i have recently purchased a 2000 gmc sonoma 4x4 with 58000 miles. occasionally i will go to start the truck and nothing happens. all of the lights come on and everything powers up but the truck wont turn over. ive noticed that if i go and check the fuses under the hood and put them back it will sometimes start right up. i have checked the battery, changed the starter, checked the fuse box under the hood, neutral safety switch seems to be brand new. i am completely lost. i have heard that it could be the ignition switch but want to be sure before i have to replace that. it seems to not start when it is cold outside. another problem i have with it is when the truck is off and the key is forward just before you start the truck is when you go from drive to third gear the truck will try to start. even when you are driving and you go from drive to third the starter will try to engage like you are trying to start the truck. i dont know if the two problems are related or not. any information would be greatly appreciated as i am ready to pull my hair out!!!
brien.clinton ... can you read schmatics? I've attached one with the starting circuit which could help you out. It sounds like two problems but if you concentrate on the first one perhaps the second will be easier (sounds like chafed wire or perhaps a problem with start / neutral switch).
I suggest you become familar with the starting circuit by starting in the upper left corner of diagram. Note that +12 is routed through the 20 amp IGNITION C fuse to the ignition switch. When switch is placed to START the +12 is passed through the 10 amp CRANK fuse to the PARK/NEUTRAL switch, to the STARTER relay.
When STARTER relay latches, another source of +12 is routed through the 40 amp IGNITION A fuse, through the latched STARTER relay, to the starter solenoid. If the power and ground wiring is good to the start solenoid the large cable carrying +12 from battery will be connected to the starter and it should spin the engine.
Here are a few things I'd try if the truck was in my driveway:
1. Use a meter or test light and with CRANK fuse removed check both sockets of CRANK fuse for +12 when the key is placed to START. Only one socket will be hot - but one of them must be HOT each time you try to START.
2. If step 1 is OK - install CRANK fuse and with key ON, place gear lever to REVERSE and make sure the backup lights are ON. If not, check the fuse, and if fuse is good, suspect the PARK / NEUTRAL / REVERSE switch is wired wrong or not properly indexed.
3. If step 2 OK ... try to start until it fails and then try moving the shift lever while trying to start, also try starting in NEUTRAL.
4. If step 3 makes no difference, listen to START relay for clicks, or remove the relay, and use meter or test light to probe socket mating with relay - pin 85. This pin should be HOT each time key is moved to START. You could also move the selector between DRIVE and 3 and check for +12 at pin 85 - you should NOT have voltage unless the key is turned to START.
5. If +12 is good to this point and with STARTER RELAY removed - "borrow" another identical relay (not related to start or fuel circuit), or use a piece of wire to jumper sockets mating with STARTER relay pins 30 and 87. Be prepared for minor sparking and for the starter to crank. If the starter turns with a different relay, or each time pins 30 and 87 are jumped, the relay is bad.
6. If starter does not crank I'd suspect solenoid, starter or power feed from battery.
I'm guessing the problem is in or near the PARK / NEUTRAL switch. The fact your truck occasionally spins the starter when you move the gear selector between DRIVE and 3 indicates unwanted +12 is being "fed" to the STARTER RELAY.