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I had posted earlier in the week about my electric speedo not working on my 87 240 wagon with M47- 5 speed manuel trans. Following the excellent advice given by David Hunter in a post titled "Maybe this will help,long and rambling" I have a few more comments and questions. Treating the transmitter in the differential like it is a generator I found that I did not have the pulsting voltage (up to 3 or 4 volts) that David describes. This was checked first at the instrument cluster, then above the gas tank inside of the car, and finally at the transmitter itself. All of this was done with one wheel jacked up, the car carefully blocked, and the transmission in first gear with the motor running. My voltage readings ranged from a negative .03 to negative .01 volts and did not increase with a faster engine speed. Oddly, I did get continuity though the transmitter although I can't say it was 3000 ohms as David describes.
If the transmitter is bad it raises a couple of questions. Am I correct in thinking that the only way to replace the transmitter is to remove the differential cover plate?? The thought that bothers me most regards gear ratios since I plan to find a replacement unit in the junk yard. I assume a transmitter from a car with an automatic trans. will not work as I would think the ratio would be different. I'm wondering about parts from an 86 with M46 - 4 speed with overdrive. Would this ratio be the same as my M47- 5 speed?? Funny thing is, my sons 85 M-46 actually has a transmitter in place on the differential. This is not hooked up as the car still use's a cable type speedo as an 85 is supposed to. Regarding the little paper "build sheet" present under the cargo floor in wagons, and in the trunk in sedans. As I recall the differential type is listed on this tag. Is this codeing the same on all years??
As always I appreciate all of your thoughts and comments.
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