What is the function of a Load Sensing Relay in a dual cab utility?
Warren-Pat_01 said
11:02 PM Jul 28, 2023
I've recently had a problem where I had no blinkers on a boat trailer, also our caravan.
I suspect something was wrong with our boat trailer wiring - mainly the blinkers. While underneath the car trying to find the problem, I heard loud clicking from a relay near the spare tyre - the clicking was obviously from a relay & corresponded with the car's blinkers. The tub light was also not working. Eventually I found a blown fuse under the bonnet of our D-Max - the fuse was marked "Trailer". Since changing the fuse, everything seems to work ok.
I visited the car's towbar installers today - they said what I heard was the Load sensing relay & they aren't known to be reliable.
So, what is it for? To assist with the electric brakes, the blinkers?
Are We Lost said
12:43 AM Jul 29, 2023
Most late model vehicles have sensors to detect if indicator lights are not working. They do this by sensing the load on the system (amps used). So if a fuse is blown the system then warns you of the problem. I am guessing there was some dash warning and maybe something audible to let the driver know.
Related to this, there is a common problem when you attach a trailer with LED lights. The tow vehicle knows a trailer is plugged in so expects to see greater load (more amps used) when the indicators are used. If that increased load is not high enough (because the lights are LED), the electronics assume a globe has blown. To warn the driver, the indicators flash at double speed. It seems LED lights in the van are borderline as to whether they will trigger as a fault or not. Mine worked fine for a couple of years and then started double flashing just occasionally.
The solution, as silly as it may sound is to get a pair of resistors and wire these in parallel with the trailer/van indicators. This draws extra current which is then sufficient to convince the electronics that no globes are blown. These "load resistors" are available at any auto parts place for around $20 the pair. Maybe that false warning is what the installers were referring to.
-- Edited by Are We Lost on Saturday 29th of July 2023 12:46:22 AM
Mamil said
12:54 PM Jul 29, 2023
Are We Lost's reply has part of the answer, but it doesn't explain the presence of the relay click you are hearing, so I think a more complete explanation is warranted...
Modern cars send control signals along the same wires that are used to actually illuminate it's lights. They are to detect whether the globes are blown or not by detecting the correct continuity and resistance on the circuit, and alerting the driver to a blown globe at the dash if they don't get the expected results. These signals, sometimes referred to as a "cold check" are of sufficiently short duration and power to check the circuit without actually illuminating the globe itself, so normally we're not even aware of them happening.
When you attach a trailer the car does the same thing on the trailer indicator circuits, but in this case open circuit doesn't indicate a blown globe, but that there's no trailer attached, whereas continuity and a certain level of resistance indicates there is one attached. It uses this knowledge to automatically turn on it's various 'smart towing features' if it detects a trailer is attached. These do things like de-activating the rear fog lights and reversing sensors on the car to prevent them being a nuisance when there's a trailer behind, and alter the car's auto transmission and braking mapping to cope better with towing a heavy load and use the car's ABS circuit to automatically correct trailer sway.
So far so good, but there's two potential issues with this scenario when the trailer lights are LED.
First, LED lights don't create as much resistance as filament globes so often aren't detected by the car, so if the car doesn't know a trailer is attached it won't activate the extra towing features. Now most of these are nice to have convenience and safety features, but aren't essential, and the car will still tow the trailer and the trailer lights operate as normal. But, there's an easy solution as Are We Lost said, put load resistors in parallel with the LED indicators in the trailer to create enough resistance in the circuit for the car to detect it's there and activate the towing features.
Second is that the control pulses though not of sufficient duration or power to illuminate a traditional filament globe, or even an LED unit specifically designed to work on that car, can make the simple LED light units on a trailer flicker annoyingly. The solution to this one is to have the trailer indicator circuit operate a relay which in turn operates the trailer LED lights. The relay being an electro-mechanical device that needs a current of a certain duration and strength to operate will not be triggered by the short weak pulses in the control signal, so effectively filters them out so the LED doesn't flicker, but will still turn the light on when the full operating current is sent down the wire.
So, a complete solution requires both a load resistor in parallel across the indicator circuit, and a relay in series to operate the lights - as in the attached diagram from a LandRover group.
It's worth noting that not all cars and all trailers with LED lights have these issues, some don't need this workaround, and some (like mine) only need load resistors and not relays. However, some auto sparkies will fit them both to a trailer just in case, as they don't know what car you may use the trailer with in the future, and the worst case scenario is that you'll just be using a bit more power in your indicator circuits than LED lights should, and you have one more element to fail - the relays!
Thanks chaps for your explanations - I was not thinking the LSR was for the current load but for weight. The towbar installer gave me that idea as it is according to him, only the utes (in my case the D-Max) have this relay.
You both made the assumption that there were LED "lamps" involved - that is not the case as both the car & caravan & previous boat trailer (sold) have globes for the blinker circuits. The car has LEDs for the tail & stop lights only, as well as the daytime running lights. And the car does not have indicators that a blinker or any other lamp is blown - unlike my MY80 Subaru Leone 4WD where it had reed relays in the stop light ccts, giving indication that a lamp had failed.
