Toyota Accelerator Sensor?

[DSGarcia]

Here are a few articles related to the Toyota "sticking" accelerator pedal problem and problems with the ECU software, etc. Interesting reading.

The first is from Design News:
**broken link removed**

and from Electronic Design News (EDN):
**broken link removed**
**broken link removed**

 

[adamey]

I think there's some confusion between a TPS (throttle position sensor) in a car with a mechanical throttle and a drive-by-wire one.

In a mechanical throttle the TPS is usually a single pot, with a ground and 5V reference and it's mounted on the throttle body. Some vehicles (Toyota does this) may also have an idle switch and/or a full throttle switch. The TPS is used to tell the ECU how much throttle the driver is requiring and to adjust fuel and timing. For example, changes in the TPS (like accelerating) would require a quick injection of extra fuel. The rate and amount of change of the TPS (along with a MAF or MAP) would let the ECU determine the amount of enrichment.

The failure of a TPS (even one putting out a higher voltage) would not adversely affect the power output of the engine, and for sure would not cause a runaway engine. Some cars run pretty good with a faulty TPS, some suffer minor drivability or rough ide problems and some have more severe drivability problems. I can think of no circumstance that a faulty TPS in a mechanical throttle would cause excessive engine speed or a runaway condition.

In a drive-by-wire setup (DBW) you would have a total of 4 sensors - 2 in the throttle pedal and 2 in the throttle body itself. This is for redundancy. The 2 sensors in the pedal and throttle would have different voltage curves. This way the ECU can compare the "known" good curves with the actual outputs and determine if a sensor has failed. If a sensor has failed, the ECU would go into a "limp" mode with reduced engine outout.

Quite often, this involves cutting the power to the throttle body motor. DBW throttles are spring loaded, and return to a position that is partially open when power is no longer applied to the motor. This position gives a high-idle, which would be enough to limp the car at a greatly reduced speed (most cars won't hit more than 15-20mph when this occurs and would take awhile to hit this speed).

Other cars will fall back to the one good sensor, but would limit drive current to the throttle motor so you would get no more than say 30% throttle opening. DBW throttles are calibrated so the ECU knows how much current is need to put the throttle in a specific position, so the car may still drive OK with a TPS fault. The check engine light (CEL) would come on immediately with such a fault.


The problem with the throttle pedals in the latest Toyota recall is a mechanical problem causing the pedal to bind or stick - it's not an electrical problem with the pots or wiring. An electrical problem like this would not cause full throttle as there's too many safeguards built in.

However, I still think there's another fault with the Toyotas. If the brake is applied, the DBW throttle should reduce power output (many cars do this). If the ECU sees throttle and brake at the same time it knows something isn't right and should act accordingly. This is allowed to a certain extent (like using light throttle to hold your car on a hill so you don't roll back when releasing the brake), but there's no reason to have full throttle and a brake applied at the same time.


Had Toyota put this simple ability into their cars, you wouldn't have that dead police office in California with the Lexus.


BTW, I'm a licensed technician and work on vehicle electrical systems.

 

[MikeMl]

I went and looked. In my Corolla, there is a single pot mounted right at the hinge point of the accelerator pedal aft of the firewall. Only three wires come out the assembly. There is no mechanical linkage or cable penetrating the firewall.

 

[DirtyLude]

It was established earlier in the thread that these were drive-by-wire. Like I mentioned though, I have no experience with newer drive-by-wire systems so I can really say anything about the failsafes the may or may not have.

 

[adamey]

I went and looked. In my Corolla, there is a single pot mounted right at the hinge point of the accelerator pedal aft of the firewall. Only three wires come out the assembly. There is no mechanical linkage or cable penetrating the firewall.

That doesn't make any sense. I'll look at the wiring diagrams for your 2006 Corolla when I get to work tomorrow, but in my experience Toyota will have 4 wires coming from a pedal sensor - ground, 5v, out1 and out2. I can't imagine any drive-by-wire system relying on only a single pot.

On some high-end cars, the throttle body is actually controlled by CAN and there's no voltage for position feedback sent back to the ECU - everything is done internally within the throttle body itself. It has its own diagnostics, it can be flashed to upgrade software and it simply goes wherever it's told to position itself.

Have you looked at the throttle body itself? Typically it will have 6 wires - 4 for the two pots and 2 for the motor.

 

[MikeMl]

That doesn't make any sense. I'll look at the wiring diagrams for your 2006 Corolla when I get to work tomorrow, but in my experience Toyota will have 4 wires coming from a pedal sensor - ground, 5v, out1 and out2. I can't imagine any drive-by-wire system relying on only a single pot.
...

You obviously didn't read my (the first) post in this thread! There are only three wires.

 

[adamey]

^ I read all the posts and I'm saying they don't make sense, which is why I'm going to look at my wiring diagrams when I get to work. I simply cannot believe a manufacturer would have drive-by-wire and only use a single sensor.

Unless your Corolla is using the newer CAN based throttles and pedals (which also use hall effect or optical sensors instead of pots), but that would be a huge surprise to me as well.

 

[MikeMl]

The partial schematic I posted is right out of the Corolla Wiring Diagram I purchased...

 

[schmitt trigger]

Mike;
I would think (although I don't know!) that Toyota engineers were smart enough to foresee an open ground wire condition could cause full voltage to be applied to the ECU.
Just like the open-circuit detector in thermocouples which prevents a furnace meltdown in case of thermocouple malfunction.

 

[adamey]

^ There are two engines for that year. One is drive-by-wire, the other is not.

The schematic you posted is for the non-DBW engine, and that sensor is located on the throttle body itself.

For the DBW engine the throttle pedal under the dash actually has 6 wires, 3 for each pot (this is odd, as you only need 4, but they might have had a reason for this). The throttle body has the usual 6 wires I mentioned, 2 for the motor and 4 for the pair of pots.

Is your Corolla an XRS or not? This will determine which engine you have. I'd love to see a picture of the throttle pedal to see why there's only 3 wires.

 

[ericgibbs]

hi,
Saw this advice on the web to day.

**broken link removed**

 

[ky8kenny]

Hi guys,

Could someone tell me what VC and VTA stand for?

And on p4 in https://www.electro-tech-online.com/custompdfs/2010/03/h33.pdf

Why does the VTA1 and VTA2 voltage signals vary and have different upper limits?

Thanks for your help.

ken

 

[crutschow]

On a car or light truck, steering to the kerb is easy with no power steering. Power steering is really not needed above about 5 mph, or when you are trying to corner hard.

I had a 1978 van, over 3 tonnes max weight, over 1 tonne on the front axle, and no power steering. OK, I couldn't turn the steering wheel when the vehicle wasn't moving, and it was hard work in town, but no problem at all on the highway. The fear of losing power steering is a lot worse than the reality.

There is a difference between a vehicle with power steering and one without, and that is the steering turns ratio. It usually is lower with power steering, and that would make it more difficult to steer with the engine off then a vehicle without power steering.

But I agree, you should still be able to steer a power steering vehicle with the engine off as long as the vehicle is moving above a few MPH (although it may take a little arm muscle).

 

[shelbycharger400]

electronics are very intersting. chrysler.. if you unplug ( or loose vac to map.. rpm goes up. 5.7 hemi's (drive by wire) have a pulse width modulated throttle butterfly that i remember failing a few, basically same technology as the ais motors. eventually the screw wears/strips out and jumps and either sticks open or closed slightly more than orig.