QuestionQUESTION: Thanks for your time. 95 Suburban 350 4wd. It is wired for a trailer with either 4 or 5 prong connector. We just got a new boat trailer, and it uses 5 prongs (my previous trailer used 4 prongs but the new one uses 5 to deactivate the surge brakes - I'm not certain the trailer light hook-up is part of the problem). The truck and trailer lights worked fine together. But about a day after I got home and disconnected the trailer my wife noticed the brake lights weren't working (except for the 'third' brake light at the top!), and neither were the turn signals and 4-ways. I checked the fuses, relays and replaced the flasher cube with a heavy duty type, but no change. A shop said it is the 'multi function switch' in the steering column and wants $634. Does his diagnosis sound right? Thanks again Mike.
ANSWER: It is almost certainly the trailer plug causing the problem. I would get a new harness that plugs directly into the truck's wiring harness. Inspect the old wiring as you remove it to make sure that there are no shorts or broken wires. It is usually a ground problem, but not necessarily. The fact that the high mount stop light still works is your clue that the harness is bad, the harness plugs in downstream of the connection to the HMSL. After you fix everything, try and find a friend with a trailer that you know works, and plug your rig into his trailer to make sure that everything's OK. If it works on his trailer and doesn't on yours, then you'll know there is a problem with your trailer. Again, almost always a bad ground, caused by corrosion.
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QUESTION: Thank you Mike, your help made me some progress. (I neglected to say, if it matters, that when the hazard switch is pushed down the turn signals operate normally.) To test your analysis I unplugged the trailer wiring harness behind the bumper, which should eliminate any short in the trailer wiring, but got no change. Someone said that might indicate a burned fusible link upstream of the harness. Could the trailer harness have a short that produced damage upstream? Thank you so much for your time and thought.
ANSWER: Yes, there could have been a short, and it could have burned out a wire. There could also be a short or bad ground in one of the rear light assemblies. Did you hook up that fifth wire to the backup lights? That might have caused a problem if it shorted the backup light voltage to one of the tail lights. I think you should pull the two tail light assemblies out and check for voltage in the sockets while operating the turn signals and brake lights.
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QUESTION: Your question 'Did you hook up that fifth wire to the backup lights?' may reveal something: before I went to get my new trailer I checked to see if I had a 5-pin trailer light coupler. I did, but noticed the white wire was not connected. Presuming it was a ground, I grounded it to the frame. Is that a problem? Later I thought it might be, disconnected it, and still had the same problems. Could that have burned a wire? Mike, thank you for staying with me on this, it could save me a bundle.
Answer That white wire is usually the ground. However, I wouldn't make that assumption without looking at the directions. Usually a plug-in adapter will have the ground built into it. And, if it's a five-pin adapter, it should have the backup wire connection built in as well. I still think I would probe the sockets with a voltmeter. Something's come undone or burned out. Is this a flat five-wire connector, by the way, or a round one with a sixth hole in the center. They are completely different, the flat one is what surge-brake setups usually use. Go to http://www.hopkinstowingsolutions.com/ they will have all of the info that you could need.
