Speedometer Cable Adapter question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

53Rick · 06-27-2016, 10:25 PM

I have a '72 GMC 1/2-ton with a Turbo 400 transmission. Recently the speedometer stopped working. I checked cable connections to be sure they were tight. Just starting to troubleshoot it. Crawling underneath I noticed the cable is tight where it connects to the speedometer cable adapter. However I noticed there is some wiggle in the adapter and it rotates somewhere close to 180 degrees when I turn it. I don't know what this adapter does and before diagnosing further, I wanted to be sure that the adapter is supposed to turn freely. If so, I will move on. If not then any help on determining why it rotates would be appreciated. I have read the shop manual and the overhaul manual and can find nothing that speaks to the adapter. The adapter I am talking about is called an adapter in the manual and the speedometer cable connects to it at the transmission. Any assistance appreciated.

53Rick · 06-27-2016, 10:36 PM

I would post a photo, but I can't figure out how to attach an image I just took.

**LockDoc** · 06-27-2016, 10:42 PM

Quote:

Originally Posted by 53Rick

I would post a photo, but I can't figure out how to attach an image I just took.

The ones I have seen were not tight. They did turn around but they didn't flop all over the place either.....

LockDoc

Feldsine · 06-28-2016, 01:37 PM

I have one of these as well, and my speedo also stopped working. There is a gear / shaft inside of it that looks like it is broken on mine, but I have not gotten to a trans shop to see if they have another gear yet. You might want to check that. Mine also rotates.

Orange Crate · 06-28-2016, 02:44 PM

For some reason I have this site saved for speedo gears:

https://transmissioncenter.net/?swoo...edometer-parts

53Rick · 06-28-2016, 06:11 PM

I did a little more diagnosis. Disconnected the speedometer cable at the transmission. Rotating the cable by hand moves the needle on the speedometer, so looks like the cable is fine. I removed the adapter (by the way it is a Stewart-Warner part 666F P 4 which is probably a ratio adapter). Turning the end of the pin on this adapter turns everything freely, so it appears to be functioning properly. I then jacked up the rear end and put the truck in drive. With the cable and adapter removed, I watched the end of the plastic speedometer gear and it does not turn with the drive shaft turning. My assumption is the plastic speedometer gear needs to be replaced which is really frustrating since it was replaced about 2 years ago when I had the transmission overhauled. This is a turbo 400 transmission and I have a spare speedometer gear that I bought when I bought the last one that needed to be replaced (43 teeth). It looks like I need a seal for it though. Does anyone know if a TH400 has any retaining clip inside that I will also need to order? And, most important, am I correct in assuming that with the drive shaft turning, the plastic speedometer gear should be turning? Seems like a no-brainer, but I just want to be certain.

06-27-2016, 10:25 PM	#1
53Rick Registered User Join Date: Jul 2004 Location: Silverton OR Posts: 20	Speedometer Cable Adapter question I have a '72 GMC 1/2-ton with a Turbo 400 transmission. Recently the speedometer stopped working. I checked cable connections to be sure they were tight. Just starting to troubleshoot it. Crawling underneath I noticed the cable is tight where it connects to the speedometer cable adapter. However I noticed there is some wiggle in the adapter and it rotates somewhere close to 180 degrees when I turn it. I don't know what this adapter does and before diagnosing further, I wanted to be sure that the adapter is supposed to turn freely. If so, I will move on. If not then any help on determining why it rotates would be appreciated. I have read the shop manual and the overhaul manual and can find nothing that speaks to the adapter. The adapter I am talking about is called an adapter in the manual and the speedometer cable connects to it at the transmission. Any assistance appreciated.

06-27-2016, 10:36 PM	#2
53Rick Registered User Join Date: Jul 2004 Location: Silverton OR Posts: 20	Re: Speedometer Cable Adapter question I would post a photo, but I can't figure out how to attach an image I just took.

06-28-2016, 01:37 PM	#4
Feldsine Registered User Join Date: Nov 2007 Location: Eureka, CA Posts: 60	Re: Speedometer Cable Adapter question I have one of these as well, and my speedo also stopped working. There is a gear / shaft inside of it that looks like it is broken on mine, but I have not gotten to a trans shop to see if they have another gear yet. You might want to check that. Mine also rotates. __________________ Stan Feldsine - Build Thread - http://67-72chevytrucks.com/vboard/s...d.php?t=673856 1969 C10 LWB Fleetside - 350 - 700R4 1971 C20 LWB Fleetside - 350 - TB350

06-28-2016, 02:44 PM	#5
Orange Crate Registered User Join Date: Feb 2011 Location: Under the Truck Dreaming Posts: 642	Re: Speedometer Cable Adapter question For some reason I have this site saved for speedo gears: https://transmissioncenter.net/?swoo...edometer-parts __________________ 72 K20 Custom Camper, D60, 14BFF, full roller 406 TPI w custom built ECU and yes..it has tow hooks...

06-28-2016, 06:11 PM	#6
53Rick Registered User Join Date: Jul 2004 Location: Silverton OR Posts: 20	Re: Speedometer Cable Adapter question I did a little more diagnosis. Disconnected the speedometer cable at the transmission. Rotating the cable by hand moves the needle on the speedometer, so looks like the cable is fine. I removed the adapter (by the way it is a Stewart-Warner part 666F P 4 which is probably a ratio adapter). Turning the end of the pin on this adapter turns everything freely, so it appears to be functioning properly. I then jacked up the rear end and put the truck in drive. With the cable and adapter removed, I watched the end of the plastic speedometer gear and it does not turn with the drive shaft turning. My assumption is the plastic speedometer gear needs to be replaced which is really frustrating since it was replaced about 2 years ago when I had the transmission overhauled. This is a turbo 400 transmission and I have a spare speedometer gear that I bought when I bought the last one that needed to be replaced (43 teeth). It looks like I need a seal for it though. Does anyone know if a TH400 has any retaining clip inside that I will also need to order? And, most important, am I correct in assuming that with the drive shaft turning, the plastic speedometer gear should be turning? Seems like a no-brainer, but I just want to be certain.



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