Weird Truck and Trailer Lighting Issue

[americankiwi]

The truck I bought had the original 7-pin trailer plug cut out and the wires were then hard-wire spliced into the 5 wire standard Narva plug. My trailer wiring is all good and works on other vehicles and from a direct battery feed, but coming through the truck it's a bit dysfunctional. Turn signals, running lights, and brakes all work fine either alone or in combination with 1 other set of lights, but when all three are engaged the brakes won't light up on trailer. I don't think it's a ground issue as I have the same issue when running an alternate ground wire direct from trailer plug to battery. Now I'm definitely no electrician so I'm not sure what to expect of the voltage at various points. When testing voltage with a multimeter at either the trailer plug, the wire splicing, and at the brake pedal switch shows the brake line at about a 1-1.5 volts lower than the other lights - is this the problem? Also, would you need to wire BOTH the Left Hand tail lights AND the Right Hand tail lights into the trailer plug (which only has a slot for Tail Lights)?

IF anyone has had luck with hardwiring a Land rover Discovery's trailer wires direct to the trailer plug, please let us know if there are any tricks I need to be aware of!

 

[Lucien Nunes]

That does actually sound very much like a broken ground connection. When at least one light function is not lit, the current from the trailer lights that are lit returns to earth via the unlit circuit and the lamps in the truck to their earth. When all functions are lit, there's none left to provide an earthy path and they go out.

Since you have tried taking an earth to the trailer plug, I suspect the earth is broken between that and the lights themselves. Take continuity measurements between the ground terminals in the lights and the ground pin in the plug. You're looking for less than an ohm everywhere for starters. Continuity through the towball can sometimes give you an earth, which could account for different behaviour on two different vehicles.

Plus or minus a volt is not significant, it can be due to greater load on one wire in the truck harness than in another. More than a couple of volts difference points to a bad connection. Therefore I would ignore the brake light voltage being different for now. If you only have one wire in the trailer cable for tail lights, you only need to connect to either the left or right circuit in the truck. Don't connect the two circuits together because they are usually separately fused.

 

[americankiwi]

Thanks for that awesome reply.

The reason I'm tempted to think the trailer wiring is all good is because all the light functions worked fine when hooked up to another vehicle and also when I direct connect the trailer plug wires to a separate battery (running through all light circuits independently as well as all combinations of light circuits with ground hooked to neg battery terminal). I also tried swapping out the truck-side trailer connector (though not for a new one) and experienced same issues as before. I went back to trailer and added additional grounding points (originally I had wired it so the earth wire for all lights was running up through trailer connector, but have since added independent grounds at lights and at junction box for wiring heading to trailer connector - so an improvement from just the 1 ground going to the plug and depending on the truck's ground to now having 3 grounding points on trailer itself in addition to the earth running through the plug.) The truck has an obvious issue with it's electrics as when I bought it the previous owner pointed out it would slow drain the battery and basically if I don't disconnect the terminals the battery will be dead if in a couple of days if left sitting. I don't know how or where to troubleshoot this issue either, so advice on that would also be most appreciated. Thanks again. Cheers

 

[Lucien Nunes]

OK then, I would start by checking the outputs from the truck using an actual bulb with two wires attached. This is a more revealing test than a multimeter because it puts load on the circuit (and responds clearly to the flashing turn signal.) With the bared ends of the wires, taking care not to short-circuit anything, connect to the ground contact and each lighting circuit, with the circuits operated individually and together. See if each circuit works normally when all are operated together.

 

[americankiwi]

Thanks for that. I will definitely be trying this tomorrow morning. And what would it mean if (as expected?) one of the circuits didn't work when all were operated simultaneously? Say the left turn signal wasn't registering, would I then just trace that line in the truck and search out a short or something?

 

[americankiwi]

Thanks for that. I will definitely be trying this tomorrow morning. And what would it mean if (as expected?) one of the circuits didn't work when all were operated simultaneously? Say the left turn signal wasn't registering, would I then just trace that line in the truck and search out a short or something?

Well I had a go with your suggestion and found the light worked indicating all the circuits were responsive both independently and altogether at the truck end of the trailer plug. Even when plugging the trailer on and using the test light on the exposed contact points on the trailer plug the light worked as expected when running through different combinations of lights. However, the rear lights on the trailer still do the same old thing - brakes will work with indicators but not with tail lights. When brakes are on and the running lights subsequently turned on, the brake lights simply stop working. Maybe they were wired up wrong? Which would be a real shame because at electrician friend had me crimp connect then cover the series of wires with thick, waterproof, glue-filled shrink wrap?

But again, when I test the trailer lights directly off a battery OR from another vehicle everything works fine!

