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A surprisingly common problem is speedometer cable failure, often manifested at first by a bouncing speedometer needle.
Paul Clark wrote:
A broken speedo is most likely because of a broken speedo cable. this is a heavy (1/4"- 3/8") cable that screws into the dash unit basically behind where you see the speedo on your dash. crawl under there and unscrew the dash end- you'll find that inside the cable housing is a core- if this turns freely to your fingers, it's broken. If not, try driving with it disconnected and see if it turns. IF not, it's broken.
New ones are maybe in the 20$ range, the other end screws into the back of your transmission, so you'll need to crawl under there. Also, I don't know how it is with an automatic, but with a stick, when you take out the cable, oil will leak out the trans until you put it back (so be warned).
Installation is easy- just reverse the procedure.
However, if the cable is ok, it could be your actual speedo unit has gone bad, in which case you'll need to find another one froma junkyard or some such. This will mean removing the dash cluster- held in with screws from the front.
Joe Harris wrote:
It is not hard to replace the speedo cable as long as you can easily and safely get under the car. The dash comes out fairly easily also, but you may find that you can reach up under the dash and screw cable in without taking the dash out. You should also check that it is not the drive gear, usually color coded nylon gears, in the drivers side of the transmission. You may also find that the cable has come of and all it needs is to be screwed on tightly.
The end of the cable should be square and not rounded of showing bare wires.The cable is cheap and so is the little nylon gear or drive unit. Many times people that have swapped out a rear end, do not have the right nylon gear so they unhook the cable. Also changing tire sizes will make it where you need to figure out your ratio's and order the right colored gear, you are going to be under the car anyway! Will only take you a hour or so if you have never done it before and about 15 minutes if you have a hoist and know what you are doing.
Alan wrote to the Slant Six Club:
You disconnect the cable from the head.You should be able to pull the cable out of the casing.In which case, I did it all by hand. Just make sure you get speedometer grease. I greased the cable a little at a time then slid it back in the casing and reconnected. Messy but effective.
Andy Wittenborn wrote:
The plastic clip that is part of the cable and attaches the cable to the speedometer is easier to remove and put on if you lower the steering column, as you have to pinch the clip to remove it from the speedometer. I usually grease or oil the cable housing with a graphite oil mixture.
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