Gauge Issues

[blakesocia]

So I'm having some problems with my gauges in my 77. When I'm driving, the speedometer needle bounces back and forth by 10 mphs. So its hard to get a good read on my actual speed. Also my temp gauge goes haywire when the truck hits an bumps. The needle starts off fine, but then basically just shoots off way right, past the red line to about 5 o'clock. These have been doing this since I bought the truck. Any suggestions? Is this something I can fix, or do I just need new gauges?

[chengny]

The speedometer issue is mechanical - the cable is getting bound up within it's sheathing. As it's rotational speed is reduced by the interference, the indicated speed slows down. Then, eventually, the cable breaks free and the speedometer needle jumps up. Release the cable housing at both ends, flush from top to bottom with some carb cleaner and then lubricate with graphite/WD-40.

A coolant temperature gauge that pegs high is generally caused by a short to ground in the sensing leg. There should be a dark green wire (the sensing leg) that runs along the LH side valve cover and terminates at the coolant temp sensor (located between 1 & 3 spark plugs). Check it for worn insulation that could cause the copper to contact the engine. Start at the sensor and work back to where it enters the loom.

[LBJ]

blakesocia, I had the exact same thing wrong with my spedo from day one, brand new truck. Turns out, the back of the spedo had a dent in it which interfered with the mechanics of the needle/pointer. It also had me going at 45 when I was doing 55. Should have had it looked at in '77 after I bought the new truck. I replaced it in 2007 with a GM authorized Chevy replacement from LMC truck. You can try chengy's suggestion also by pulling the cable housing out of the tranny. When you do this, a fair amount of tranny fluid may dump out, but it won't be the whole 4 qts. That seal leaks a bit and collects in the housing.

[blakesocia]

Thanks for the help guys! I actually just bought a "new-to-me" cluster and housing from Dave, so hopefully that will fix my speedometer problem. (Assuming this new one doesn't have the same issue.) I'll still check out the old speedo and see if I can get it working.
As far as the temp sensor goes, I found the sensor on the side of the valve cover. It looks like the copper tip on the end of the wire is starting to come out of the fitting. Should I be able to push the end in and out of the fitting like that?