Peterbuilt 389, turning on the "fog" light on hi beam?

You're running a truck with an SAE J1939 multiplex system. Try adding wires and jumping it off, and you're gonna start getting a bunch of annoying error messages, not to mention voiding the warranty. Unlike older trucks, everything goes to a chassis node, then the programming assigns a value for each input and for each output. The program they use for this is Electronic Service Analyst (ESA)... a great program, but only in the hands of someone who knows what they're doing, as it's also a very easy way to blank box a CECU by those who don't. They can set it up for you at a dealership, if the law doesn't prohibit them from doing so.

 

I never heard of that. Do you have a reference to that regulation?

 

There is no parameter to allow fog lights with high beams, a dealer cannot help. The only way to make it work is to take the chassis node out of the fog light circuit. You need to disconnect the signal from the switch to the CECU and disconnect the power from the chassis node to the fog lights. It will not set a fault if you disconnect both because it will never get a signal to turn them on and wont try to power them up, it will just thing that they are off. Once you take the control off the chassis node, you can wire the fog lights to the switch through a relay hooked to a spare fuse slot.

 

Fastest way is to unplug factory harness off switch. Then add a spare switch harness, plug it into either keyed or constant. Add fuse to correct spot in fusebox and run 2 wire cord to the bumper. No hacking of factory wiring needed

 

I would not do that without a relay. Fog light bulbs average around 55 watts, so a par of them will draw almost 10 amps. The factory spare switches don't last long over 5 amps, they are designed to activate relays or LED add on lights, not run large loads.

No butchering is needed. You just have to remove the wire from the factor plug, insulate and secure it out of the way. Use that wire to activate a relay. Unplug the wire to the fog lights and install weather pack caps to keep them from corroding up, then wire in new wires to the fog light.

You can use a spare switch, but removing the wire to the factory switch will let you use the fog light switch to turn on the fog lights.

If you want to get fancy, you can use a diode and keep everything working as factory and use a spare switch to override them with the high beams on.

 

I would use the factory switch with a spare switch harness. No need for relays or anything. We have run up to 20amps on the switches with no issues.

 

You are definitely a minority there. I've seen several dozen switches melt and burn up at as low as a 10 amp draw. Bet if you check the backside of the switch its started melting.

 

Older Pete switches had a 20 amp rating, the newer ones when they went to the Namux system have a 10 amp rating. I know this because I got with Pete engineers when we added back up electric hydraulic pumps. The solenoids require 15 amps to activate and 8 amps to hold. Peterbilt deferred my question to Pollak (the ones who make the switches) and they said that the switches are rated for 10 amps continuous, 20 amp peak, but always recommend a switch capable of 25% over the rating of the fuse. So a solenoid that calls for a 20 amp fuse needs a higher capacity switch or a relay. Get a partial short and the switch could melt. Get a bad connection and the extra heat will melt the plug. It's not worth risking melted wires and a possible dash fire to save less than $10 and a little time. I've seen more than my share of melted plugs on switches.

You can do whatever you want on your own truck, I'm just posting the facts so people can make informed decisions.