Clutch Mystery
 

Buckle up and put on your thinking caps. This is tooooo weird!

My truck is a 1968 GMC cab and vin on a 1972 3/4 ton FWD frame, stick shift.

The motor was recently replaced with a 1999 350 Vortex by a very competent guy who has probably done 1000 R&R's over the years.

Starting Sequence:

1. Put the tranny in neutral
2. Turn the key, engine starts and runs smoothly.
3. Depress the clutch Pedal.
4. Engine dies.

There is no neutral safety switch on this vehicle. I have looked for vulnerable loose wires that could be pinched by the pedal and linkage and have found none. The engine does not seem to be "loading" it just dies.

Other symptoms:

If I put rearward pressure on the stick and depress the clutch pedal half way, the tranny slides into gear. If I then continue to depress the clutch pedal fully, the engine dies. In other words, depressing the pedal ALL the way triggers the engine to die.

Comment:

This feels and sounds like an electrical issue but as I said, I've searched for loose wires and found none.

Any ideas for me?

ccstelmo

Re: Clutch Mystery
 

New engine and the old transmission?

I think there is a bellhousing, clutch or clutch fork misalignment problem. Which unfortunately means the transmission is going to have to come out in order to identify what is binding up.

Did you drive the truck home from the shop?

Re: Clutch Mystery
 

HO455:

I forgot to mention that.
I drove the truck home 115 miles. I've had it on and off the dirt roads around here, in and out of 4WD, for a few weeks without having this problem. It started suddenly after I had been using the electric winch to pull a couple of stumps.

ccstelmo

Re: Clutch Mystery
 

Disconnect the wiring to the winch and see if the problem goes away.

Re: Clutch Mystery
 

I will try that. I'll be out of town for several days and will get back to you when I do.
ccstelmo

Re: Clutch Mystery
 

The winch shouldn't really make a difference on the engine running unless some real strange wiring was done. I would check for a wire that grounds out when the pedal is depressed. Its pretty easy to get one that gets chafed.

Re: Clutch Mystery
 

Are the ground straps, from the firewall to the valve covers, still attached?
I would run a wire from the + side of the coil to a bulb and install it where I could observe the bulb from the drives seat. Ground the bulb to a ground on the firewall and see what happens to the circuit when the clutch is pushed in. Then do the same thing with the ground on the motor. Seems the grounds should be fine because the starter works but need to start somewhere. One has to assume your have compression and fuel. That only leave electrical. I would also pull the engine harness plug from the fire wall and confirm it's copper terminals are in pristine condition. Yup that's a good one!
Cheers

Re: Clutch Mystery
 

90% of electrical problems are grounding issues.

Re: Clutch Mystery
 

Well that does change things. As others have stated check the ground wires all the way from the battery's negative post to the engine block and to the frame. And for good measure check the ground wires for the winch and its controls.

I would also inspect the motor mounts, one may have failed.

Oh and Welcome aboard! :metal:

Re: Clutch Mystery
 

Did you keep all of the fuel injection or put a carb manifold on the vortec?