chris@umcp-cs.UUCP (Chris Torek) (08/27/85)
I was driving back from New Jersey early Monday morning, going faster than usual (~55 [I have a Chevette]) when my speedometer started acting up, as if the cable were sticky: the needle started bouncing up and down around my actual speed. Having seen this happen before, I was unworried. Anyway, it finally decided to bounce all the way up to the stop, then all the way down to zero, and settled on 30 MPH for a few minutes. It eventually returned to 55. Well, it seems to be OK now except for one small problem: the needle "sticks" at 30. If I go 50 MPH, it goes up to 30, says there for about one minute, then goes up the rest of the way and remains accurate so long as I remain above 35 MPH or so. When I stop, it goes down to 30, sticks there for a few seconds, then plummets to zero with a little "clink" noise as it hits the stop. I am curious as to the mechanism involved here. If the speedometer were a simple ammeter, this wouldn't happen. How do speedometers work inside, and how might it have been damaged such that it sticks at one particular speed? -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251) UUCP: seismo!umcp-cs!chris CSNet: chris@umcp-cs ARPA: chris@maryland
bsisrs@rruxe.UUCP (R. Schiraldi) (08/28/85)
Sounds like the speedo cable is in need of replacing. The cable can be
replaced by yourself, but ti involves (sp?) removing the ENTIRE dash
and the instument cluster. To do the job yourself will probibly
cost one afternoon and about $20. A GOOD mechanic will charge about $50
or maybe a little more.
Rich Schiraldi
rruxe!bsisrsjohnbl@tekig5.UUCP (John Blankenagel) (08/29/85)
> Sounds like the speedo cable is in need of replacing. The cable can be > replaced by yourself, but ti involves (sp?) removing the ENTIRE dash > and the instument cluster. To do the job yourself will probibly > cost one afternoon and about $20. A GOOD mechanic will charge about $50 > or maybe a little more. > Not necessarily. On cars I have done this on, you could remove the cable from the transmission, pull out the center, and put in a new center and it would work fine. If the case is bad then the whole cable would need replacement though. However, since the cable sticks at 35, it kind of sounds like something inside the speedometer is sticking. After awhile of driving it breaks loose and is free above 35. I can't imagine how that could happen to a speedometer, but maybe when the needle was bouncing madly around the needle got bent slightly. Then if there were some irregularity in the face plate at 35 . . . no maybe not. Anyway, replacing the cable core only may fix it. You can get cores from any auto parts store and just cut them to the right length. John
tmorris@convex.UUCP (09/03/85)
If you were going to have to work on a speedo on any vehicle,
(of modern manufacture) you couldn't get an easier one than a chevette.
It is quite easy to remove and repair , and if you aren't afraid ,have
a blast and save bucks.
You asked how it works. The cable is connected ,thru a bearing
and shaft arrangement ,to a magnet . This causes the magnet to rotate
in proportion to the vehicle speed . Surrounding the magnet is a steel
shell , which is on another shaft connected to the needle. there is a
very fine spring around this shaft that attepts to pull the needle
towards zero by rotating the needle shaft counterclockwise. The magnet,
spinning clockwise opposes the action of the spring , and the balance
between these forces causes the needle to indicate your speed.
Now , what is wrong with yours ?
POSSIBILITIES :
1. Bent needle shaft . There is an emission service required on 81 and later chevettes in which a yellow indicator on the odometer
is reset. The needle or needle shaft can get bent at this time....
ask me how I know...
2. Bent speedo faceplate . They are very thin , and almost any
under-dash activity which involves pulling on the speedo cable to get it
out of the way can bend it.
3. Grease from the speedo cable slinging out into the magnet
shell. From excessive lubrication of speedo cable at wrong points.
4. Bad needle shaft bearing.
5. Rust in magnet shell
6. Worn bushing.
WHAT ARE YOU GONNA DO ?
SUGGESTIONS:
1. If you elect to remove the speedo head yourself , put in
fresh cable while you're there.
2. If you dont see anything real obviosly wrong , take the
removed speedo to a speedo shop. They don't charge nearly as
much when the units are brought in minus the car. Also they
can calibrate it for you.
3. Before you even pull it out , see if reaching under the
dash and pushing UP on the speedo cable fixes it. I've seen a
lot of chevettes fixed that way. If it goes away when pushed up
, use a ty-wrap to hold it in place.
4. If you have an 81 or later model , there will be a sensor
on the back of the speedo head with a couple of wires on it.
Remove the entire sensor by taking out the 1/4 " hex - head
screw. Be sure to hook it back up , it tells the computer
how fast you're going , and if you forget , the check-engine
lite will tell on you.
5. If you replace the speedo cable ,be SURE to rout it like
the original. They like to get in fights with the wiper
mechanism....
HAPPY MOTORING !
.