A simple way to monitor when the Prius is injecting fuel has already
But what if you don't want to inject any fuel, including during some of
those times when the hybrid system annoyingly keeps the engine running for
no good productive reason? That leads to inefficiencies, so being able to
manually cut off the engine's fuel supply occasionally has its uses. As
simple as the injection monitor is, inserting a cutoff switch in the lead to
the electric fuel pump is even simpler.
This describes a surprisingly easy method and location to install one in the '04 - '09 2nd-generation car. The mod has actually been in place in my own car as shown for over a year before this page was finalized, so this description also carries happily flawless long-term experience with it. There is an oblique reference to usage in my '08 roadtrip account where it came in especially handy through some mountainous-terrain traffic congestion. When you're in stage 4 and fully warm there's not much need for it, but it's been a useful research tool in the meantime. It also allows easy exploration of some odd running conditions that the PHEV community has been interested in.
Small images lead to larger ones, as usual.
The harness connection runs through the block of connectors behind the
driver's kick-panel. Pull up the long door-sill panel first, which frees
up the lower end of the kick-panel which then basically just pulls away,
along the plane of the doorframe, from where it's spring-clipped into
position. Don't be shy, just yank.
No other panels really need to be removed, although a bit more working room above the connector block may be obtained by removing the lower dash panel and moving the hood-release cable aside.
The fuel pump lead, IF1-11, is the physical connection as far to the lower
right as you can get when looking at the entire block of stuff from where the
driver sits [red arrows]. Once the connector toward the rear of the car is
disconnected, the half toward the front of the car can be freed from the
plastic mounting bracket using a small screwdriver to release the retention
tab [blue arrow].
The wire really does change color from red to black going through the connector, which is unusual for Toyota to do at inter-harness connections where they usually try to keep wire colors consistent all the way from origin to destination. Furthermore, it is incorrectly documented in manuals prior to 2006. There must have been some internal confusion as to how this piece of the harness was originally manufactured.
|In '06 the manuals were finally corrected to match reality. Compare these two page segments, noting what's circled in pink:|
[Why it is called the "circuit opening relay" is beyond me, as any of the
numerous other relays in any car also open and close circuits. But this one
is apparently special somehow and has deeper safety implications by cutting
off the fuel pump power if an airbag is deployed in a collision. That in
conjunction with opening the battery relays is part of disabling all of the
onboard energy sources in the event of vehicle damage, and that's the name
Toyota chose for this particular functionality.]
The electrically astute observer may look at this and say "well, why not break the coil *input* and use the relay to do the high-current switching?" ... a very good question, as that would be the "elegant" way to do this and more compatible with the way that the *car* frequently controls the C/OPN relay during engine starts and stops anyway. However, there's a small problem with accessibility.
Consult the big picture and look at where harness connector IA3 sits, which
would be one obvious point to pick up the relay coil wire. It's way up under
the main dash panel, which is removable but only through many more disassembly
steps. Punt. Connection block 3 that carries 3G-7, where the relay itself is
also located, is out under the hood and thus impractical for in-cabin control.
And area 4 that would presumably give access to 4H-14 is that huge rats-nest
block of wires to the right of the stereo, with very little in the way of
service-loop leeway and something even I have to date been fearful of messing
with to add connections.
Ultimately the fuel pump control lead goes back to the "FC" pin from the engine ECM, which is a bit more accessible behind the glove-box where all my other ECU taps arrive, but that's getting a bit far away for easy driver-accessible control and for the moment I'd rather not bring such a critical connection up through my gauge-panel harness and back down.
So using the direct fuel-pump power lead via IF1 is by far the easiest path
to access, and besides, it's still sort of an experiment to play with which
doesn't need a lot of formalized dash-tearing-apart work to implement and
may well be un-done later if it proves problematic. Early testing of the
connection characteristics, by just backing the pins out of the connector
halves and using a clip-lead to make and break, reveals no large sparks on
disconnection of the power that one might ordinarily expect when interrupting
the connection to a motor -- clearly, the pump already has a built-in reverse
diode across it to soak up inductive kick and all we need to have is a switch
that can pass its 2.5 or so amps. There's a very brief startup spike of 10
or 11 amps but that quickly settles down. There is no dedicated fuel-pump
fuse, unlike that found in many other cars; the pump simply feeds off the
15A AM2 supply.
