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Post Info TOPIC: Help with TH400 Transmission speedometer gears


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Help with TH400 Transmission speedometer gears
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Working on the 51 Pontiac

Looks like the TH400 may have come from a GMC Truck?

Any way is has a electronic pickup for the speedometer, not too compatible with the 1951 speedometer.

Looks like with the yellow/white 19 tooth drive a older style speedometer

is it really that simple or must the digital drive ring be removed.

Hoping someone has actually done this and can confirm my suspicions 

Thanks 



-- Edited by BLyke on Tuesday 14th of September 2021 06:51:28 PM

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Bruce L. - Lakeville MN

1971 Malibu Convert

1951 Pontiac Chieftain Super Deluxe Catalina Hardtop 



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Also, what would need to be done if anything with the other electrical plug?
Sounds like it may be part of the kickdown?

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Bruce L. - Lakeville MN

1971 Malibu Convert

1951 Pontiac Chieftain Super Deluxe Catalina Hardtop 



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They typically can be converted to mechanical speedo. I have done it on a couple 700r4 transmissions. This should not be much different.  The driveshaft will have to be pulled and the 5 or 6 bolts holding the tail housing on and pull the tail housing. The gear on the tail shaft will have to be pulled off there may be a clip holding it or it might be friction fit I can‘t remember. Then tap on the new drive gear which does have a clip that holds it in position. Reinstall the tail housing and driveshaft and then install the Speedo housing with the driven Gear. Sounds complicated but it is probably a 15 minute job.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe



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The other plug is for kick down function, there should be a switch on the gas pedal or the carb that would send Power to the solenoid in the transmission at wide open throttle.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe



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Thanks Chris!


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Bruce L. - Lakeville MN

1971 Malibu Convert

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67ss wrote:

They typically can be converted to mechanical speedo. I have done it on a couple 700r4 transmissions. This should not be much different.  The driveshaft will have to be pulled and the 5 or 6 bolts holding the tail housing on and pull the tail housing. The gear on the tail shaft will have to be pulled off there may be a clip holding it or it might be friction fit I can‘t remember. Then tap on the new drive gear which does have a clip that holds it in position. Reinstall the tail housing and driveshaft and then install the Speedo housing with the driven Gear. Sounds complicated but it is probably a 15 minute job.


 so all 3 of these will be removed and replaced?



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Bruce L. - Lakeville MN

1971 Malibu Convert

1951 Pontiac Chieftain Super Deluxe Catalina Hardtop 



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Yes, the digital drive ring is not compatible with a driven gear and is what Chris is referring to replacing by removing the tail housing.

You will need to get a drive gear for the output shaft to take the place of the digital ring. Figure out your rear gear ratio and tire size and match the drive and driven gears for that combo.



-- Edited by Lost in the 60s on Wednesday 15th of September 2021 04:54:02 PM

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Mitch D.   River Falls, WI

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Some Assembly Required

1966 Chevelle SS 396 M20

1970 Chevelle SS 396 M20



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BLyke wrote:
67ss wrote:

They typically can be converted to mechanical speedo. I have done it on a couple 700r4 transmissions. This should not be much different.  The driveshaft will have to be pulled and the 5 or 6 bolts holding the tail housing on and pull the tail housing. The gear on the tail shaft will have to be pulled off there may be a clip holding it or it might be friction fit I can‘t remember. Then tap on the new drive gear which does have a clip that holds it in position. Reinstall the tail housing and driveshaft and then install the Speedo housing with the driven Gear. Sounds complicated but it is probably a 15 minute job.


 so all 3 of these will be removed and replaced?


 The yellow gear is the governor I believe which stays along with the gear driving it. It is the ring behind those that has to be removed and then a gear installed in its place to drive the speedo housing.



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Chris P
East Central, Mn

66 Chevelle 300 deluxe



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Thanks Chris,
will the yellow gear come off when the rear housing is removed?

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Bruce L. - Lakeville MN

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No it won’t. If you search YouTube there are some videos of replacing the gears to give you some idea of what to do.



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Chris P
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67ss wrote:

No it won’t. If you search YouTube there are some videos of replacing the gears to give you some idea of what to do.


 Yep,

Found several and looks like it will be doable 

Planning to go with a steel drive gear form LA Speedometer Gear. it is a press fit and Supposedly was part of a GM recall long ago

 



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Bruce L. - Lakeville MN

1971 Malibu Convert

1951 Pontiac Chieftain Super Deluxe Catalina Hardtop 

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