75 K10 No Speedometer-NP203

[93FatBottom]

Hey guys, asking for a friend.

1975 K10, 400/TH350/NP203, recently the speedometer stopped working. Factory everything. He doesn't want to trouble shoot this or that, he wants to start replacing everything from the transfer case forward. I'm trying to find out what parts are between the cable and the transfer case speedometer gear. Did some searching but only found a lot of incomplete/contradicting info.

Any help is much appreciated it! He already has a new cable but wants to try other things first!

Thanks guys!

[rich weyand]

OK, follow along. Inside the NP203 is a speedometer driven gear. I have seen them with 18 and 22 teeth, stamped "H" and "M" respectively. The driven gear has a square hole in the end that is a mate for a speedometer cable. This is a heavy duty steel part and is very unlikely to be a problem. They are also hard to find.

The driven gear is retained in place by a threaded sleeve with an internal O-ring oil seal. Part number is AC DELCO #326561, and is easy to find. This sleeve is threaded to accept a speedometer cable.

Onto the sleeve is mounted a ratio adapter. This adapter ratios run from.73 to 1.36, and there's a bunch of them. This corrects for axle ratio and tire size. The part number varies with ratio, as the ratio is built in to the part.

Onto the ratio adapter is mounted the speedometer cable.

The ratio adapter and the NP203 driven gear are both females: there is a "key" that connects them. The correct key is HR124. On the other side of the ratio adapter, the ratio adapter is female and the speedo cable is male, but not the same. The adapter you need is HM146. You get them here: http://www.texasindustrialelectric.c...on_Adapter.asp. No internet ordering, you call them on the phone. They're like $10 for the pair.

If you know what the axle ratio and tire diameter is, I can figure out the right ratio and give you the part number. Or just give me the ratio (stamped on the adapter) and I'll look up the part number if he wants an exact replacement.

[93FatBottom]

Originally Posted by rich weyand

OK, follow along. Inside the NP203 is a speedometer driven gear. I have seen them with 18 and 22 teeth, stamped "H" and "M" respectively. The driven gear has a square hole in the end that is a mate for a speedometer cable. This is a heavy duty steel part and is very unlikely to be a problem. They are also hard to find.

The driven gear is retained in place by a threaded sleeve with an internal O-ring oil seal. Part number is AC DELCO #326561, and is easy to find. This sleeve is threaded to accept a speedometer cable.

Onto the sleeve is mounted a ratio adapter. This adapter ratios run from.73 to 1.36, and there's a bunch of them. This corrects for axle ratio and tire size. The part number varies with ratio, as the ratio is built in to the part.

Onto the ratio adapter is mounted the speedometer cable.

The ratio adapter and the NP203 driven gear are both females: there is a "key" that connects them. The correct key is HR124. On the other side of the ratio adapter, the ratio adapter is female and the speedo cable is male, but not the same. The adapter you need is HM146. You get them here: http://www.texasindustrialelectric.c...on_Adapter.asp. No internet ordering, you call them on the phone. They're like $10 for the pair.

If you know what the axle ratio and tire diameter is, I can figure out the right ratio and give you the part number. Or just give me the ratio (stamped on the adapter) and I'll look up the part number if he wants an exact replacement.

Wow! Thanks for the info! The truck in question has 8 lugs swapped under it, the guy he bought it from told us they were 4.11's, but tomorrow we're gonna jack it up and see for ourselves. I'm also going to get under there and check these parts and get part numbers. The truck is on 35's. I'll post back tomorrow at some point! Thanks again!

[48cj2a]

Also check and see if it has a speedometer cable adapter like this coming off the T/C.

I've had the edges ware down on the little key piece sticking out and its no longer making enough contact to work.



This one is a Dodge T/C but this is what it looks like mounted:

[rich weyand]

From right to left in the picture: speedometer cable, ratio adapter, sleeve/driven gear retainer, transfer case.

[slotard]

Rich, I have a related problem - my tranny isn't original to the truck, and the gearing is wrong. I have 350/350, 4.10s, 30.5" tall tires. Currently, my speedometer reads ~10 over GPS at freeway speeds. What adapter do I need to get it to read correctly (or at least closer)?

[rich weyand]

Originally Posted by slotard

Rich, I have a related problem - my tranny isn't original to the truck, and the gearing is wrong. I have 350/350, 4.10s, 30.5" tall tires. Currently, my speedometer reads ~10 over GPS at freeway speeds. What adapter do I need to get it to read correctly (or at least closer)?

OK, so you could put the right gears in the tranny to make it read right. Or you can use a ratio adapter.

If you are going to use a ratio adapter, I need more info. I have no idea what gears could be in there now, and what you gave me isn't enough to nail down the correction you need.

So here's what you do. Take it out on the interstate somewhere. Note the odometer reading (to tenths of a mile) when you pass a milepost. Go ten miles. Note the odometer reading when you pass the ten-miles-along milepost. Subtract the two odometer readings.

You should get something like 10.5 miles, which means your speedometer is 5% over. Easy from their to get the right adapter.

On the other hand, adapters are a lot more expensive than getting the right speedometer gear. If you pull the speedometer cable, and pull the driven gear, tell me how many teeth are on the driven gear. I will still need the amount you are off in the ten-mile test.

[hatzie]

I made a fairly complex spreadsheet to calculate RPM at various speeds based on Tire Size, Transmission Ratios, Transfer Case Low Ratio, and Rear End Ratio.
It automatically calculates a table of possible speedo drive and driven gears based on Tire Size & Rear End Gears. The Drive gear is the end of the transmission or transfer case output shaft. The Driven gear is the gear on the end of the speedo cable. The Driven Gear is the only one you can easily change.

Many of the calculated gears are not necessarily available even if they were 30-40 years ago. This is why I made a large table of possible values with a percentage deviation from ideal column.

Ratio correction adapters can be calculated as well but that table is not automagic. These will make up for the fact that you may not be able to get the perfect gear set.

You need to edit the spreadsheet cells to match your tire size, transmission ratio, transfer case low range ratio, and rear axle ratio. The transfer case low range ratio is only for the RPM tables.

It's saved from Microsoft Excel as an Excel 97 spreadsheet. If you don't have Excel... Open Office and Libre Office suites are free and they will open and edit the Excel 97 (XLS) and the newer (XLSX) file types.

[slotard]

https://docs.google.com/spreadsheets...it?usp=sharing is a version of your sheet that anyone can open/use online. Thanks for providing it.