Speedometer , Odometer, and VSS sensor testing ( Vehicle Speed Sensor):
My pages show how to work with the Speedo driver gear ratio's , cable and head , as seen below , aka: I/P cluster or Instrument Panel cluster.
The system is 100% mechanical , except the VSS switch itself, and is only a simple reed switch. "no electonics at all"
If you want to see the cable gear ratio's fast , CLICK HERE
VSS testing , I/P cluster on the bench (bench testing it) Slide show here.
The car can have two VSS sensors
( 1 in a 3 speed or 5 speed, and 2 for a 4speed transmission) I call the 4speed sensor VSS2, at its tail shaft , this page is for the VSS1 in the cluster.
Your ECU depends very heavily on the VSS1 to run the injection system properly. ( used to calculate load, cruise data, and is a fall back device for the 4speed)
It is not optional.
The VSS acutal , almost never fails.
What fails is the Cable, feed gears .
If the CEL lamp glows and you see code 24 flash out, or the P0500 (OBD2 scan tool DTC)
DTC code 24 THROWS at : ( loss of VSS for 5 seconds during fuel cut , deceleration)
Code 24 can be set
with a binding speedo cable! Oil it with motorcyle cable oiler.
a broken cable.
a broken speedo gear at the transfer case (4wd) or at tranny tail shaft housing on 2wd.
Never attempt to spin the cable at the disconnected end of cable at the speedo cluster , with any tool / or machine, using fingers only , it will be impossible to turn (agood sign)
If you remove the cable at the bottom end , you can spin cable CCW and see speedo needle go above 0 mile per hour (km/H?)
Do not spin the cable backwards ! (only CCW facing HEAD)
Never pull on cable from top side (head) as that invites destroying the bottom cable crimp key. way bad. that is.
You can remove the internal cable (pull it out from beneath the car and lube it and put it back.) Twist it gently on re-insertion, so it dogs in to the speedo cluster port
The speedo end is a square drive.
The tail end is a key shape terminal that likes to shear or come uncripped from the end of cable, End of life on said cable..
Test 1: ( car on flat ground)
Speedo cable, you pulled loose at I/P head, , chocked front wheels.(front and rear) , get tranny into neutral or transfer case in neutral , then raise the left rear wheel.
Never in gear Park, you release the hand brake. (car is free to float so use effective chocking)
person one spins left rear wheel ,the rear prop shaft spins, the speedo cable spins in your hand, and it is impossible to stop cable with fingers.
yes?, then cable is not broken nor ar the gears in tranny tale shaft.
Actual reed switch failures are rare.
Of all the functions of this head the VSS can be a real show stopper. IT MUST WORK. Or the ECU will go quite crazy.
What is it?: It is a reed switch (magnetic) that sits inside the speedo head, that click close x amount of clicks per the turn of the rear wheels.
Its function is to tell the ECU how fast you are going and to report this to your insurance company. ( that last part, is a fib, LOL !!).
This is your speedo head, the VSS switch is located to the right. 93/94 This is a USA Fed 48 Head.
See my 91 speedo head photo.
The 91 speedo head wiring photo, is here. others are simular.
See my detailed speedo head page and see the speedo gears and choices, for all transmisions.
The reed switch rarely fails but these screw pads surely corrode, just polish them with an pencil eraser.
The speedo cable , head connector, one just tweaks the tabs lightly and pull the cable shell forward.
The main electrical 2 or 3 big connectors, just squeeze the lock tabs to release the locking connectors.
The SES Milelage NAG conn. (4pins/ 3 wires) switch connector (federal cars only) , has a backwards lock ! Spread it to unlock it ! Dont put it back it , is nothing but trouble.
Clean the 2 VSS contacts C1 AND 2 AND chean the 2 screw pads and replace the 2 washer if corroded.
Here are the ECU connections VSS shown here.
What is a reed switch? :
Good general write up on the VSS:
Here is how the whole thing works end to and.
THE, main tranny shaft spins. ( rear wheels spin, the rear prop shaft spins and the end of transfer case spins (if 2wd substitute tranny for xfr case)
the plastic worm gear #35 on same spins the speed plastic gear 35b, in the port , id 41
this gear 35b, spindle , has a hole slot and key.
the end of the speedo cable ,at xfr case (pretending 4wd)
this end has a crimped on key device on the cable end, if not crimped on any more, that cabe is bad, this device inserts into the gear. the gear spins , that cable spins.
The cable must spin at all times moving, never try to stop it from spinning while rear axle is in motiion. (breaks things)
Never pull on cable from top side (head) as that invites destroying the bottom crimp. way bad. that is.
the cable spins speedo head ( it is mounted to rear of speedo head)
the head has a gear to the odometer and trip counters.
and the same speedo head input shaft, spins a magnet
this magnet spins and the needle (not connected) is magnetically deflected upscale CCW from rear.
the faster the cable moves the more magnetic coupling energy to the needle .
the VSS clicks 4 times per cable turn.