Paul Woods
10-10-2011, 07:56
Now i presumed i knew everything there was to know about the 3vz fuel cut circuit, we've been working on and wiring up these engines for nigh on a decade but i made a discovery recently that might help a few people.
A brief and basic outline of how the Circuit opening relay (COR) works. The COR's job is to cut power to the fuel pump in the event of an accident, when the engine stops running etc, it also serves as a basic fuel pump relay. It supplies power to the pump when the engine cranks (it has a starter signal wire going to it) which latches the relay during cranking which in turn delivers fuel.
After the engine has started the relay needs to be kept latched, and that is were the fuel cut (FC) circuit comes in. On most engines the FC pin is supplied directly from the ECU, so when the ECU sees the engine running (via IGT signal or crank signal etc) it will then turn the FC terminal into an earth. This earth then latches the COR until the ECU says otherwise, so if the engine stops pin FC loses continuity to ground and the COR unlatches, fuel pump stops.... all good so far.
Now on the 3vz the ECU doesn't have control of the COR, it's far more basic than that. The COR still requires an earth path on its FC terminal (Green/red wire) to keep latched and it gets this earth path via the Air flow Meter (AFM).
As soon as the mechanical flap moves off its parked position a small microswitch inside the AFM provides an earth path from the White/black input wire out to the G/R wire which then runs to the COR.... with me so far?
The AFM flap will only stay open while the engine runs, so it's a good system. Now all W/B wires on any Toyota harness are ALWAYS a full time earth, never switched or anything like that, basic full grounds to the chassis or engine block.
We have always known that sometimes when a homebuilder or ourselves wire up a 3vz that the W/B earth wire going to the AFM would be dead, and always put this down to differences in 3vz looms, perhaps there was an earth grounding point on certain Camrys that we weren't including during the conversion process? Either way the answer was never a problem because we just supply a direct earth to the W/B AFM wire if it was missing.
Now here's the newly found info.... drum roll.... :) , i found out by pure accident that the W/B AFM wire does not run straight to ground on ANY 3vz loom, it does in fact run via the oil level switch in the sump!
So why did Toyota do this? Pretty simple really, you run low on oil, and Toyota prevent you from killing your engine by cutting your fuel supply. As soon as the oil level switch trips you instantly lose the W/B earth path to the AFM, which in turn unlatches the COR.
The reason we were seeing some engines with no earth on the W/B AFM terminal was simply due to faulty oil level switches, or broken plugs on them.
Now the reason i'm sharing this is because while it seems an awfully good idea to have an engine saving switch in the event of no oil, this was in the Camry, we are putting these engines into Sportscars, which get thrown around the roads/track... sloshing oil all over the place and there is a risk of tripping the oil level switch.
Who fancies losing all power right when you need it? I'd personally rather run without the oil level switch deciding my fate. So if you want to run without oil level control the answer is to permanently earth the W/B AFM wire, it will still have fuel cut control via the AFM, just no oil level control.
They say everydays a school day and they aren't wrong, hopefully this little nugget of new info will help others even if it's just a basic wire up COR issue you have.
A brief and basic outline of how the Circuit opening relay (COR) works. The COR's job is to cut power to the fuel pump in the event of an accident, when the engine stops running etc, it also serves as a basic fuel pump relay. It supplies power to the pump when the engine cranks (it has a starter signal wire going to it) which latches the relay during cranking which in turn delivers fuel.
After the engine has started the relay needs to be kept latched, and that is were the fuel cut (FC) circuit comes in. On most engines the FC pin is supplied directly from the ECU, so when the ECU sees the engine running (via IGT signal or crank signal etc) it will then turn the FC terminal into an earth. This earth then latches the COR until the ECU says otherwise, so if the engine stops pin FC loses continuity to ground and the COR unlatches, fuel pump stops.... all good so far.
Now on the 3vz the ECU doesn't have control of the COR, it's far more basic than that. The COR still requires an earth path on its FC terminal (Green/red wire) to keep latched and it gets this earth path via the Air flow Meter (AFM).
As soon as the mechanical flap moves off its parked position a small microswitch inside the AFM provides an earth path from the White/black input wire out to the G/R wire which then runs to the COR.... with me so far?
The AFM flap will only stay open while the engine runs, so it's a good system. Now all W/B wires on any Toyota harness are ALWAYS a full time earth, never switched or anything like that, basic full grounds to the chassis or engine block.
We have always known that sometimes when a homebuilder or ourselves wire up a 3vz that the W/B earth wire going to the AFM would be dead, and always put this down to differences in 3vz looms, perhaps there was an earth grounding point on certain Camrys that we weren't including during the conversion process? Either way the answer was never a problem because we just supply a direct earth to the W/B AFM wire if it was missing.
Now here's the newly found info.... drum roll.... :) , i found out by pure accident that the W/B AFM wire does not run straight to ground on ANY 3vz loom, it does in fact run via the oil level switch in the sump!
So why did Toyota do this? Pretty simple really, you run low on oil, and Toyota prevent you from killing your engine by cutting your fuel supply. As soon as the oil level switch trips you instantly lose the W/B earth path to the AFM, which in turn unlatches the COR.
The reason we were seeing some engines with no earth on the W/B AFM terminal was simply due to faulty oil level switches, or broken plugs on them.
Now the reason i'm sharing this is because while it seems an awfully good idea to have an engine saving switch in the event of no oil, this was in the Camry, we are putting these engines into Sportscars, which get thrown around the roads/track... sloshing oil all over the place and there is a risk of tripping the oil level switch.
Who fancies losing all power right when you need it? I'd personally rather run without the oil level switch deciding my fate. So if you want to run without oil level control the answer is to permanently earth the W/B AFM wire, it will still have fuel cut control via the AFM, just no oil level control.
They say everydays a school day and they aren't wrong, hopefully this little nugget of new info will help others even if it's just a basic wire up COR issue you have.