I have an interesting fault on the speedo of my BMW 635csi. When you put you foot down the speedo will drop about 5-10 mph and then when you take your foot off the pedal then it springs back to what I assume is the correct figure.
There seems to be no middle position, either it is in the correct reading or the 5-10mph too low reading, and it swings in between very rapidly.
I've switched the speedo closk unit for another with no effect so I assume its is either the cable drive on the gearbox drive.
Any ideas?
Where does the cable meet the gearbox on this car?
many thanks
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Is it a mechanical speedo on a 635 ?? Regards Peter
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I would think its an electronic speedo and there is a fault with the sender in the gearbox.
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I think you wiil find that the speedo is driven from a sensor in the rear axle.Check the wiring & connections to the 2-pin plug in the centre of the diff.
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Yes, apologies, you are quite right. The impulse sender mounts in the diff. on the 635. They are not too dear (about £25 I think). I am assuming this car is mid-late 1980's?
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This could be one of two problems, either the sensor i.e. faulty or not correctly paced form the crown wheel in the diff, thus the error when accelerating as the crown moves under load and is missing pulses, or the wiring to the sensor either at the connector or a fracture is being effected by the load in the and subsequent movement in the axle. I am surprised you change the cluster as the first issue, you must have had one lying around or something. Regards Peter
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Thinking some more, some of these older sensors where magnetic not Hall Effect types and if you think the cabling and the connector is OK I would remove and clean the sensor as it may have debris on it. Regards Peter
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Early 635s had a cable speedo where a 2 piece cable ran up from the gearbox. 9 times out of 10 though if the cable failed your speedo just wouldn't work.
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