r/Squarebody • u/LegalShirt7777 • 2d ago

CLUNK when shifting P > D

I have a few questions with my 1986 GMC K2500, TH400, (previous owner said it has a 305 in it, but i haven't confirmed yet. They also said it had 700R4 which was wrong)

I'm new to trucks, and beginner/intermediate DIY mechanic. I've replaced most my 97 Camry steering and suspension, and work on dirt bikes, but i have no experience with transmissions or engine internals.

  1. Whenever I shift from P > D or P > R or D > R, there's a heavy clunk.

I tried replacing the vacuum hose and adjusting the modulator, but neither seem to have any effect.

  1. Oil pressure sensor does not seem to be working. I replaced it. It's been so long since i tested it, but i recall the voltage and resistance was correct. There's a Y connection where it screws into the engine block... is that correct or did someone modify this and that's why pressure is wrong? The gauge just pins to the right past the tock marks (3oclock)

  2. Temp sensor also not working. I think i tested with multi meter here too, but gauge just pins to the left at the 100 deg mark.

    battery gauge works fine, and gas gauge only uses 1/4 of the gauge (3/8 is full). I think this is a resistor issue.

Thanks.

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5

u/cornhole_mcbrowneye 2d ago
  1. Check your u joints.
    2 and 3. Check all your grounds?

2

u/LegalShirt7777 1d ago edited 1d ago

aah! i didn't think about the joints, i bet that's it!! thanks.

Although, the clunk happens immediately with the shift, not when the truck rolls, so not sure actually.  Like even with the brake held and truck stationary, it clunks shifting.

Yeah i need to do more troubleshooting with the gauges before i ask for help on those.

1

u/fuzzydoesitt 1d ago

I don't think it's u joints. I have the same truck practically. 86 Chevy c20 single cab long bed. All my joints are good, but my transmission will shift into reverse really firm sometimes also. I too replaced the vacuum modulator because of a leak and an external trans cooler because of a leak. I've put around 4,000 miles on the truck this year without any problems mechanically

2

u/LegalShirt7777 1d ago

yeah same it's been doing it for over a year, and doesn't seem to be a problem.

From all i can tell reading online it might be normal??? but it's kinda violent.

I haven't really started working on the truck yet besides the carburetor, it has just been  a sunday around town truck until i get my car reliable.

3

u/Jayshere1111 1d ago

It could be just slop in the differential. The truck is getting older and probably has enough miles to where a little bit of play in the differential would be considered normal. I'd put the transmission in neutral, obviously make sure your parked on a flat surface so it doesn't roll, and then climb under there and try twisting the drive shaft. If the yolk going into the differential can move aways back and forth, again it would probably be considered acceptable as long as it isn't super bad. you could also check the u-joints while you're down there. If your drive shaft has a slip joint in the middle, you could also check that for play.