78 k5 speedo problem and reverse delay

[76k5blazerr]

I currently have my buddy's 78 K5 because he had me replace the rear u joints. While I have it I was gonna see if yall could give me some insight on what might be causing some of the other problems it has. 1- the speedometer doesn't really work, the needle is always bouncing around and when you hit 50mph it pegs out a little past 100. 2- when you shift into reverse there is about a 5-6 second delay before it actually goes into reverse. Shifting to drive doesn't do it, it shifts fine, it's just reverse that has the delay. Any ideas? The tranny is a th350 and if matters-the truck is a 78 k5 Cheyenne so it's got all the options.

 

[dyeager535]

Speedometer sounds like it's toast. The one I had that failed the speedometer would randomly start bouncing around, finally doubling in speed. You can somewhat test those things by taking the cable off at the transmission and using a drill to turn it. Bouncing could be issues with the drive/driven gears, but if it stays steady after a point, probably not that.

 

[76k5blazerr]

So you are thinking that the actual speedometer needs to be replaced, the gauge itself?

 

[diesel4me]

Unfortunately,in my experience,when a TH350 starts "delaying" going into reverse,its the first sign that reverse will soon never engage again,and then the forward gears may or may not still work...

I had 3 of them die that way..last one I had that did that was my '77 K2500 that I plowed with for years..at first it took its sweet time going into reverse,and then one day,all I heard was a buzzing noise when I put it in reverse,and it would not back up...forward gears still worked fine though..

Greg Ducato can probably confirm that symptom...(reverse usually going out first )..

If the fluid level is correct,the filter is clean,and the modulator is good and hooked up right,I'd start saving up for a rebuild or another transmission..all 3 of mine had nice clean fluid and filters replaced and no "burnt" smell too..
Maybe it was something "easy" like a bad valve body gasket or valves sticking,but I doubted it..so I junked one truck,and bought used transmissions for the other two from salvage yards..

 

[dyeager535]

I would say the gauge needs replaced if when you run it with a drill, and after inspecting the drive/driven gears, you can eliminate the cable and gears as the problem.

 

[76k5blazerr]

Just checked with the owner and he said the delay has been going on for 5 years, and maybe 15k miles. Does that make you feel any differently about the problem?

 

[76k5blazerr]

Paging Greg Ducato!!!

 

[diesel4me]

I had one TH350 in my '72 K5 that refused to go into forward gears for as long as it took the engine to get to normal operating temps every morning,or after it sat over 8 hours or so..once it went into gear,it shifted normally,didn't slip,and I could drive it 500 miles with no issues..next morning,the same BS again..

I tried every additive known to mankind in it,with no improvement,along with filter and fluid changes..

I drove it that way for almost 3 years,until I almost got fired for it making me late for work too often..I ended up converting it to a SM465 manual transmission..

So yeah,they can go for a long time "sometimes" ..a neighbor of mine had a TH2004R in a Buick that took as long as 15 minutes or more to finally shift into reverse ,the colder it was,the longer it took...he drove it that way for a couple years ,till he could afford a new car..

I think mine had leaky lip seals on the forward clutch,at least thats what more than one transmission shop told me..it may have lasted years that way,but I was tired of worrying about it not going into forward gears at all far from home someday..

 

[76k5blazerr]

You know anything about sea foam transmission treatment? It looks like pretty good stuff. I think we are gonna give it a try, put some in, drive a couple hundred miles and then change the fluid and filter and see if that helps. Now, what do you think about the speedometer issue? Need a new one?

 

[diesel4me]

I've had a few speedometer's start freaking out like that,I just went to the junkyard and bought another cluster..today that might not be easy,seeing there isn't many trucks that old left to pick on any more..

I tried taking a few speedometers apart,they have a "cup" driven by the cable ,that surrounds the needle,and I guess its magnetic and somehow moves the needle...

I tried messing with it and it only got worse-said screw this and bought another dash cluster..by doing that I not only fixed the speedometer problem,I also made my truck "lose" about 50,000 miles!..

 

[Fordum]

Actually, on most of them, there is a flexible magnet wrapped around a spinning cylinder inside an aluminum cup.
Even though the aluminum is nonferrous and is not actually attracted to the magnet, it is a conductor and the spinning magnetic field sets up eddy currents in it that cause it to be dragged in the same direction the magnet is spinning.

The cup is attached to the needle and its movement is held towards zero by a weak spring.
The faster the magnet spins, the stronger the current, and the more the spring is wound up.
If the needle just jumps a lot, its often the cable needing lube. Since its flexible, as it binds in the sleeve, it slows and then jumps ahead causing the needle to jump back and forth.
If the needle is reading really high, then the spring has come loose, or the magnet is slightly loose and touching the sleeve.
I have seen dirt or lint between the spinning cylinder and the sleeve causing friction. The magnet has to be fairly close to the cylinder for it to work. Not a lot of clearance.

If its the cable, I have had lots of luck with pulling the cable out of the sleeve, cleaning it with solvent, then coating it with a really light lube or graphite and putting it back in.

Not as much luck with fixing the speedo its self. The guts are fairly delicate, and if the magnet is coming loose its usually going to self destruct sooner or later.

Worth a try though.