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Brake light fuse blows out

After helping others here in the past I now find that I need help from the good folks here. Hopefully someone here has the answer to my problem.

My '02 Chrysler T&C Limited AWD van with towing package keeps blowing out brake light fuse 26 (20A) only after leaving the garage. The fuse supplies power to the brake light switch and that passes power on to other things besides the brake lights. The testing I have done so far hasn’t pointed me to anything specific yet because when I replace the fuse, things work fine in the garage. The load resistance at the fuse position is about 75 ohms when the pedal is pushed down so that is no problem. The fuse only blows out when I get out on the road. The wiring at the lights has no obvious problem but haven’t got to the wiring between the switch and the rear end yet.

I found a post on YouTube that seemed to be the answer to my issue but it didn’t help. The poster had shown how to replace the brake light switch and that apparently fixed his fuse issue but it didn’t work for me. The switch is very unusual and it can’t be reinstalled once it has been removed from the base. There is a alignment mechanism inside the switch that gets set once the switch is installed.

It seems there should be a plugin for the trailer tow harness but so far I haven’t seen it. I know that is a likely area for trouble like this. I have the factory manuals for the van and will keep looking it over for possible trouble areas. If anyone here has any good ideas please let me know.

Are any of the brake lights (especially the third brake light) on the tailgate itself? If so, I’d start by looking for damaged wiring (due to flexing) where the wiring bundle goes from the body into the tailgate.

Good thought @lion9car, I have already checked that out as there are two wiring channels going into the lift gate. I moved the ducts all around while the brakes were on and the van inside the garage but there was no issue. At that time I was watching the resistance at the fuse position and saw no change in the 75 ohm resistance. Of course the hatch was open then so I suppose the issue could be happening while the hatch is closed and there is more tension on the wiring. I’ll see if I can pull the ducting back to inspect the wires.

I also just checked to see if the fuse would blow out with the ignition ON and moving the shift lever. The lights had no problem that way either.

This is a common problem with Chrysler vehicles.


Thanks for the info @Tester. That is a pretty interesting incident report and the kind of info I am looking for which may be helpful here. After reading the report I have to wonder what happened since the shop couldn’t find any real problem. In their case the power to the lights was found to be okay. The power for the brake lights passes through the brake switch and then on to the lights so there is no module control of the lights themselves. Now the manual does show splice S304 makes connection to the lights after the switch and to a couple of modules in parallel with the lights. If the splice had a bad connection I could very easily see that as a cause for their issue. I have since found out the location of the splice location, near the rear hatch in the middle of the floor. I will try to get access to it and see if perhaps there is a problem there. Please keep the ideas coming, I need to get this fixed as fast as I can so I can drive it safely. The help is really appreciated, thanks.

75 ohms with the brakes applied seems lower than I"d expect in a brake light circuit. That’s only 2 watts. Power = volts ^ 2 / R. 144/75 = 2 watts. Seems like it would make for pretty dim brake lights, but maybe you have some super efficient LEDs or something. Do the brake lights actually turn on when you do the same thing as you do for that test? That 75 ohm thing wouldn’t explain the blown fuse problem, but might mean there’s something about the way you are doing that test that is giving you a false reading for some reason. Which could be masking what’s causing the blown fuse problem. Maybe give that test a re-do.

My first guess btw is the cause is exactly as you suspect, since you have a tow package, the trailer light connector is shorting out. I’ve had that happen on my truck after I installed a trailer light connector for lighting my utility trailer. After that my next guess is something is shorting out the socket that one of the brake lights plugs into. Best of luck.

Resistance readings do not tell you the wattage. For incandescent bulbs, the resistance goes way up when they have the proper 14 volts across them, so the cold resistance readings would be very low. For example, a 20 watt bulb would measure (20 = 14²/R) 10 Ω hot, but perhaps 1 Ω cold.

If they are LEDs, the low voltage resistance readings are meaningless, as the diodes are conducting only minimally.

Thanks for the replies guys.

The resistance test I did was from the protected side of the fuse connection (no fuse in place of course) and ground. Then step on the brake pedal. The bulbs are just standard bulbs. The resistance does seem a little higher than it should be to me but then we are looking for a short, not a high resistance. When I replace the fuse in the garage and step on the brakes the lights are good, no problem. It is very strange.

I know what you say is true about the lamp resistance. I can’t really explain the higher resistance reading I have seen so far.

Besides tying to the lights, the brake switch also ties to the antilock brake controller and to a front control module. It seems that if there was an issue with one of those modules the trouble would always be there but perhaps not, depending how it is designed internally, I have no clue.

I found splice S304 near the rear and in the middle of the floor area. The wires then split to both sides of the rear area. I found no damage or signs of trouble there. I also looked the wires over in the channels to the hatch and found nothing there. After we did those checks we checked the resistance of the circuit again and now the reading is about 50 ohms. It seems we did something to the circuit by doing these checks but I don’t know what caused the resistance to change…

Tomorrow I may replace the fuse with a headlight bulb to see if I can at least see when the trouble happens while on the road.

Another story but something to check anyway. Back about 1978 I had a 67 Buick wagon. Coming back from the library one night, I got stopped because my tail lights were out. I had no idea. So replaced the fuse and they worked. Problem was they would work as long as you didn’t drive the car much. Once the car was driven a few miles, the fuse would blow. After some frustration, I crawled under the car and started to trace the wires and found a bundle of wires that had dropped down on the exhaust pipe. As soon as the pipe warmed up, it would melt the insulation enough to short the wiring and blow the fuse. When it cooled off, it was ok again.

