Loose speedo cable?

[LuckyLightning]

I have a 1979 K20. My speedometer and odometer do not work. Looking under the truck, you can see where the cable is loose where it connects to the transfer case. I took a video of it.

It doesn't look like it can be tightened any more (although I have not tried yet). Anyone see this before? Any recommendations for a fix?

[rich weyand]

OK, so this is how the thing works.

The TH350 and other trans have a myriad of speedometer gear combinations -- drive gear and driven gear -- to accommodate different rear-axle ratios and tire sizes. Not so with the NP203 and NP205 (like yours) transfer cases. Accommodation of different rear-axle ratios and tire sizes is through ratio adapters, which is what the widget is that plugs into the transfer case and has the speedometer plugged into it.

The ratio adapters have female connections on both ends. The speedometer cable has a male connection (though not the one that matches the female in the ratio adapter) and the transfer case is also a female connection internally. So there are little widgets, called keys, that connect the rotating parts.

The speedometer cable has a square end, while the female in the ratio adapter is a round hole with a keyway along the side. There is a cylindrical sleeve that has a key on the side and a square inside diameter. The sleeve plugs into the adapter, and the speedometer cable plugs into the sleeve.

On the transfer case side, the ratio adapter also has a round hole with a keyway in it, while the transfer case accepts a square connection like the speedometer cable end. There is a piece that goes in between that is square on one end and on the other is round with a key.

If the keys get sideways in there -- say someone pulls the speedo cable out of the sleeve from the dashboard end -- there is no way the cable will go back into the key. Simillarly, if the key on the transfer case side is damaged, or is the incorrect one, there is no way that the speedometer drive will work, and the nut may not pull the adapter up tight to the transfer case.

Finally, there is a replaceable piece in the side of the transfer case that has an O-ring in it to seal the transfer case oil/grease from going out the speedometer cable. This is usually shot by the time a truck is this old. This replacement is pretty easy to find.

So, the procedure at this point is to take the cable off the ratio adapter, and the ratio adapter off the speedometer cable adapter, and the speedometer cable adapter off the transfer case. Make sure the keys are in place and undamaged. Turn the ratio adapter by hand in both directions and make sure it rotates easily in both directions. Inspect the O-ring seal in the speedometer cable adapter.

If the ratio adapter is jammed -- the gears and or gear shafts can wear out, and then it locks up solid -- you will need to find a new ratio adapter. They can be had, but are not cheap. The ratio adapter will be stamped on the side with the ratio built into it. This is the ratio needed to get the speedo indicating correctly for a given axle ratio and tire size. Your truck may or may not have the right ratio adapter in it, because the tire sizes or axle ratios may have been changed.

If the keys are damaged, you need to get new keys.

If the O-ring is damaged or shot, you will need to get a new speedometer cable adapter.

I can help you determine the correct ratio adapter if you give me your tire size and axle ratio. I can also help with sources for all of the above parts. But before I do that, perform the inspections above and let me know what you find out.

[rich weyand]

This is the only place that has all the keys:
http://www.texasindustrialelectric.com/speedo.asp

This page has the ones you need if they're busted.
http://www.texasindustrialelectric.c...on_Adapter.asp

You need the HR124 and the HM146.

The part with the O-ring in it is the driven gear retainer. It's part "B" in the photo, AC Delco #326561, opaquely called "sleeve" in the parts book. See this page:
http://coloradok5.com/forums/showthread.php?t=269149

#326561 is not hard to find: https://www.google.com/?gws_rd=ssl#q=Delco+%23326561

In your inspection, you also need to pull the driven gear. Part C on the page above. It just comes straight out the hole once the retainer sleeve is removed. It will have a letter stamped on it denoting the number of teeth, like H (18) or M (22). I need to know that as well to calculate the correct ratio adapter in case yours is bolluxed.

Note that pulling all this apart and doing the inspection takes about 15 minutes.

[slotard]

I need to lube my speedo cable (or something from the gauge area seems to squeak at least), how do I do so? 350 trans.

[LuckyLightning]

Wow, thanks Rich. Lots of info here! I'll dig through the links you posted and (try to) crawl under the truck today to inspect as recommended.

- The transfer case is an NP205 (I believe because it's not aluminum)
- The transmission is a 4-speed manual (L and 1-3, R; no idea if it's a Muncie or NP)
- Tires are 315/75 (basically 35's)

Until I can order replacement parts, can I just remove the ratio adapter and plug the cable straight into the transfer case? I understand the speed gauge most likely will not be accurate, but at least I would have a ball-park idea of how fast I'm going when driving.

[rich weyand]

Originally Posted by slotard

I need to lube my speedo cable (or something from the gauge area seems to squeak at least), how do I do so? 350 trans.

Unclip the cable from the back of the speedo by reaching up under the dash. Pull the speedo cable housing down to where you can pull the cable all the way out of the housing. Coil the cable up in the bottom of a coffee can with enough mineral spirits to cover it for an hour or two. Swish the can around to rinse anything off it, then wipe dry. Soak the cable with speedo cable lube (OReillys has it), except for the last 6 inches by the gauge. Reinsert the cable into the housing and re-attach to the gauge.

