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Speedometer is erratic and out of cal after transmission rebuilt


petef
09-11-2010, 12:09 AM
After having my transmission rebuilt the speedometer acts erratically (bouncing) and it's also out of calibration by about -20 MPH when it's not bouncing. This a 1985 Chevy G20 Van with a Turbo 350 automatic transmission and mechanical drive speedometer cable.

My mechanic tells me the problem is in the dashboard end but I totally disagree. First off the speedometer worked just fine before the transmission was rebuilt. So far, I replaced the speedometer cable and also checked the following:

* Removed the Speedometer cable at the transmission end and attached a portable drill. With drill driving cable, the speedometer does not act erratically and registers steady speeds up to 50 MPH.

* Made custom square drive tool that mimics the square speedometer cable end allowing me to apply rotational pressure to the speedometer gear on transmission. No slippage detected in either direction.

* Removed speedometer "bullet" and gear at transmission and visually inspected them. No abnormal wear or broken teeth detected.

I'm thinking that the mechanic screwed up and either lost the original speedometer gear and/or he put in the wrong gear from another transmission when he rebuilt mine.

Any other troubleshooting suggestions or ideas on what may be wrong would be appreciated.

---pete---

MT-2500
09-11-2010, 08:01 AM
Confirm that the speedometer gear is not slipping and turning good.
Remove cabel and stick a small pocket screwdriver into end of gear and hold a little pressure on it and make sure it is turning good and steady.
If so check cable at end of end for good fit.
And also where cable fits into speedometer head for good fit and turning good under little pressure.
Does it have a adapter box in cable to dash?

20 mph off indicates wrong gear set up.

speedometer bouncing can be in speedometer head.
If all else fails try another speedometer head on it.

petef
09-17-2010, 02:24 AM
Does it have a adapter box in cable to dash?

20 mph off indicates wrong gear set up.


No adapter box.

I found a great speedometer website....
http://www.transmissioncenter.net/speedometer_calibration_______va.htm#350

I have the TH350 transmission. If you page up a few screens from the link above, they have all the formulas for calculating the correct gears. I ran the calculations for my van and I do have the correct gears, which makes sense because the speedometer worked just fine before the transmission was rebuilt.

The only clue that something is worn is the speedometer gear housing seems loose and it's leaking transmission fluid, which means the black O ring seal is worn. For about $9 I can buy a whole new speedometer gear housing which includes the black O ring and also the inner seal that keeps the driven gear from leaking fluid up into the cable.

I'll keep you posted on my progress but it might be a few weeks or more.
Thanks for the suggestions!

---pete---

petef
11-29-2010, 10:14 AM
Update:
I installed a new speedometer housing (cable screws onto housing, housing inserts into trans) and gear that goes on the cable. This dramatically improved the speedometer operation and accuracy of the reading but it's still a bit erratic at times.

Close inspection of the old gear indicates some wear because the teeth are slightly more worn in the center than the edges and the new gear was perfectly straight along each tooth. . I also suspect that the worm gear inside the trans has some wear and needs to be replaced. It's working good enough for now, but I do plan to change the worm gear too. My advise to anyone, if you are going to buy one gear because you suspect it's worn, buy both gears.

That's all for now.

---pete---

MT-2500
11-29-2010, 10:51 AM
Update:
I installed a new speedometer housing (cable screws onto housing, housing inserts into trans) and gear that goes on the cable. This dramatically improved the speedometer operation and accuracy of the reading but it's still a bit erratic at times.

Close inspection of the old gear indicates some wear because the teeth are slightly more worn in the center than the edges and the new gear was perfectly straight along each tooth. . I also suspect that the worm gear inside the trans has some wear and needs to be replaced. It's working good enough for now, but I do plan to change the worm gear too. My advise to anyone, if you are going to buy one gear because you suspect it's worn, buy both gears.

That's all for now.

---pete---
]
Thanks for poating back how it went.
Sounds like you hit one sore spot on it.
And 10-4 on the gears.
Good Luck

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