C2 -Powerglide/Rear end noise/clunk

[Stephen P.]

My 65 coupe (327-300hp) with powerglide and 3.36 positraction rear makes a noticable clunk when slowing down at downshift. Since I'm a fairly new corvette owner, I had someone with more experience look at the underneath while on a lift. This mechanic didn't identify any problem with looseness in the differential or U joints and indicated the trailing arms were OK. He only identified a problem with the rear spring being loose. The clunk seems to go away after driving the car for 30 minutes or so, which to me wouldn't relate to the spring. I haven't changed the fluid in the differential yet, but have the GM fluid and positraction additive for next weekend. I would welcome any ideas to what the problem might be.
PS: The NCRS forum has been a great learning experience for me, a new corvette owner, over the past couple of months. Thanks.

[Roy Braatz]

Re: C2 -Powerglide/Rear end noise/clunk

Check to see if the vacumn line going to the Modulator came off
or pluged up and if not the Modulator is shot. The Modulator controls your shift points going from first to second and back down.

[Stephen P.]

Re: C2 -Powerglide/Rear end noise/clunk

Roy, I'll check the vacuum line, but would a problem with a malfunctioning modulator go away when the temperature of the fluid increased (since problem is not as noticeable after driving 30 minutes plus)? Also, there does not appear to be any problem with the timing of the upshift or downshift, but rather the abruptness of the downshift or the gearing down transfer through to the rear wheels.

[Roy Braatz]

Re: C2 -Powerglide/Rear end noise/clunk

the heat from driving can some times loosen up the modulator or I just thought that the heat could also make the band ajustment piston to seal better.If not the mod. re=ajust the band, turn it in tight and back off 1 1/2 turns out.It's easy , it's on the right side of the case.But I would replace the Mod. first, it's cheap.