puzzlement - 86 NA speedo/odometer inop

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William Noble - 05 Mar 2007 08:28 GMT

new car (to replace daughter's car that I destroyed test driving it) - 86
NA - has bad motor mounts (ok, easy, just change them), one bad rear shock
(?? how can that happen), a darned K&N filter (need to find oem filter box
and put it back right), but here is the real puzzle:

tach works fine - speedometer and odometer inop. Shift light inop.
"mileage" meter reads wrong (shows way too much fuel consumption) - changed
sensor with a known good one, no change in symptoms. I don't think it's the
speedo because the light is driven by the computer so something is affecting
both - any suggestions beyond a wire? I'm sure there is one cause for all
these symptoms, I'd like to find it.

thanks

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bill
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Fred Aston - 09 Mar 2007 12:55 GMT

Hi Bill,

I'm still sick over you putting your daughter's car in the ditch. Anyway,
one of my repair jobs to my '89 many years ago involved replacing a plastic
"module" that is attached to the transmission. Apparently the plastic
becomes brittle and it broke when my mechanic was doing associated work. I
had no problem with paying for the replacement since I truly trusted this
guy, and felt he shouldn't have to absorb the cost of the part. I'd poke
around under the car for something like this.

Best of luck to you.

Fred

> new car (to replace daughter's car that I destroyed test driving it) - 86
> NA - has bad motor mounts (ok, easy, just change them), one bad rear shock
[quoted text clipped - 9 lines]
>
> thanks

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William Noble - 10 Mar 2007 02:53 GMT

that "module" was the first thing I changed - pulled the one from the
wrecked car, didn't fix the problem, thought I checked the gear too, but
maybe not - so tomorrow, I'll check that the gear is rotating - the module
to which you refer contains a hall effect sensor and a round magnet - the
magnet is rotated by a plastic gear in the transmission, each rotation
causes the sensor to put out a pulse (or is it two?) which the odometer
counts and the speedometer uses to determine speed (more pulses per second =
higher speed). I'm going to bring an oscilloscope with me too, so if the
gear is working, I can check at the gauge cluster and see if I'm getting
pulses. It's gotta be one of four things:
1. the sensor (I changed it)
2. the gear
3. the wire from sensor to gauge (I checked and there is 12V on the sensor
pos terminal)
4. the gauge itself

we shall see if I can find it tomorrow - a full day planned - exchange motor
mounts, shocks, clutch cylinders, diagnose and repair speedo/odo, and other
things as time permits - we'll see how far I get - wish me luck. I think
I'll leave the test driving to someone else...... :-)

> Hi Bill,
>
[quoted text clipped - 23 lines]
>>
>> thanks

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William Noble - 12 Mar 2007 06:07 GMT

following up - I have now verified that turning the wheels causes the speedo
gear to turn, I've verified power to the sensor - so it's nothing wrong in
back. Anyone traced out a schematic for the speedo assembly?

> that "module" was the first thing I changed - pulled the one from the
> wrecked car, didn't fix the problem, thought I checked the gear too, but
[quoted text clipped - 44 lines]
>>>
>>> thanks

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N8N - 12 Mar 2007 20:59 GMT

Have you tried simply installing the whole cluster from the other car?

good luck

nate

> following up - I have now verified that turning the wheels causes the speedo
> gear to turn, I've verified power to the sensor - so it's nothing wrong in
[quoted text clipped - 62 lines]
>
> - Show quoted text -

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William Noble - 13 Mar 2007 03:04 GMT

that's next on the list, but it looks like the clusters are different
between 86 and 87 - the 86 has the "fuel mileage" meter, the 87 does not, as
I recall, the 86 has two connectors, the 87 has four (or three?), but I'm
doing this from memory. I do have access to my old 85.5 which should be the
same as 86, but I don't want to take a third car apart for this goat rope -
there has got to be a single cause for all of this. It looks like the fuel
mileage meter is driven off the speedometer somehow, that's why a schematic
of hte instrument cluster for 85/86 would be helpful.

> Have you tried simply installing the whole cluster from the other car?
>
[quoted text clipped - 31 lines]
>> > pos terminal)
>> > 4. the gauge itself

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