Speedo erratic My speedo works from engine start for about 15 - 30 mins and then zeroes. can anyone tell me if it is the instrument or a sensor. And if sensor problem where located. Toyota Celica ST 1.8 7AFE 1995 (N)

input and out put speed sensors tell the TCM and the ECM the difference between engine rpms and transmission input roms so that the correct gear ratio can be selected
A speedo sensor reads the out put shaft (drive line ) shaft and refers to road speed for speed and odometer readings and to calculate the fuel consumption rate
it is possible that you have replaced the wrong sensor
Talk with an accredited transmission specialist shop and have the problem diagnosed

Depending on the type an year vehicle an the electronics on the car , the instrument cluster could be at fault ! The stepper motor that moves the speedo could be the problem if a GM product ! As far as the speed sensor goes a few other things wouldn't work very well if it were bad an the check engine light would be on more then likely because this is an input to the PCM - engine computer ! The transmission would shift erratically !
Speedometer
The IPC displays the vehicle speed based on the vehicle speed signal from the PCM. The PCM converts the data from the vehicle speed sensor to a 4000 pulses/mile signal. The IPC uses the vehicle speed signal circuit from the PCM in order to calculate the vehicle speed. The speedometer defaults to 0 km/h (0 mph) if a malfunction in the vehicle speed signal circuit exists.
So I would say the instrument cluster is bad !

it is possible that it could be the instrument cluster that is at fault

most cars including yours, you have a speed snesor on the gearbox, which tells the computer (ecu) and transmission how fast you are going....the fact that you have included that the speedo is erratic, is telling me that the sensor needs to be replaced, as it is telling the ecu (which tells the speedo) the incorrect signal as well as the transmission......hopefully, easy fix

this reminds me that cars are like humans IN A WAY, the same symptoms may lead to a cause, the car not downshifting and the erratic speedo are the symtoms, just like a runny nose and cough are symptoms, and the sensor is the cause of the faults, where the cold is the cause of the faults in us....how cool

sounds like to me you have a wiring harness problem

no but they do suffer with the float sticking in the sender unit and check the sender unit on top of the diff unit for the speedo

sounds like a bad speedo drive gear or speed sensor mounted on the rear of the trans there is no speedo cable, its electronic its not expensive take it to the dealer they will know exactly whats wrong with it

I don't know what is causing this problem, but I know phil140701's answer is completely wrong on two counts. First, 740 Volvos starting at least in 1986 have electronic speedometers and no cable. There is an electronic sensor on the differential which sends a signal to the speedometer cluster. Second, if it were strictly mechanical with a cable then the voltage regulator could have no effect on it at all.

Check and or replace your speed sensor if that does not work replace the instrument cluster - you can find one at almost any salvage yard.

anyone that can afford to code read for free needs there head looking at my equipment has cost me thousands of pounds and returned me a lot more !!!!!! does the rev counter do funny things as well ? ...