If you're implementing your own different type of mod and have convenient access to the coil side of the relay instead, go for it. Staying in the low-current area for control hacks is always recommended; physical layout simply suggests otherwise for this and after a year+ of having it this way I see no need to go back and redo the work. The switch doesn't get used all that often.
To connectorize the hookup, I've found an old trailer-lights type connector
in the junkbox. The switch only needs two leads and since this has four,
I have the luxury of adding a ground connection and an LED to show when the
fuel pump is energized. Just barely peeking out from behind the plastic
flange is a 560 ohm resistor, and the small LED mounts on the front face of
the kick-panel. LED wiring is simply gaff-taped into place, as it's one of
the few types of tape that holds up fairly well in the automotive environment.
The LED conveniently duplicates one of the earliest Prius "engine run light"
mods, which simply tracked fuel pump power but didn't really indicate actual
injection of gas..
The LED positive is connected *before* the switch, because I actually want to see when the ECM is *trying* to power the fuel pump regardless of whether I'm going to permit it to or not.
The cut-in is done in the red lead before the connector block, and all snugged
down against the small harness piece that comes down the corner of the cabin.
To add to the confusion, I'm using the red and black leads of my connector.
There's a convenient threaded hole right above the area that serves admirably
as a ground point for the green. Now the kick-panel connection has to be
installed and the switch turned on, or the engine won't run.
Beware of the generous wads of grease on the parking-brake assembly when working in this area, which can get all over your hands if you brush the pedal pivot or ratchet.
The LED is strategically located in the panel so that it's visible from the
normal driving position of my eyes. At this point I mostly ignore it but
being able to see this has been essential for the early observations after
the switch went in:
|Take note of the foot through the steering wheel in the previous picture. With the switch at this position in the kick-panel, it is quite easy to access with my little toe. This picture also shows the fairly oddball item I found for the application -- some sort of splash-protected mil-spec toggle switch with a waterproof and EMI-shielded cover. Under the rubber condom is a metal mesh that goes around the actual toggle lever, presumably delivering a total TEMPEST-shielded solution if the switch were mounted in a metal box. Whatever -- I can still toggle it on and off on the fly by feel without reaching down with my hand, and that's proven handy in the few situations when the switch is needed to force-stall the engine and float along in Neutral.|
Okay, so what happens when the Prius engine goes tango uniform in the middle
of normal operation? The short answer is usually P3190 or P3191, but it all
depends on what the running conditions are at the time.
Let's remember that the definition of Neutral in a Prius is, "no control is applied to the electric motors". That's what it is, electrically. Both inverter gates drop, and the motors free-spin. Mechanically, of course, nothing changes at all since there's no physical gear-shifting. Starting or stopping the gasoline engine is a secondary consideration, which cannot happen if the hybrid ECU is commanded to not have any control over the electric motors. This also implies that the best method available for sensing engine output torque is electric motor/generator current vs. switching phase, as the motors have to essentially fight the engine's rotation to transfer torque through the PSD and deliver usable power to the wheels.
Thus, if the engine up and and dies when the Prius expects it to be delivering power, an error condition is set. If the fuel-cut switch is left turned off long enough to stall in any shift position other than Neutral, the resultant "christmas tree" of dashboard lights is similar to what happens when the car runs out of gas.
If the shift state is in Neutral, then fuel flow to the engine can freely cease and it eventually stutters and rattles its way to a stop and the hybrid controller just doesn't care. The hybrid controller *knows* the engine isn't running since it also monitors a copy of the crank sensor for RPM and position, but it's not an error condition unless the hybrid system was depending on solid engine output at the time. If control is later granted by shifting out of Neutral, the hybrid ECU has the option of restarting the engine if needed.
If the pump switch is off and the Prius goes to start the engine but finds that it won't stay running, it's a slightly different error condition -- the dreaded P3191, "engine failed to start". Owners of older cars whose throttle-bodies have never been cleaned may be all too familiar with that one, as that can cause a similar no-start condition but for the wrong reason. The car may try to light the engine up to three times before deciding it's out to lunch and throwing the DTC.
