Toyota Repair: ABS light on, 195 65 r15, toyota carina

Question
QUESTION: I have a 1997 Toyota Carina diesel wagon (FWD, a model up from a Corolla), bought used a year ago (imported from Europe). Never had a problem till now, love the car.  I live in W. Africa, the roads are often dirt and gravel, and the car bottom often hits debris.  I needed 2 tires so I replaced the 195/55-R15 in the rear with 195/65-R15 which give more clearance.  After driving about 50km the ABS light came on.  Is the ABS system now non-functional? Do I risk damaging the car by continuing to drive it this way? I suppose the bigger tires are the cause. If that is the case and I exchange the tires for the original size, will the light go off and the ABS function?  Thank you.

ANSWER: Different size tires will cause the abs light to turn on since each wheel has a speed sensor and the computer will see enough of a difference in wheel rotation to cause it to turn the warning light on.

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QUESTION: This has gone on for a couple of days now and I notice a pattern.  The abs light does not come on at low speed driving but only when I get up to 50-60mph.  Is the abs system disfunctional when the light is on? I have tried bigger tires to get more ground clearance.  Is the way I'm going about it going to risk damaging the car's systems?   Do the wheel speed indicators measure the wheels against each other and set off the light, or do they measure the wheel speed and set off the light relative to the wheel size of the original size tires?   If I put a little bigger tires on the front, say 195-60-R15, would the light continue to come on?  
PS The car has a 5-speed manual tansmission.  Here in this country speedometer and odometer accuracy is of little importance.  Thank you.

Answer
Each wheel has a speed sensor, the computer compares these sensors to each other monitoring the revolutions, if one or more sensor reads more or less within a predetermined parameter then the light will turn on. It may be a good idea to have a diagnostic scantool connected to find out what rouble codes are in the abs system.