My GMC P3500 will not start. There is no spark, either. What do I need to check?
First, check for battery voltage as the recharge may be low. You could also try jump starting it with another vehicle or a battery charger. If the recharge is indeed low or if the battery isn't revived from the jump start, then you will need to replace it. It is also possible that that the battery cables are loose or corroded. Simply clean them and tighten both ends of both battery cables before trying to start your GMC P3500 again. However, if the battery doesn't start up or if fixing the cables doesn't help, then the starter relay wiring connections or ground connection may have gone bad. To confirm this, you need to check for voltage at the relay. If the relay has voltage but there is no clicking sound when you turn the key to start, then this component must be replaced.
The gears of my GMC P3500 won't change at all. What seems to be the problem here, and how do I fix it?
The problem lies in the transmission itself, and you have to check it step by step. First, inspect your transmission fluid. Remember, you should be checking this at least twice a year, but even though you are on schedule with this maintenance task doesn't mean you'll skip this part. Check the transmission fluid if there's enough left, and top it off just to make sure. However, if you think that your van is losing more fluid than usual, then it's possible that you have a leak, and you will have to look for the source. The most common sources of leaks are at the filler tube base, the drain hole underneath the transmission, and between the transmission and the engine. Once you find the source, you can either seal the hole or crack or replace the damaged component. If there is no leak, check the transmission filter if it is already clogged. This wear item is usually replaced every 30,000 miles, but if you haven't done so on your GMC P3500 in a while—or ever—then you need to inspect it now, see how dirty it has gotten for being neglected, and replace the dirty filter.
I've been having this annoying problem with my GMC P3500 wherein I sometimes go 60 mph uphill without a problem, but there also occasions when I have trouble driving past 45 mph. How can I find out what's causing this?
Unfortunately, intermittent problems like this are hard to diagnose. The most often-used approach is to wait it out the next time your GMC P3500 has a hard time speeding up again. However, if you cannot wait anymore, then you can go ahead and check the components that usually cause engine hesitation. First, your van's engine might be sucking too much air, not getting enough fuel, or misfiring. However, this is usually accompanied by a Check Engine light, so if there is none, check the throttle position sensor to see if it's faulty.