Hot Rods leaking speedometer sender

Sharing knowledge, I have been building cars for over 40 years. Since we always had a speedo cable going to the speedometer, leaks were not a issue. So I have a 46 chevy panel that started leaking tranny fluid at the classic industries speedo sending unit mounted to the side of the th350 tranny. In all my years I never noticed that the tranny bullet has a pair of O-rings inside of it to seal the speedo gear. So if you have a cable or sending unit leak, it is highly possible that the bullet seals are shoot.

 

Yep-and the bullet may be worn as well. There are two types of seals as I recall-not sure.

 

If you have an aluminum bullet they are more prone to wear than the steel ones. I would buy a new bullet and a driven gear and that will likely fix it.

 

Strangest leak I saw was back in 74 on a brand new Dodge Challenger A/T. A young woman who had to wear nice clothes to work, kept getting oil stains on her legs from under the dash.

I don't recall what was missing for seals, but the spiral grooves on the speedo cable were like a conveyor, bringing ATF up to the speedo connection.
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There was usually a single o-ring on the outside of the sleeve to seal it to the bore in the extension housing. On the inside of the sleeve there was a small lip seal that sealed it to the shaft of the driven gear.

The inner core of the cable was spiral wound in a specific direction such that when rotating in the proper direction it would tend to push any fluid present back down into the transmission. Every now in then you might run into a cable that was wound in the wrong direction and when rotating they would pull any fluid upward in the cable housing toward the speedometer head and cause a fluid drip under the dash.

 

Yep, new bullet and gear on the way. The small things can be the crazy fun stuff. I had a 69 cuda in the early 70's that had a leak at the speed-o. Fun part of this event is, when I talked to a young service rep, "not at classic" he thought my gear should be ok. I had to remind him that the front seal on the sbc timing cover would grove the machine steel crank after some time. If the bullet seal is leaking, I would bet the plastic gear is grooved also. Replace them both.
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