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Do you have the VIN available? Is this a sienna van and has the vehicle been exposed to rust and corrosion in the Northeast?
Ok let me look into some things in the mean time have you checked for codes in the ABS system or know how to? DO you have a code reader to check for codes in the engine computer?
There will be codes set in the ABS computer. You do not need a code reader to retrieve those code numbers. Click the link that I've attached there will be a picture of the diagnostic port that you normally plug your scan tool into. Use a small piece of speaker wire or a small paperclip being careful not to damage the female pins place a jumper wire between the two terminals shown in the illustration. Turn the key to the on position and monitor the flash pattern of the ABS light. For example if there is a code 31 it will flash three times a slight pause than one flash if there is a second and third code they will follow in a two digit format. Let me know what the code numbers are in the ABS system.
31 and 35 are for the right front speed sensor. 32 and 36 are for the left front speed sensor. 33 and 38 are for the right rear speed sensor. 34 and 39 are for the left rear speed sensor.
The bot***** *****ne here is three of the four wheel speed sensors have failed or the sub harness going to them. The four inputs are detected by the ABS computer. It then converts those signals into one signal and sends it on to the combination meter that in turn sends the signal on to the engine control computer. When you lose one wheel speed sensor or even two I've seen it still does not affect the speedometer but if you loose three out of four typically it sets all the lights. The System has to see the wheel speed sensors to effectively measure the diameter of the tire because as the pressure drops the diameter changes. That's why the tire pressure warning light is on the check engine light will come on because it does not see the signal from the speedometer for the speed. The bot***** *****ne is you will have to replace three of the four sensors the right rear sensor is the only one you don't have a code for.
The best way to check with an oscilloscope but most don't have one for your next best option is to Ohm test the sensor. The front sensor should be between 900 ohms and 1300 ohms. The rear sensors are 900 ohms to 1800 ohms.
The next step is to on test the wires from the ABS module to each individual sensor. Do you have a wiring diagram or pinout of the connector at the ABS module to check resistance of each set of wires from that connector at the ABS module to each individual sensor?
Standby I will find one.
Click on the attachment. This is looking at the face of the connector. You'll need to disconnect electrical connector from the ABS unit then orient the connector accordingly. With all of the sensors plugged into the sub harness at each wheel pin number 4 and 5 are for the right rear wheel speed sensor pins number six and seven are for the left rear wheel speed sensor pins 13 and 26 are for the left front and pins 27 and 28 or for the right front. Ohm test between these sets of pins and see if you have the same readings that you got directly at the sensors.
You are still getting the code numbers watching the ABS light flash pattern?
If you are still getting wheel speed sensor codes and the wiring tests good as you have indicated this only leaves the ABS module as being faulty.
The only way to know for certain is to use a scope and test the inputs directly at the ABS module while driving.
If you remove the sensor you will see there is a ring gear with multiple teeth for grooves on it that generate the pulse. A scope is the only way to see any fluctuations. The ohmmeter generally will not pic up any change. The only time the ring gear gets damaged is if the axles have been removed or rust is an issue.