Just got back from another day of watching my speed needle wave between 120-140kph when the tach @ 2000 says I'm at 110. I've replaced the ratio adapter, (all worked fine for about an hour), then on Ace's advice loosened the VSS (all worked fine for about an hour), then tightened the VSS finger tight (all worked fine for about and hour), wrenched it to maybe 10 lb-ft (all worked well for about an hour), yesterday loosened the VSS so it could wiggle in the nut, then today all worked fine for about an hour. You think this is boring to read, try looking at that speedo for a few hours!.

I thought of this today: how about trashing that whole crappy adapter nausea and put a speedo cable in, which is more in the Getrag's era anyhow, and replace the speedo head with something from maybe a pre-90 Chrysler product. For a cruise control I could run one of those cheap JC Whitney dash-mount jobs off the tach signal. Cheaper in the long run as my next logical step, short of dynamiting the truck, is getting a new VSS at about $120, on top of the ratio adapter which was about $130 when the dust settled. Failing that bolt the VSS directly on the Getrag, where it works perfectly except it reads 150 when I'm doing 100, and the cruise control doesn't work.

Sorry about rabbiting on so long but I was too angry to write shorter, .... Dennis

 

Try doing a search in the archives regarding the output shaft nut on the back of the Getrag. I've posted this several times - it will cause the same problems you are describeing. The speedo drive gear is is only held in place by preasure exerted by two spacers on either side of that gear. The rear trans "U" joint flange is held in place by a nut that needs to be very tight or the drive gear will slip as it turns causing eratic speedo readings and problems with the cruise control.

Bob

 

Thanks Bob, I know exactly where you're at here, that nut spec is something like 200 lb-ft. I was bopping around there again and remembered the off-set of the speedo driven shaft journal to give more or less tooth contact between your worm and the shaft gear teeth. I had it set with the holding yoke in a set of grooves on the journal. I rotated the journal a few degrees clock-wise and it made it much worse. Eureka! So going back and rotating half-a-few-degrees CCW from the marks it's now been perfect for several road hours, a three-year record!. I thought those journal marks were to hold the yoke claws and serve as index marks, but I'm now at about 10 degrees CCW of where it was and so far so good. I can't believe there isn't more about this on the forum as some of these kids are breaking Getrags and clutches like I break drill bits, and there's nothing my manuals literature to guide this installation. My Haynes manuals show only NP and NV models anyhow, no mention of Getrag, and the VSS shows up as a comment in the Emissions chapter, with no sign of a ratio adapter. Now I can finally fuel up and get an accurate fuel economy calculation.

 

Glad I could help. The problem was first discovered on the W models but is also on the the D models.

Bob

 

My 89 has an adapter on the transfer case then has the cable after that. Looks just like the one on my 93 where the vss screws in.

 

after all the grief I had with my speedo setup I'm pleased to announce that it's still working perfectly, with the cruise control back to its smooth let-off and take-up. Many thanks to those who chimed in with tips

 

Im not getting this here.
My speedo quit on me several years ago, I tries 3 ddiferent adapters in the transfer case none worked any better.
My speedo will fly all over the place at a stop light but not move when I am moving.
I have had my dash out for a long time, not going to put it in untill my speedo works again.
No cruise control on my truck.
What does this have to do with the output of the Getrag ?

 

Sounds eggzackly like mine was. The spooky part was the needle waving around when idling! The solution turned out to be a rotation of the speedo output shaft journal, the piece that's held in place by the 1/2" cap screw and yoke. The shaft is off-center in the journal so it's position is related to the shaft gear contacting the drive worm on the tail shaft. I rotated mine about 20* clockwise and that made it way worse. So going back to original position and rotating the journal about 10* CCW it became perfectly perfect! I nordled around with this enfuriating problem for over two years before getting it right. Leave your ratio adapter attached to it and use it as a handle to rotate the journal after loosening the 1/2" cap screw and yoke. Lemme know how you make out.

 

..oh yeah and the cruise control not working is due to the PCM comparing tach reading with road speed so that,for example throwing your clutch in will kick it off cruise and prevent overspeeding the engine. That's also why the CC won't work when the VSS is screwed directly onto the speedo output shaft = tach reading outside what the PCM expects for that road speed, if that makes sense.

 

Hey PigPen
I gave my sending unit a little twist today at lunchtime, about a 1/4 turn. didnt have any wrenches so I couldnt move the ratio adapter

well about halfway home tonight I noticed i was doing 65 mph

cool

I wonder if the drive part or gear is worn inside, I will have to unscrew everything to check them out.

thankx a bunch

 

check and see if the wiring harness is wrapped around or touching the down pipe and melted it in two, just fixed my wiring harness today and the spedo works like a charm, had the same simptoms. if they are melted, you can just splice connect them back together, 3 wires i belive are 16 gauge and 3 are 14 gauge