AH! Confusion. In my ignorant laymen's terms, it feels like not enough power is going to the trailer from the truck to allow for all the lights to run simultaneously - (but the lights are leds, low power requirement, right?).

Oh of curiosity I took voltage readings at the trailer plug under various combinations of the lights being on (or off):
When off the aux power line read 14.4v (all others negligible - .03v
With the Running Lights on: Run Lights - 13.72v
With Brake Lights on: Brake Lights - 12.6v
With Brake and Run Lights: Run - 13.68v; Brake - 12.5
With Brake, Run, and L-Turn Lights: Run - 13.5v; Brake - 12.2-12.6v (bouncing around with indicator pulse)

Does this tell us anything useful???

Thanks again for your feedback!

 

[Lucien Nunes]

Lots of info, not much I can extract from it. The fact that the test lamp behaves normally when connected to the trailer plug terminals (good test idea BTW) when the trailer is also connected is fairly concrete proof that the fault is in the trailer.

What truck is it? If a USA model, did / does it have combined brake / turn lights?
Can you confirm that the trailer connects via a 5-pin socket with just ground, tail, brake and the two turn signals?
When you have the brake and tail lights on, and you add the turn signal, do the brake lights flash off in sync with the turn signal flashing on, or do they stay off all the time the turn signal switch is on?

Can you post some pics of the lighting units and connections? And/or post links to their details?

 

[americankiwi]

What truck is it? If a USA model, did / does it have combined brake / turn lights?
Can you confirm that the trailer connects via a 5-pin socket with just ground, tail, brake and the two turn signals?

It's a 98 Land Rover Discovery 1. The previous owner had cut the factory wire harness out and hardwired the trailer plug (rather than use an adapter from the small round 7-pin to the standard 7-pin flat we use in New Zealand). The truck does have a combined brake/running light bulb with the double filament. But the wiring for the trailer is all separated - Left, Right, Running, Ground, Brake (plus additional unused wires for Fog, Reverse).

When you have the brake and tail lights on, and you add the turn signal, do the brake lights flash off in sync with the turn signal flashing on, or do they stay off all the time the turn signal switch is on?

This has changed over time as I've messed with the wiring trying to find a problem. Originally the brake light would just go off completely when the full combination of running lights, indicators, and brakes were engaged. Then later the brake lights would flash on and off in sync with the indicator flashing. Now I'm back to the brake light just going off after I engage the indicator while the running lights are on.

Can you post some pics of the lighting units and connections? And/or post links to their details?

Which lighting units? On the truck or the trailer? The trailer lights have three wires: ground, indicator, and running light wires. It's on a large 6.5m trailer so there are 3 junction boxes - one above each set of wheels connecting the back lights to the electric trailer brakes (which work but have been disconnected through this whole ordeal while I'm trying to troubleshoot just the lights without the additional load/component on to think about) and then another junction box at the front of the deck where 7 wires come together (L, R, Brake, Running Lights, Ground, E-Brake, Aux Power). They are watertight with connector blocks inside (rather than crimp butt connectors or whatever else).

http://cmtrailer.co.nz/sites/default/files/103_-_electrical_led.pdf

I think it's the first set of lights on that page.

 

[Lucien Nunes]

Sorry dropped the ball here, work is getting a bit intense, perhaps you've already fixed it. The way you have been testing sounds organised and coherent but the results are inconsistent.

Is it possible to repeat the test-lamp tests at the junction boxes nearest the trailer lights? We're trying to find out whether, when the rear lights are malfunctioning, the test lamp operates correctly or also malfunctions in the same way, on a circuit-by-circuit basis. If the malfunction is the same at the LED and the test lamp, the fault probably lies with the vehicle or the trailer wiring to the junction box. If the test lamp is OK but the trailer lights malfunction, then either the wiring to the trailer lights is bad, the lights are faulty themselves, or there is an issue with voltage that is affecting the LEDs but not the (filament) test lamp. Although, so far the voltages looked OK.

 

[americankiwi]

Thanks for jumping back in! No I haven't sorted it out, but I did bite the bullet and take the truck to an auto electrician who supposedly found nothing wrong. I will take the trailer out in a couple of days to more closely examine all the wiring. My main question is...Does testing the trailer lights with a 12v battery directly hooked up to the terminals of the trailer plug verify it's (dis)functionality? I have done this multiple times by with the set up below and the trailer lights always work as expected. I did this with another trailer and had the same results. However, with both of these trailers, when I hook up my truck all the weird issues pop up. The auto electrician added a diode to allow the truck's independent left and right tail light circuits to be connected to the single tail light wire on the standard NZ trailer plug. The only difference now is that when I turn the indicators on at the same time I have the brakes and/or tail lights on, these other lights blink along with the indicators.