If you have trouble getting the cable in all the way, or if you have a transfer case, you will have to disconnect the cable at the other end to get it enghaged properly.

[rich weyand]

Originally Posted by LuckyLightning

Wow, thanks Rich. Lots of info here! I'll dig through the links you posted and (try to) crawl under the truck today to inspect as recommended.

- The transfer case is an NP205 (I believe because it's not aluminum)
- The transmission is a 4-speed manual (L and 1-3, R; no idea if it's a Muncie or NP)
- Tires are 315/75 (basically 35's)

Until I can order replacement parts, can I just remove the ratio adapter and plug the cable straight into the transfer case? I understand the speed gauge most likely will not be accurate, but at least I would have a ball-park idea of how fast I'm going when driving.

Transmission is probably a 465, with the granny low gear that you don't use in normal driving.

The case is a NP205. I can tell by looking at it. Note the NP203 is also a steel case: it weighs 200 pounds. NP205 came with the stick in 1979, while NP203 came with the automatic.

315/75R15 should be 33.6" diameter. I'll use that later to calculate your correct ratio.

Yes, you can remove the ratio adapter and the keys, and just plug the speedometer cable into the transmission case for now. To calibrate your speedometer, drive 10 mileposts on the interstate, taking not of your odometer reading at start and stop. Let's say the odometer says 8.5 miles: then you are reading 85% of your actual speed. That number will also be a check on my ratio calculation, since, for example, 1/.85 is 1.18, so you would need a 1.18 (or close) step-up ratio adapter to get the speedometer up to the right value.

[LuckyLightning]

Thanks Rich. I'll plug the speedo cable directly into the transfer case to see if that works.

Also, 315/75/R16 tires should be 34.6" in diameter, which are basically 35's (I'll measure for myself here in a bit). I got the info from here.

[77K10Silverado]

Nice thread here. Rich thanks for the wealth of knowledge you continue to gift us.

[LuckyLightning]

Just to provide an update: I removed the ratio adapter and noticed the cable end that goes into the transfer case was shorter than the cable end coming out of the speedo cable itself. Thinking that might be the issue, I removed the adapter and plugged the speedo cable straight into the transfer case.

No difference: still no speedometer working.

So next I took off the gauge cluster. The cable end (again) seemed short so I used some needle nose pliers to try and pull it out a little. I got maybe 1/8 of an inch more out of it. I carefully replaced the speedometer (so as not to push the cable back in) and got everything bolted back in.

No difference: still no speedometer working.

I guess the next step is to ensure the cable is rotating like it should. Otherwise, it's to The Google for other causes to troubleshoot.

[rich weyand]

The speedometer is calibrated to 1000 turns per mile. That is, the cable turns one revolution every 5.28 feet. So if you just pull the cable out of the back of the gauge, and route it to stick out from under the dash, you should be able to see it turning easily by just idling up and down in the driveway.

[LuckyLightning]

Nice Rich, that's a great way to test for proper cable rotation! I'll be able to do that this Saturday when the weather is forecast to be more tolerable.

[LuckyLightning]

Just following up on this: I removed the gauge cluster and drove the truck around the block to check for cable rotation: there was none. So now I need to figure out if the issue is lubrication for the cable or if it's something inside the transfer case.

[rich weyand]

Could be the key is off the other end of the cable, the cable is broken, the key is broken, or the ratio adapter is seized up, in which case the key from the transfer case to the ratio adapter is now broken.

I have had both the first and last of those in my time with the truck.

[LuckyLightning]

I already removed the ratio adapter, so I can at least eliminate that as a potential issue.

How can I check to see if the key is broken? Is that inside the transfer case?

[rich weyand]

The socket on the driven gear is a .152" square hole in the end of a steel shaft. Unlikely to be broken, but it is simplicity itself to unscrew the sleeve that retains the driven gear and pull the gear out for inspection. Take the speedometer cable off and remove the sleeve with the large hex.

Another thing to check is that the cable is down into the driven gear. It can miss off to the side, and it can also not mesh and be pushed out in the cable sleeve.

[LuckyLightning]

Been a long time, but I wanted to post an update. A friend and I were under the truck last week and decided to poke around on the speedo issue.

Did like Rich said and removed the part that the cable goes into. I believe this is the key/driven gear component ("C" in the photo of the third link from Rich's second post here). We removed it, about the size and length of a large finger with a gear on the end. It looked fine, so my friend stuck his finger inside the transfer case and he said it felt like the gears inside were worn down. Not sure what this part would be called.

Rich, in a few days I should be able to get under the truck again and count the teeth on the driven gear, as requested.

[Shugalou]

Wow Rich this is great info! I am dealing with this part leaking right now and this info has given me an understanding of how this works...I looked through the manual and couldn't find any info in the transmission section.