Here is the issue with my speedometer. It operates normally up to 60 kph (40 mph) and then sharply falls to 40 kph (25 mph). It continues to climb slightly as I continue accelerating but is always indicating 30% more or less than the speed I am travelling. If the car slows below 60 kph (40 mph) the needle jumps back to the correct indicated speed. I have gone through the previous owner's receipts and it appears like my local Mazda dealer can not get this fixed. He had the car in twice for the issue, once no fault detected, the second time they replaced the speed sensor.

I don't mind springing for a new speedometer if that is what the problem is but given that (up to a certain speed) I am getting voltage from the speed sensor and the speedo is reading it, I could use some advice on better diagnosing this.

Could I not use a multi-meter to read the voltage coming to the speedometer; if the multi-meter sees the drop in voltage at 40 mph, then I could point to the wiring or the speed sensor itself.

All help is appreciated.

my baby

 

I know this is probably too late, but maybe it will help somebody else, here goes.

the speed sensor is an electrical generator that sends 8 pulses per 1 rotation of the speed sensor's gear. The voltage will stay the same no matter what speed you are going. however the pulses per minute will increase as the speed increases. The amperage won't change either, as long as the resistance and voltage are do not vary. In this case, the resistance and the voltage don't change, so the amperage will stay the same. Using a DMM is out. Here is what you can do, though its a pain, drive your car . the speedometer and the odometer work off of the same pulses. so if you are driving down the road counting the mile markers and your odometer is off by as much as the speedometer then your back to guess work. if the odometer is off by only 5% you know the speedometer is wrong.