1999 Chevy Astro Blown TRANS Fuse w Trouble codes shift pro

CERAMICAT

MEMBER
1999 CHEVROLET ASTRO
6 CYL
2WD
AUTOMATIC
208,000 MILES

Had the transmission in our Chevy Astro rebuilt a month ago and it drove great! Two weeks later we go to start it and the lights on dash where trans indicator not viewable and the trans feels like it is in second gear upon throttle instead of first gear. Take it back to the trans repair specialist and they had it two days. Trouble code stored and read by their comp is a 'PO760 Shift Solenoid C'. Called us to pick it up and they said it was working temp and we needed to replace the ignition and they were 75% sure it was that, but could not say positive though. We had NO prob with ignition before. It is tight and key goes in smoothly. No electrical or dash light problemss or any difficulties with ignition or starting van. They say it is causing TRANS fuse to blow. Aprehenious, we say we will wait for $250 repair to replace ignition switch and do some checking. Two weeks go by and no problem but then fuse blows again and the trans acts like it is in second again. Wiring harness in trans was replaced when rebuilt(according to trans repair specialist). Ran our own SCAN tool and came up with PO 740, PO753, PO 758, P1860 stored codes today. We think it is indeed a bad wire to trans or a bad wiring harness and the problem is not in the column or not directly related to the ingnition. We have the GMC pro service manuals and the wiring diagram doesn't show the TRANS fuse connected to the ignition but straight back to the trans harness. Please give this very capable DIYer confirmation or retraction to which direction should be taken. Are we right or is the trans place right? If we are right we would like the trans place to put in new wiring harness or to do thourough inspection of the trans wiring rto find the short.

1 Reply

FACTORYJACK

MECHANIC
4,159 POSTS

I am thinking you are on the right track with it being wiring, you should also be losing the instrument panel cluster as well, it is on the same fuse. The ignition switch could be a source of power loss, but to blow a fuse is unlikely. The trans fuse is supplied its power in run from the ignition switch on the white wire circuit 1390, it is shown as a stand alone circuit for the trans fuse. After the fuse the circuit splits(circuit 1020 pink wire) and goes to the cluster and the transmission. You could have a short in the wiring, or possibly a shorting solenoid. One way to determine which area of the circuit to focus on, would be to install inline fuse holders in each branch of the circuit, using lower rated(7.5, or 5) fuses than the 10 amp trans fuse. When the fault occurs, it would be narrowed down to a branch of the circuit.