1998 merc c class 250 td accelerates fine, get to cruising speed and pulls back, get rev counter sweep but will accelerate to another speed. changed fuel filters and tried injector cleaner. After a while will run great, untill you turn the engine off and re start.
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Re: loosing power at cruising speed
Have you had the transmission flushed? it seems to be caught in a limbo. where the calibration of the speedo and the transmission are not compatable. Now as wierd as that seems it happens usally like when going up a hill if you have a manual trans. and cruise locked in. it cant hold that speed and moves to a different speed if you dont down shift. Or perhaps your cruise control cable is slipping. is it vacume over mechanical? pop the hood and make sure all your hoses to cruise control are tight and no leaks.
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the rev counter on the 300tdi disco takes its speed signal from a wire connected to the back of the alternator (single spade type connector) this wire usually breaks rewire and should be ok
cruise faults are usually caused by a vacuum pipe splitting check all the vac pipes to the cruise control actuator in the engine bay and at the brake pedal switch of end are split cut off and refit pipe
I have this same car. The place you set speed control is on the windshield wiper switch on the right side of the steering column. The button with the indicator light has to be pushed in order to use the cruise control but just because the indicator light is on, it does not mean the cruise is on. Hope this helps a little. If you need more info and someone else doesn't answer more eloquently.. I'll check back and try to give you more info. :-)
run the fault codes
could be a transmission actuator problem as it senses vacuum changes when moving the accelerator and tries to change ratios ( 2000 to 2500 rpm indicates a ratio change which is the speed increase momentarily until the Transmission range sensor puts it back in the correct ratio for the road speed
check the wiring to the transmission
it is a sensor problem not a transmission failing
Your ford has power assisted breaks this power is delivered from the motor, at the point when the accelerator is inactive. There for it sounds like your idle speed setting is too low, if you have a rev counter then it should be reading around 1000 revs at idle, if not the idle needs increasing. Hope this helps
Pull up on the brake pedal. If that does not work check your brake light bulbs (LED brake lights will also keep your cruise control form working). Finally check all of the fuses. This system needs at least 3 different fuses to work properly.