Backup fuse for trailer brake light.

Hi!

"Occasionally, I get older cars brought to me for service, like Golfs, Passats, and Audi A3s, where the glow plug indicator light is flashing."
It's common practice to first check the brake light and then, possibly, the brake light switch.

Often, the vehicles involved have a trailer hitch, and when you ask the vehicle owners, they often say they have recently used a trailer, and it's often suggested that there might be a connection.

Well, we still have a lot of people around here who are holdovers from the East German era, with their corresponding outdated and dilapidated electrical systems, and I always attributed it to that.

Yesterday, the fuse was the only thing that was defective in the A3.

The fuse is 10A.
When towing a trailer, 4 incandescent bulbs, each 21 watts, illuminate, and a third one does as well. I'll leave the LED brake light for now.
So, we have 84 watts, which at 12 volts, equals 7 amps. Incandescent lamps are cold resistors that draw significantly more current when switched on.

Can we assume that such a system is accurate? Should 15A fuses be used there, or what would be the technically correct solution?

The strength of the fuse depends solely on the cross-sectional area of the conductor. If the new fuse doesn't blow immediately when towing, then everything is fine, and I wouldn't worry. I would not arbitrarily overload the conductor with an additional 5A.

VG
Andreas.

The original VW wiring harness for the Passat 3C Variant (Hella) includes two 15A fuses, each of which protects approximately half of the trailer lighting. In addition, there is a 5A fuse for the power supply to the trailer control unit.

Could you please explain the connection between a flickering filament and a trailer?

"When I received my 3C, the trailer hitch was installed very sloppily. The control unit wasn't screwed to the body, but was instead wrapped in foam and hidden behind the trim." After 4 years, the control unit was damaged from being loose, the incorrectly routed wiring was frayed, and it had a short circuit.

I experienced the following symptoms: The engine doesn't automatically start with the start-stop system, the glow plug indicator flashes occasionally, the parking assist system shows an error, there are unexplained alarms, and a fuse has blown. Due to the heating element, the AGR valve and pressure sensor were replaced, and wiring harnesses were damaged at the wrong end of the car.

After identifying the glow plug relay as the culprit using VCDS and replacing it, all the problems disappeared, and the glow plug indicator no longer blinked, even after a visit to the dealer.

cj77 wrote:

Could you please explain the connection between a flickering filament and a trailer?

This was common practice for vehicles from the PQ34 platform or vehicles of that era. Since they had lamp monitoring in only 2% of cases, at the very least, the brake light, which is extremely important, was monitored by the MSTG, and a complete failure was indicated by a flashing filament (=serious error).

Steffen G wrote:

So, we have 84 watts, which at 12 volts, is 7 amperes. Incandescent lamps are cold resistors that draw significantly more current when switched on. Can we assume that such a system is accurate? Should you use 15A fuses there, or what would be the technically correct way to do it?

It all fits together like that. Keep in mind that the fuse is also a cold conductor and has a very slow response characteristic. The current can safely exceed 10A for up to 1 second, until the brake lights are fully illuminated.

A current that is ten times the normal level can flow for approximately 1 second before the fuse trips. The current was doubled, and this lasted for approximately 100 seconds.
While this is just an example, the values are likely to be roughly in this range.

Your problem is likely not caused by overload due to too many light bulbs, but rather by a short circuit in the trailer.

Greetings.
Guste.

Hello!

Thank you for your explanation.

guste100 wrote:

Your problem is likely not caused by overload due to too many light bulbs, but rather by a short circuit in the trailer.

So, I naturally told the vehicle owner that as well.
However, I've said that to others before. It's happening more and more frequently, which is why I'm asking.
And the idea that all those fans are experiencing a short circuit, especially in the brake lights, seems rather doubtful to me.

Are there perhaps any diagrams or statistics showing how many times a fuse can withstand such "stress tests"?
It's like with heating elements; the weakest point will be the most stressed, overheat, possibly develop cracks, and then it will fail. Someday.

Is it certain that this is even related to the fans?

For the brake light and vehicle generation of the Golf 4 and Passat 3B(G), I immediately think of the cable routing to the rear hatch. And those who drive with a trailer, my own experience tells me, are often found near the trunk area.

I also believe that the observations represent an inexplicable accumulation of potentially different sources of error. However, in the case of a trailer, I find it unlikely that a cable would break if it is properly routed. I could imagine that corrosion or a short circuit caused by water ingress might be the issue with the rear lights of the vehicle.

Quote:

However, this is not the first time I have said that to someone. It's happening more and more frequently, which is why I'm asking. And that all those supporters supposedly have a short circuit in their brake lights seems rather doubtful to me.

I've experienced this before; the plastic interior of the taillights deteriorates over time. Then, potholes can cause further damage. Therefore, I would not rule it out completely.

Best regards, Rainer.

Hello!

Thank you for the helpful answers.
Okay, I'll just wait and see if, and when, the car will be back.

I'll find out then if it happened again during a trip with the trailer.

Otherwise, the tip about the wiring harness for the tailgate is also good; I hadn't even thought of that.
Judging by the age of the vehicles, there's likely to be a huge mess of cables there as well.