Either way, once the system is in one of these error states it allows much more use of the battery for propulsion -- close to its current limit of 100 amps, and down to a frighteningly low state of charge before ultimately cutting off that energy source too. This is the "high speed EV mode" that some of the plug-in conversion vendors are claiming to be able to do, but it's a cheat -- they deliberately throw the car into an error condition and take advantage of the fact that it allows substantially more battery use for a while. Toyota probably allows that slight excursion outside the normal parameters as a safety feature, allowing a driver to limp off a highway and go a short way in EV after running out of fuel. But it really isn't the way you want to be routinely running the car.
The PHEV hacks generally don't go after the fuel pump, as it takes too long to generate the engine-croaked condition and shudders a lot in the process; they instead do things like cutting 12V ignition power to the whole engine ECM which is an instant error for the HV controller. Some of them even go as far as immediately clearing the error condition via an OBD-II mode 4 command on the CANbus, just to turn off the big red triangle, but at that point the hybrid ECU has given up on using the engine at all and that state doesn't clear -- the entire car has to be shut down to restore normal engine operation regardless of whether the dashboard is lit up or not. And if you were hoping to go to emissions inspection later that day, forget it -- none of the EVAP and catalyst monitors will show as completed after such a recent OBD-II reset. So it's still a klunky non-solution that is being routinely misused by these folks and sold as a product feature. Buyer beware. They'd be much better off to build up [and/or spoof] enough SOC to widen the car's warp stealth control threshold and encourage it to push more meaningful battery current into highway cruise with the engine still turning but not injecting fuel, as it's still a very low-friction state. There's really no need for any extreme hacks; the car already has a perfectly good high-speed zero-consumption running state available. At this point I can offer some seat-of-the-pants pseudo-proof that we don't need to be so uptight about having the engine at a standstill.
For a little fun on highway downhills, as touched on in my mountain journeys, the fuel switch provides an alternative to warp stealth if your foot needs a rest. It's the same state as reached by gliding at < 42 MPH, going to Neutral, and coasting up to faster than that. But this can be initiated at higher speed. The shifter can be put in Neutral and the switch cut, and in about ten seconds the now-idling engine will cough and die and the hybrid controller just goes on happily being in Neutral and ignoring it. At this point the switch can be re-enabled for the next run. At highway speed, the stall spins MG1 up pretty high in the reverse direction [see the nomograph as a guideline]. This can actually be felt as a perceptible slow lurch. Interestingly, the difference in mechanical drag between engine-spinning and engine-still seems almost nonexistent, as though pulling the engine around at 960 RPM or MG1 at 9600 or wherever with the associated rotor windage and stator reluctance creates about the same power loss. It's actually a bit of a mystery why this doesn't slowly pull the engine around against compression, because there's *got* to be some amount of leftover torque applied to the crankshaft even with electrical motor-control disabled. Going back into Drive immediately brings the engine back up its warp-stealth spin speed as MG1 slows down under re-established inverter control, as though nothing were amiss.
My informal butt-dyno and MPG-tracking conclusion is that there doesn't seem to be any particular advantage to "warp neutral" coasting at interstate speeds, other than to relieve the "lock-in" demand on one's right ankle for a while. If I had to guess, I would put the tradeoff point below which warp-neutral makes sense at around 50 MPH based on what I've seen and felt on numerous rolling country backroads. If the PHEV red-triangle aficionados really want to trade the inconvenience of having to pull over and reboot the car for ten more MPH of engine-stalled operation that gains an experienced driver little in the way of meaningful fuel economy, more power to them but I suspect that quite a few discerning customers might disagree.
It is interesting to watch the short-term fuel trim on the Scangauge after the switch is turned off. It immediately rockets upward as the ECM lengthens the injector time to try staying at stoichiometric burn. This can be done for a very few seconds during normal travel without reaching an error state; there's enough pressure left in the lines and rail to keep going for a little while, but the system is immediately aware of decreased available fuel volume and starts compensating. Fuel trim exceeding something like 20% in either direction is generally considered to be a problem by most cars' control units, so this is a short-lived game.