T-400 Speedometer only works when cold - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-26-2011, 06:18 AM
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T-400 Speedometer only works when cold

The speedometer on my '78 Chevy Big10 works fine for about 10 minutes and it is cold. After the initial 10 minutes the speed starts decreasing slowly until it reaches zero. Mind you I am probably going 55-60 on the interstate when the speed starts decreasing. Checked the gears in the trans they look good. The cable is new and appears ok. Anyone ever see anything like this ?
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OTG

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Old 01-26-2011, 06:44 AM
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Since you state a new cable then we are talking mechanical not electrical pickup. The plastic gear at the transmission has a square hole that the end of the cable slips into. Is this hole rounded and the cable slips? Positive that the cable is inserted as far as possible? When did this start? After you installed new cable? Why did you install a new cable?
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Old 01-26-2011, 07:52 AM
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Quote:
Originally Posted by heyjude076
Since you state a new cable then we are talking mechanical not electrical pickup. The plastic gear at the transmission has a square hole that the end of the cable slips into. Is this hole rounded and the cable slips? Positive that the cable is inserted as far as possible? When did this start? After you installed new cable? Why did you install a new cable?
Guess I am stuck in the '70s never saw an electric pickup only cables.

I swapped the trans from a C20 to a C10Big10 which is why there are little differences. The C20 was a 84 and the Big10 is a 78. If this matters.

After swapping the trans the speedo didn't work at all.

I checked the gear etc a couple of days ago and replaced the reduction
box(?) (unit between speedo cable and trans) The one that was on it had a cable that was smaller than the square hole in the gear. This reduction unit was shaped like the letter "Z". I had another reduction box that was an "inline" and the cable into the trans was bigger and fit better. Everything is snug now.

One would think that if the cable was slipping the speedometer would be erratic. This one slides down to zero over a few miles distance and even takes a pause at 15mph sometimes.

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OTG
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Old 01-26-2011, 08:10 AM
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Look again at the speedo drive gear on the output shaft. When the speedo cable starts to go bad and drags, the plastic teeth on the gear start to wear. Since the gears are helical, the wear is only in the middle and may not be apparent if you're just looking through the hole for the driven gear. When the trans fluid is cold, there's enough viscosity that the driven gear will turn, but once the trans fluid heats up and thins out, the driven gear (and thus the speedo) stops.

If you are sure the drive and driven gears are good, then the only other thing I can think of is that the holder for the driven gear is the wrong one or is not inserted all the way so the gears aren't making full contact. The viscosity of the trans fluid is again what makes the driven gear turn when the oil is cold.
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Old 01-26-2011, 05:36 PM
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Another possibility is the speedo head.

It works by a rotating magnet (hooked to the speedo inner cable) 'pulling' the needle along- the needle itself isn't mechanically connected to the inner cable, in other words.

If any of the friction points are sticky, this can cause the needle to slow or stop altogether.

If you can reach the back of the speedo to unclip the cable, you might be able to see if the cable stays turning after the amount of time that it usually has stopped. If the cable quits turning, that eliminates the speedo head.
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Old 01-26-2011, 06:10 PM
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Thanks for the quick response. I will pull the drive gear and replace it. I am going to try NAPA for a replacement. Think they have it ? Didn't see it listed online for them. Anyone know where I can find it ? Hopefully I can find an o-ring also.
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OTG
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Old 01-26-2011, 06:29 PM
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Quote:
Originally Posted by otgdy
Thanks for the quick response. I will pull the drive gear and replace it. I am going to try NAPA for a replacement. Think they have it ? Didn't see it listed online for them. Anyone know where I can find it ? Hopefully I can find an o-ring also.
Thanks
OTG
Just to be clear, the drive gear is the one on the output shaft. You need to pull the tailhousing off to change it. The o-rings should be available at a hardware store.
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Old 01-27-2011, 05:36 AM
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Quote:
Originally Posted by joe_padavano
Just to be clear, the drive gear is the one on the output shaft. You need to pull the tailhousing off to change it. The o-rings should be available at a hardware store.
I am thinking about the gear that comes out with the round housing that has a distributor like clamp holding it in.Your referring to the one that this one mates up with ? Now that would suck. It is a new trans hopefully your wrong
even if Murphy says your probably not. Any idea where I can get the one that is easier to remove ?
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OTG
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Old 01-27-2011, 08:55 AM
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Most Trans shops keep a few Gears on hand or a Trans Parts House.
If no luck check here:
http://www.transmissioncenter.net/sp...____va.htm#400
http://www.tciauto.com/Products/Tech..._gears.asp#400
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