The auto electrician said there could be a faulty trailer module on the truck but he doesn't know if the truck even has one and if it does he wouldn't know where to locate it - which wasn't particularly encouraging.

Anyway, I'll let you know how I get on with the trailer wiring!

Thanks again for the feedback.

 

[Lucien Nunes]

Other lights blinking in time to the indicators really does sound like a high-resistance ground in the truck. I know that was the first thing I said, and you more or less eliminated it by taking a replacement ground wire to the trailer socket, but I want you to do a test for it.

Use your test lamp, with one wire grounded to something reliable. Prove that the lamp works when the other wire is connected to a feed. Now, connect it instead to the ground terminal in the trailer plug, so that the lamp is connected between the ground that the trailer sees, and a ground you know to be good. If the lamp lights even dimly, short-circuit it so that the trailer is now grounded to the point the test lamp demonstrated was a functional ground.

As for a trailer signal module, it's normally connected in series with the indicator feeds to the socket, to sense whether the trailer indicator lights are taking current and activate the tell-tale light on the dash and/or an integral buzzer. Older units especially do not always respond to LEDs as the current is too low, and therefore fail to signal that the trailer lights are working, but are unlikely to affect the lights themselves. I struggle to understand how the module, even when faulty, would affect things in a way that makes one set of lights dependent on another. I would expect a damaged trailer signal module to simply break the circuit and stop the lights working.

 

[americankiwi]

Thanks for test - here's what I did:

Took my test light and hooked one end up to the negative battery terminal. I took the other end and hooked it into my trailer plug (truck side) at the tail light terminal with the lights in the truck on - the test light came on as expected. When I shifted the test light positive to the earth terminal on the trailer plug there was no light, dim or otherwise.

Would it be significant that the Land Rover's tail/brake lights are on a dual filament bulb?

I came across something about installing load resistors - that some older vehicles require a bigger draw than LEDs have and these load resistors address that. All the lights on my trailer are LEDs - maybe this is part of the problem?

 

[DPG]

You tend to need load resistors when replacing filament bulbs with LEDs when a thermal flasher unit is in use. Otherwise there isn't enough current drawn to cause the flasher to operate at the correct rate.

 

[marconi]

americankiwi: Here are a few things, which if I may suggest, you carry out.

1. Please confirm whether one or two diodes have been used to combine the left hand and right hand tail light circuits for connection to the appropriate terminal on the trailer socket. If you study the attachment you will see two wiring diagrams - the left hand one shows two diodes and the right hand one one diode.

2. Could you also confirm they have been connected the correct way - if you use your multimeter on the low Ohms range they should conduct when +ve lead is on the truck lamp terminal and the -ve lead on the trailer plug terminal for the tail lights - the trailer to be unplugged during the test.

3. Further to 1 and 2 above - now reverse the leads the diodes should not conduct.

4. Now disconnect the ground from the trailer plug and insulate the end. Using a thick cable which is a short as necessary - to minimise its resistance/impedance - connect one end directly to the -ve terminal of the battery. Connect the other end to the two ground leads from the 'multi-voltage led submersible lamp fittings' on the trailer. This thick cable might be a length of the same cable used to wire the trailer plug with all its conductors paralleled together. The greater the cross sectional area the better it will be for this temporary ground.

5. When you have made this direct path between the -ve battery and the trailer LED fittings please investigate to functioning of the tail, indicators and brake lights as you have done already. The trailer plug of course needs to be plugged in to the truck but the ground is being supplied via the temporary cable to the battery. Do not hook up the trailer to the tow bar for this trial. If all is well we stop there. If not, then move on to step 6.

6. Disconnect the diodes. Now connect both trailer tail lights to only the left hand tail lamp on the truck - repeat the functioning trial of the trailer lights. The repeat but this time with the trailer lights powered only by the truck's right hand tail lamp.

I look forward to hearing about the results.

For my EF colleagues. I wonder whether the 6 multi-voltage led arrays circuits (L, R, 2 Stop and 2 Tail) for the trailer lights are failing to work properly for the OP because of interaction between their constant current driver circuits because they share the same weak ground return path which is upsetting the necessary current sensing feedback circuitry of their buck-boost type driver - see second attachment which shows a typical driver and you can see where the current sensing resistor (R1 6.3 Ohms in my example) is placed - ground side. So since all the sense R1s are terminated to the same ground return, any voltage variations of the ground caused by the drivers as they turn on and off will be fed in to each led driver chip at pin FB - or something along these lines - messing up their current sensing. Got to go because I am being nagged for being anti-social by my wife. I hope you get the gist.