I'm gonna end up with a knot on my forehead from banging my head against the wall. The backup lights won't work. 98 Blazer with auto transmission. I changed out the park/neutral switch but they still don't work. It keeps blowing the fuse as soon as I try to start it. I'm thinking it's shorted/grounded someplace. Is there any common problem area in the system or does anyone have any ideas?

OK, does anyone know where I can get a good schematic of the back up light wiring. It's grounded someplace and I could sure use a diagram to go by when tracing it out.

 

A wiring diagram isn't going to show you wires are run from the front of the truck to the back. Which fuse is blowing? There are 2 fuses used for the vehicle backup lights. Knowing which fuse is blowing will tell you where the problem lies.

Power runs from the underhood fuse block (B/U LP fuse 25A) to the park/neutral position & backup lamp switch back to the underhood fuse block (VEH B/U - 15A & TRL B/U - 10A). From there, the wiring goes back through the main grommet on driver side of the firewall near the brake booster. The VEH B/U fuse is also used by the auto dimming mirrors if your truck has them.

 

Thanks for getting back to me. The B/U LP fuse 25A is the one that keeps blowing. The other two are fine. When I put a new 25A fuse in the B/U LP spot it's ok until I getr ready to start the Blazer. Once the key gets to the on position the fuse lights up. I took my meter and with the key off I read no voltage in either side of the B/U LP. But, Looking down at the fuse block from the drivers side the right side of the socket will show 12 volts with the key on. The left side of the socket is zero. Just for the heck of it I checked for continuity on the left side against a ground and sure enough I got a reading/tone just like I was putting both leads together. I put a new park/neutral position & backup lamp switch in yesterday. It seems to be in correctly, It only starts in park or neutral. All the other lights including the daytime running light setup seem to be working fine.

 

It really sounds like there is a short somewhere between the fuse box and the park/neutral switch. When the transmission is in park, there shouldn't be any continuity to ground on the fuse output to the switch.

 

A huge thanks for the help! Last night I got back under the truck and pulled the plugs out of the switch. I checked the continuity on the output side of the fuse to the switch and the ground was gone. When I plugged the connections back in to the switch, the ground was back. I was thinking it was between the switch and where it comes back to the fuse box but I thought it would blow the other fuse also. I feel like I'm getting closer to the problem spot but it doesn't feel like it adds up just yet.

 

The other fuses only deal with the wiring from the fuse box out to the lights/trailer harness so the problem is likely either in the switch (doubtful because you replaced it already, but worth checking) or in the return wiring from the switch to the VEH B/U & TRL B/U fuses in the fuse box.

 

Still no backup lights! I replaced the switch again, Replaced the pink wite from the UBEC to the switch. Checked the grey wire that goes from the switch to the UBEC and they seem fine. I turn the key to on and the 25 amp fuse blows. I unplugged the 6 wire plug from the switch turned the key to on and the fuse was fine. I plugged it back in and the fuse blew again. I have been trying to check out the other wires in the plug but can't find much info on them. I know about the pink and grey wires but there is a green wire in B, what looks like a black and white wire in D and the E and G look like they have purple with white. Can anyone give me any more info on the other wires or maybe even point me towards a schematic that shows them?

 

Those other wires don't have anything to do with the fuse you are having problems with. I would replace the grey wire as well to rule out a short in that wire.

Does the fuse still blow if both the VEH B/U & TRL B/U fuses are pulled? If so, then it definitely has to be either a wiring problem, or an internal short in the fuse box.

 

The fuse will still blow with the other fuses out. I know the modules come out of the Fuse box that is under the hood but does the top of the box separate from it so I can see inside? I have a feeling the box under the hood can be expensive to replace.

 

I have never tried to open one up before, but I imagine that it is sealed up pretty well. The harnesses do unbolt from under it though so you could give it a try.

The box itself is pretty expensive if you are going to the dealer for it. You can pick up a used one from the junkyard, but you have to match the box. There were two levels, base & uplevel. The base box didn't have all of the contacts in it that were necessary if you had the uplevel box.