Propelling drive control apparatus for working vehicle

A propelling drive control apparatus for a working vehicle comprising:

• a pump for supplying hydraulic oil to said apparatus,
• hydraulically operable friction clutch means for providing a propelling drive transmission state,
• first clutch control means for operating said friction clutch means,
• second clutch control means connected in series to said first clutch control means and having three control positions (off, on1 and on2), wherein one of said control positions is a declutching position (off) which is established for stopping a hydraulic oil supply to said friction clutch means and for releasing hydraulic pressure oil from said friction clutch means, a second one of said control positions is a first clutch engaging position (on1) which is for supplying pressure oil supplied to said second clutch control means directly to said friction clutch means, and the third one of said control positions is a second clutch engaging position (on2) which is for stopping the hydraulic oil supply from said second clutch control means to said friction clutch means, means for releasing the hydraulic pressure oil from inside said friction clutch means in an amount corresponding to an amount of operation of a control device, and thereafter stopping release of the hydraulic pressure oil to maintain an oil pressure in said friction clutch means, and
• an oil pressure adjusting mechanism including a variable relief valve and an adjusting means mounted in an oil feed passage from said pump for the first and second clutch control means,
• said oil pressure adjusting mechanism being connected to the oil feed passage with the first clutch control means being in an operative position whereby relief pressure of the relief valve increases with increasing oil pressure supplied to the oil pressure adjusting mechanism,
• said oil pressure adjusting mechanism being connected to a return tank with the first clutch control means being in a neutral position whereby the relief pressure of the relief valve decreases to a lowest level, wherein
• said second clutch control means is disposed adjacent said friction clutch means and is supplied with hydraulic pressure oil from said first clutch control means,
• and said oil pressure adjusting mechanism cancels the hydraulic oil pressure for engaging said friction clutch means in response to a declutching operation of said first clutch control means and returns to an initial state.