel up near the engine boiling. Why that was only needed on the 454 trucks, no clue.

If you use an aftermarket electric oil pressure gauge, do you need the switch?

 

[dyeager535]

Yes, because the switch has nothing to do with any gauges. It is there solely to keep the truck running and driveable if the fuel pump relay circuit dies.

 

[NEK5]

Yes, because the switch has nothing to do with any gauges. It is there solely to keep the truck running and driveable if the fuel pump relay circuit dies.

Hmm..So the oil pressure circuit will keep the truck running if the fuel pump relay dies?

So it isn`t something that should be removed?

The hot fuel module ok to remove? Seems like, from your description, it`s useless for my setup.

 

[dyeager535]

I wouldn't remove it, seems kind of dumb to me to remove redundancy that GM thought necessary, especially in a system as critical as fuel. What if the relay decides to take a dump descending a steep hill while 4 wheeling...

Hot fuel module, I'm not sure WHY GM used it. Well, I know why GM says, but not sure when it's *necessary*/ As with the above, they must have needed it, but I'm not sure if it was something so folks in AZ didn't have problems, and it just simplified the wiring harnesses/diagrams to include on all, or what. I can't understand what would make a 454 truck more special than every single other GM EFI system out there that didn't use one.

 

[468k5]

doesn't the oil pressure switch cut power to the fuel pump relay if the oil pressure drops too low?

 

[dyeager535]

Nope. It's a failsafe for the fuel pump relay, that's it.

As an edit, you will find a LOT of people that think the oil pressure switch is used like you say. Don't believe them. Any GM EFI system from about '85-92 is wired exactly the same, (and it is reflected in the wiring diagrams clearly) and the oil pressure switch simply bypasses the relay, gets power from the same source, but is electrically "upstream" of the relay.

Not sure if it was a post here or an email, but someone on the board was nice enough to pull their fuel pump relay connector while the truck was running, to prove the oil pressure switch would continue to run the truck, which it did. This of course assumes that switch is working, which you'd never know until the fuel pump relay failed.

 

[southernspeed]

Nope. It's a failsafe for the fuel pump relay, that's it.

As an edit, you will find a LOT of people that think the oil pressure switch is used like you say. Don't believe them.

Another urban myth dispelled! I always 'thought' it was to shut the motor down when no oil pressure was present too. But I 'thought' that because I'd read that. I've never looked at it on a wiring diagram. Interesting. Makes good sense though.

 

[newyorkin]

Nope. It's a failsafe for the fuel pump relay, that's it.

As an edit, you will find a LOT of people that think the oil pressure switch is used like you say. Don't believe them. Any GM EFI system from about '85-92 is wired exactly the same, (and it is reflected in the wiring diagrams clearly) and the oil pressure switch simply bypasses the relay, gets power from the same source, but is electrically "upstream" of the relay.

Not sure if it was a post here or an email, but someone on the board was nice enough to pull their fuel pump relay connector while the truck was running, to prove the oil pressure switch would continue to run the truck, which it did. This of course assumes that switch is working, which you'd never know until the fuel pump relay failed.

I always thought it was a safety cutoff, too... Good to know what it's really for.

 

[bp71k5]

As I'm thinking of converting from a carb, is the oil pressure switch just a switch or another relay? If the fuel pressure relay dies, does the oil pressure switch just keep the fuel pump running solid instead of cycling on/off depending on pressure?

I ask because it sounds like GM must have thought its relays were not very reliable. Otherwise, why not remove the relay entirely and run it off the oil pressure switch in the first place? One problem would be the pump wouldn't turn on when the engine was off and you tried to start it.

I ask this knowing I'm probably missing something important aspect of the system so feel free to educate me.

 

[dyeager535]

Simply a switch. The pump runs solid whether it's off the relay or the switch...that's why there is a return line and a pressure regulator.

GM probably did this because it was one thing that would leave you absolutely dead in the water without a backup. Remove the MAF, TPS, temp sensor, etc., the thing will still run. No fuel, no run. Of course you've got the ignition module, but that's not a cheap/easy piece to have a failsafe on.

As you mention, with no ECM control, cold starts are longer.

 

[bp71k5]

I guess it makes more sense now after thinking about it. What happens if you shut the motor off after the relay died while the motor was running. I assume since there's no oil pressure or fuel pump relay operation, the motor won't fire back up? Or it would just take much longer since youd have to keep the starter cranking long enough to get the oil pressure up?

 

[dyeager535]

The engine won't die with the switch in place if it's working right.

You'd notice the problem once you shut it down and let it sit for a bit. It only takes roughly 4PSI for the switch to close, so while it will crank for a bit longer, it's not an inordinate amount of time. Just enough for you to notice the difference.