Thanks for the post Bing. I will look things over, especially in the engine compartment area. I did look under the rear area of the van and didn’t see any wiring. I was looking for wiring to the trailer harness.

Have you tried brake lights with turn signals activated?

I haven’t looked into the turn signals or emergency flasher circuits as of yet. I think both of them are fed from different fuse circuits but I will look those things over in the manual and try them out. Thanks for the idea @OldcarsRbest.

Had this happen with my Nissan PU, but it turned out to be in the trailer itself, only blew the fuse when the trailer was attached. I would look at the pigtail for the trailer plug very closely. Cut open the shield because there is a ground wire in there with the various hot wires.

I have looked for the trailer wire harness plug @keith but so far I haven’t found anything yet. From what little info the manual shows about it the plug seems to behind the left rear trim for the side of the van.

Update on latest testing:
One of the power connections after the brake switch ties to what is called the Front Control Module. The manual shows that it is located in the same power panel the fuse is in in the engine compartment. The panel is designed to flip on its’ side to expose the wire connections to the panel on the under side. This is a pretty neat design I think. Looking things over on the under side of the panel I didn’t see any signs of trouble but the wire harness does go into the left front wheel well area so that may be where the trouble is at. I would need to remove the plastic wheel well cover in order to get to the harness. In order to flip the power module over you have to remove the battery. Out of curiosity I decided to recheck the load resistance again. With the battery disconnected now the resistance to the lights was under 1 ohm instead of 75 ohms. I didn’t think it would be a problem leaving the battery connected and checking the resistance while the fuse for the brakes was removed. But I didn’t think about the connections to the two other modules that are tied to the brake circuit. My suspicion is there was some current coming from one or both of the modules and that caused a false reading on my meter.

On my first reading without the battery the resistance seemed to be a dead short. The resistance only about .8 ohms to the negative battery connector. That later changed to a little less than 2 ohms after moving the wiring around some under the power panel. After putting things back together and replacing the fuse the lights are working just like they should be, but I will see what happens when I get back on the road again.

that should clue you in to where the short is…

No idea if this would help in your case, but there’s a relatively inexpensive gadget you can buy — I’ve never used one but they have it at HF on the shelves there I noticed – to help find where a short is occurring in a complicated wiring harness. Not entirely sure how it works, but it somehow allows you to follow where the current is going, so if it goes off-track you can spot where that happens in the harness. You could imagine doing this by hooking a small battery up the isolated circuit and using a magnetic compass as a sort of magic wand to sense the current’s magnetic field, to determine where the current goes in the harness. I’ve seen technicians in electronics companies use a similar gadget to find shorts in wiring harnesses on the production line.

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As far as the resistance of the brake light circuit goes, I was surprised to see the 75 ohm reading I first got when I made the test. I think there are 2 bulbs on each side of the van plus the middle brake light which makes a fairly good load for the brake circuit. I was thinking that each light was about 25 watts going from past memory and checking the specs for a 3157 bulb show the brake light is 26 watts. So if there are 4 lights total that’s over 100 watts, not including the middle light. That is going to be a pretty low total resistance (most all us here know Ohm’s Law very well). When I saw the reading slightly above 1 ohm I wasn’t too concerned since that is more in line what I thought the reading should be in the first place. Further tests showed that the original reading of 75 ohms was caused by the battery being connected to the car. The other modules the brake light circuit is tied to had to be supplying some power back to the line and influencing the resistance reading as power to those modules was not disconnected. I’m sure if I isolated the brake line connections to the modules my reading would have been correct then.

When I first made a reading of the load I got about .8 ohms and that was a little concerning to me. Doing a lead resistance check it showed .5 ohms. After moving the harnesses around looking for a problem the resistance went a little higher, between 1 and 2 ohms. When things were back together and the fuse reinstalled, once again, the lights worked. I even moved the van out of the garage and parked it in the street so I could get to the snow blower and clear my driveway. When I put the van back in the garage again the lights were still working. I will do another road test today and see what happens. After seeing that the harness of concern goes into the wheel well of the chassis I think that is the next best place to look if the trouble still occurs, unless someone here can think of something else. Once again, thanks for all the help and ideas on this.

I know the tester you are talking about. In fact, thanks very much for mentioning that as I have one those nice tester kits that I had purchased in the past just for a situation like this. I had forgotten about that kit. With all things going on right now in my life (like moving) I had forgotten about it. I believe the probe works by sensing the current induction of the wire. Thanks George.

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Well after another real road test the fuse blew again. It looks like I am going to have to dig into the left front wheel well area and check for the trouble there.

I feel for you. A while back I had a wiring harness problem on my Corolla, the alternator wire to the battery became intermittent. I had to trace it out by cutting into a big bundle of wires in the middle of the harness, until I found the problem. The battery apparently was leaking battery acid a little at a time, and the acid day by day was decanting down the middle of the wiring harness, b/c that wire’s insulation was torching the top surface of the battery, providing an initiation point for the decanting into the harness. It probably took years for the acid to reach a point where it could do any damage, but eventually it came to a splice, and promptly ate. it. And of course that splice was in the most inaccessible place in the entire wiring harness … lol…

It sure seems if there is a weak point in the wiring somewhere, something will find a way to get at it and cause a problem.

So far in my case it seems that the problem isn’t in the rear area. I tried isolating the rear lights at the splice in the rear area one at a time but the trouble is still occurring. I am going to try and look at the harness where it goes into the wheel well area and look things over there. If I don’t find the problem there then I plan to bypass the brake circuit wiring going to the other modules so I can at least get the lights working. Then work on finding the problem later on at a better time for me to dig into it.

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