Electrical problelm solved

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Jun 10, 2004
first i must thank all who responded to my post and especially Dean--review
i have a lx470 2004 with the trailer package, i installed an aftermarket hitch to pull a travel trailer, when i in went in the trailer and stopped the lights remained on and the rear wipers would move slowly, and the door locks failed to work with the key.

i took it to many places on my travel but none had a definitve answer --please see the original post, to solve the problem i just unpluged the harness from the trailer, and waited to return home, went to lexus they checked the wiring and said all was well, but when i got home the lexus electrician emailed me and asked me to perform a test or i could return and he would do it, simply the test was use a spare battery and connect it to the power terminal that cmes from the harness on the car and see if the problem could be duplicated --it was --meaning the problem was in the car,he pulled the wiring diagram and found the problem, the relay was closing but this was in the design, only an 18gauge wire is used in this power feed, also he told me that if my car battery was low this circuit would allow the starting battery to draw power from the trailer battery and this is a 18 gauge wire which could pose a fire hazard, he installed a new harnes with a 10 gauge wire that goes through a solenoid that is activated by the ignition, --- end of problem-- i hope -- brought it home connected the trailer and all is well. they said they would inform lexus. it seems that some other owners would have noticed, but how many pull a travel trailer?

i do have the towing schematic which the electrician copied for me, again thanks to all ---you were right --i might visit camperworld on my next trip and tell one of the techs there that my problem was not impossible.
I don’t thank a 10gauge wire will start your car, but way better than the 18 gauge. I think a battery isolator is the way I would go so it won’t drain your cars battery.
I’m glad you got this solved. Did they fix this for free?
electrical problem resolved

hi Dean yes they fixed it free in fact they told me that i could take it to any hitch place i wanted and they would pick up the tab for up to five hundred dollers, i thought i should get it done at the dealer just in case of problems, but i was tempted.

I'd go buy a box of cookies, donuts, whatever, and drop it off to the mechanic! That guy spent some time thinking of your problem and came up with a solution. That's someone I would want working on mine. He called you, that's rare.:cheers:
As I'm in the garage right now wiring up my trailer brakes and 12V to the plug, thanks for the info. Back down to Orchard Supply for 10 gauge...:bounce:
sounds like he did exactly what I did, put a relay/solenoid in to disconnect the trailer battery when the key is not in.
The part about the 18 gauge wiring and the current going backwards is noteworthy. The thing that's odd about this is that a relay based on the ignition as I did and as it seems, your guy did, would not resolve the issue of the starter drawing from the trailer battery, since the ignition is on when you start the engine... which is why I'm not entirely happy about my solution. Of course, if you have a 10 gauge that's better. I don't know if I have an 18 ga wire though. I'll better check.
And to think that my Toy dealer was so unhelpful about this....

I think the rationale is that a 10 ga is OK to do trickle charging of a battery since the resistance of the wire would limit the current through.
electrical problem resolved

not being great at automotive electrics i do not know what would be the best solution, i just had it fixed at lexus so that if any problems happened i could return, acording to the electrician, he did consult a local hitch shop that has a good reputation and they worked out the solution together, i do have the schematic of the towing part and if i can put it as an attachment i will, thats if i can figure out how to do it.

I think there should be a battery isolator or diode to prevent drawing current from the trailers battery. But the solenoid will work. (I’m curious if it cuts power off when starting)
The tow vehicle should charge the batteries while driving, but not allow the trailers battery to power the vehicle (Key on or off).
With the situation that was happening with David, if the trailer was plugged in while camping, he could have drained the batteries down to the point that it couldn’t start the car.
I think there’s a safety issue allowing an 18 ga wire to draw current from the trailers battery. There’s probably going to be a recall on this one IMO.

David, did he change the fuse size on the new larger wire? Just curious.
electrical problem resolver

hi Dean he put fuse wire in, ive never seen it before just a piece of wire about 6 inches long, all the things you said, the electrician verified could happen, power going from car to trailer,and power going from trailer to car through an 18gauge piece of wire, a definate design flaw, the japanese dont pull traileres, so he said.

Yes, very good, now you’re safe. The wire he used is also known as a fusible link, pretty common in the automotive industry.
Since you are towing a heavy trailer, and we are on a safety topic, let’s make sure your brake controller is on a self-resetting circuit breaker. Was your 100 pre-wired for a brake controller?
I’m curious to see if they used a self resetting circuit breaker. If it’s just a fuse, then that’s no good. But let’s see what’s being used first.
electrical problem solved

yes the 470 2004 is pre-wired for a brake controller although it came as a suprise to the dealer, i had phoned lexus and after three weeks they called me and told me it was not pre-wired, i told the girl it was as i have just installed the brake controller the harness is up by the brake pedal- i got this info from this forum-dont know about the fuse but i will check- to there credit the dealer, again the electrician looked through the diagrams and found that it was indeed pre-wired- the dealer told me that it was useless asking lexus about towing, they dont have a clue

Users who are viewing this thread

Top Bottom