towbar wiring box?

[missioneer]

Hilux doublecab, 2004 D4-D, 75kW, LHD, factory installed towbar.
I am tracing down a grounding problem. All wires at the towbar up to the tail lights were checked with an ohmmeter and are ok, ground on the trailer connector is also good.

When car lights are off and the trailer is connected, everything works fine.
When car lights are on and I add another light - indicator or brake lights - the foglight switches on. So it looks like some bad grounding, maybe a corroded ground cable.

A retired Toyota mechanic advised me to look for the cigarettebox-sized box of the towbar wiring, there´s some relay inside. I should check its connections and its grounding. But he cannot remember where that box is installed on my 2004 model.
He suggested behind the driverside footwell area or behind the glove compartment. I didn´t find any there, though I removed all panels I could.
The wiring harness from the factory runs all the way up to the engine bay and is connected to a fuse and 12V power but I cannot identify it.
Does anyone of you know where that relay box sits?
Thx!

[martynvella]

It is usually under the bonnet on the LHinner guard.
What fog light switches on?
That box only has a couple of relays that takes the load of the trailer indicators off the flasher module so the flash rate doesnt change.

[missioneer]

It is usually under the bonnet on the LHinner guard.
What fog light switches on?
That box only has a couple of relays that takes the load of the trailer indicators off the flasher module so the flash rate doesnt change.

Thanks a lot for that hint!
The fog light on the back of the trailer switches on. I have a 7pin socket on my Hilux with the microswitch that detects if a trailer plug is connected and deactivates the cars foglight.
Then I can turn off the foglight manually by its switch on the dashboard. But if I hit the brake or start the turning light and more current is flowing to the lamps of the trailer, the foglight comes up again. So it looks like a bad grounding connection or a defective relay box.

[missioneer]

It is usually under the bonnet on the LHinner guard.
What fog light switches on?
That box only has a couple of relays that takes the load of the trailer indicators off the flasher module so the flash rate doesnt change.

what part do you mean by „inner guard“? Sorry I’m not a native english speaker and google search didn’t help me either. Under the bonnet, I only see the insides of the fenders and the firewall. No idea what a guard could be under the bonnet.
Thanks a lot!

[Gipsy]

Fog light? Do you mean reversing light? The box is probably near the battery in the engine bay.

[missioneer]

Fog light? Do you mean reversing light? The box is probably near the battery in the engine bay.

no, I really mean the rear fog light.
My Hilux has two batteries - one on the left and one on the right.
Do you know what martynvella means by "on the LH inner guard"? What part is an "inner guard" in the motor compartment of a car?

[Gipsy]

Ok I thought that was what you mean, we don't have the rear fog lights in Australia. look close to the battery, the original one, it's a small black box the size of a cigarette pack. Your problem may be with the 7pin socket, in particular the microswitch. Or the wiring to the manual switch for the rear fog lights. Can you take that out of the circuit?

[dave g]

Have you tried a different trailer to see if it works
If it does the thought could be in the trailer. And check the wiring in the plugs loss or damage wire
All so make sure the plugs are making good contact had bad plug before had similar fault.

[Gipsy]

Or have a look at the trailer plug wiring and remove the wire that powers the rear fog lights.

[missioneer]

Have you tried a different trailer to see if it works
If it does the thought could be in the trailer. And check the wiring in the plugs loss or damage wire
All so make sure the plugs are making good contact had bad plug before had similar fault.

yes I did as a first analysis. The fault is not with the trailers, its within my car. I even replaced the old worn 7pin socket with a correct new one, measured the each wiring resistance of the harness at the back of the car, all with low resistance and good contact. So I came to the conclusion that it must be somewhere else and then the old mechanic told me about the relay box.

[missioneer]

Ok I thought that was what you mean, we don't have the rear fog lights in Australia. look close to the battery, the original one, it's a small black box the size of a cigarette pack. Your problem may be with the 7pin socket, in particular the microswitch. Or the wiring to the manual switch for the rear fog lights. Can you take that out of the circuit?

the wiring to the dashboard switch of the foglights ... hm, I didn‘t consider this as a potential root cause. Good hint, many thanks, I will check that, too.
Thanks to all hints, folks. Will keep you updated on the progress...

[Gipsy]

Did it all work before you changed the old 7pin? Try a different one.

[missioneer]

Did it all work before you changed the old 7pin? Try a different one.

no it didn‘t work with the old plug either. Thats why I replaced it when checking and cleaning the wires.

[missioneer]

Thanks to all for your hints!
I have the solution and its quite interesting!

First - there is no relay box in my 2004 model, only the later ones were equipped with them. That‘s why I couldn‘t find one.

Second - My problem as described is not limited to the Hilux and not to my model! Several generations of Hilux and Landcruisers have that as they share the same wiring harness, though in different lengths. But the layout is the same.
Toyota used wires with too small diameters and therefore too high resistance for the ground cable. Therefore, if an additonal load (= indicator lights, brake lights) is connected, the ground potential shifts, causing a small fault reverse current which activates the fog light relay! That relay has a low sensitivity. That combination causes it to switch on the fog lights consistently!

That problem is well documented in other Toyota forums like buschtaxi.com and several Toyota workshops are aware. As replacing the wiring harness is a lot of costly work and won‘t solve it, the common workaround solution is to serially insert a diode into the cable of the foglight inside the trailer plug socket. Kathode on the light side, anode on the foglight switch side. That blocks the reverse current. I used a standard 5A diode for 30cents like every electronics shop sells them. Worked!

[DeadlyBeast]

Great outcome
Thanks for posting the fix

[martynvella]

Wouldnt it make more sence just to ground the trailer socket earth pin to the chassis rather than use the earth wire in the rear harness?

[missioneer]

Yes, if it would provide less resistance than the grounding cable. But that would have to be the case and I am sure that was tried by the Toyota repair shops. So I followed their workaround.