Join Date: Feb 2017
Location: St. Louis, Missouri
"Yes but also in both cases those first few wheel revolutions after you turn the key on are used to work out what the ratio of front wheel to rear wheel ticks is."
The ABS control unit logic has a set parameter for that ratio. That parameter is wide enough in range to take into account the differences in tire diameters--to a point. Outside of that range, you have a TC/ABS malfunction indicator lamp on with a diagnostic trouble code stored in the control unit's memory.
Many times Ive had vehicles come to me with a ABS/TC light on. The 2nd thing I did for diagnosis, after verifying the light on and retrieving the trouble code was to inspect the tires for circumference and pressure. The usual customer complaint was "My ABS lamp is on and I just got tires replaced." Usually it was the fronts OR the rears. All 4 changed with the same brand, series, and size, not a problem.
I never had to drive a car to get a TC or ABS lamp to go out unless it was after a repair for which the light came on in the 1st place. Once repaired, the lamp was on until the control unit received acceptable signals from the wheel speed
sensors. If all was a go, the light would go out. Or running a car in gear on a lift. Same deal on a motorcycle running the engine in gear with the bike on front and rear stands. The light would be on until the control unit judged the wheel speed sensor reading to be in the correct range. Doing that with a car on the lift with TC engaged would have the the engine bucking and surging, not revving thinking that wheels were slipping. Back on the ground, driving it would turn off the light as long as the ABS control unit saw what it was looking for.