When I occasionally borrow a normal 7x5 trailer for green waste disposal, I do check the lights for correct operation - they are LEDs but they work ok every time.
jegog said
05:07 PM Aug 5, 2023
My car, a Mitsubishi Pajero sport 2022 model, has a seperate sub-system for trailer lights.
It is plugged into the wiring loom at the back, same compartment as the jack, and the trailer lights have their own fused power supply.
If the fuse is blown then there are no trailer lights.
There is a little black box controller unit and I suspect the lights are turned on via opto-couplers, a form of solid state relay, or maybe just driver transistors, or mechanical relays.
I would imagine that most late model cars would use something similar.
jegog said
05:17 PM Aug 5, 2023
Mamil
"put load resistors in parallel with the LED indicators in the trailer to create enough resistance in the circuit for the car to detect it's there and activate the towing features."
That is completely wrong. Because Rt = 1/R1 + 1/R2
Which means that the total resistance is less than either R1 or R2. Basic Ohms law
The network current will increase as the network resistance is less.
Are We Lost said
06:14 PM Aug 5, 2023
Correct. LED lights have more resitance. And putting the load resistors in parallel will result in less resistance in the circuit. This will result in more current.
Maybe Mamil meant current. The purpose of creating less resistance is so more current will flow, which the vehicle detects as "no problem with the globes".
jegog said
10:48 PM Aug 5, 2023
Actually the formula I gave for total resistance in a parallel circuit, Rt = 1/R1 + 1/R2, is incorrect
Rt = 1/(1/R1 + 1/R2).
I was relying on my memory of schoolwork from 1957.
Greg 1 said
02:04 PM Aug 7, 2023
Fixed the issue with my Ranger being upset by the caravans LED'S.
Bought two 120 ohm resisters from Jay Car for a monumental 0.69 cents.
Fitted them in the two indicator circuits in the caravan plug wired between an indicator pin and the earth pin. Works like a treat and far less money than any other solution.
Are We Lost said
03:40 PM Aug 7, 2023
I would not recommend other people follow that suggestion.
While getting resistors from Jaycar is a great idea if the $15-$20 is too much to justify, the size you have chosen is far from ideal. While it may work for you that 120 ohms will make very little difference to the current drawn. Others may find it is insufficient.
An incandescent globe of 21 watts (the normal wattage for a single indicator globe) has a resistance of 6-8 ohms. The goal of fitting resistors in the circuit is to simulate the power drawn by such a globe instead of the low current drawn by LED.
The Narva load resistors, designed for the purpose, have a 6 ohm resistance and 25 watt capacity. For $18 a twin pack, with terminal fitted I suggest getting the correct ones is better than saving a few dollars.
-- Edited by Are We Lost on Monday 7th of August 2023 03:43:50 PM
Greg 1 said
03:37 PM Aug 8, 2023
If you are not happy with 120 ohm then just buy a lower resistance.
Jay Car have a range of them and far cheaper than any of the made up boxes which are only basically a resistor in a box for 100 times the price.
The ones I have fitted have been in there for 8 years now and certainly cured the Ranger's problem with it's canbus system.
I have also done the same with an adaptor that I have to convert from flat pin to round pin for my other two trailers. Perfect solution given to me by an auto sparky that I know.
So A. It works and B. Suggested by an auto sparky so must be on the money.
I've recently had a problem where I had no blinkers on a boat trailer, also our caravan.
I suspect something was wrong with our boat trailer wiring - mainly the blinkers. While underneath the car trying to find the problem, I heard loud clicking from a relay near the spare tyre - the clicking was obviously from a relay & corresponded with the car's blinkers. The tub light was also not working. Eventually I found a blown fuse under the bonnet of our D-Max - the fuse was marked "Trailer". Since changing the fuse, everything seems to work ok.
I visited the car's towbar installers today - they said what I heard was the Load sensing relay & they aren't known to be reliable.
So, what is it for? To assist with the electric brakes, the blinkers?
Most late model vehicles have sensors to detect if indicator lights are not working. They do this by sensing the load on the system (amps used). So if a fuse is blown the system then warns you of the problem. I am guessing there was some dash warning and maybe something audible to let the driver know.
Related to this, there is a common problem when you attach a trailer with LED lights. The tow vehicle knows a trailer is plugged in so expects to see greater load (more amps used) when the indicators are used. If that increased load is not high enough (because the lights are LED), the electronics assume a globe has blown. To warn the driver, the indicators flash at double speed. It seems LED lights in the van are borderline as to whether they will trigger as a fault or not. Mine worked fine for a couple of years and then started double flashing just occasionally.
The solution, as silly as it may sound is to get a pair of resistors and wire these in parallel with the trailer/van indicators. This draws extra current which is then sufficient to convince the electronics that no globes are blown. These "load resistors" are available at any auto parts place for around $20 the pair. Maybe that false warning is what the installers were referring to.
-- Edited by Are We Lost on Saturday 29th of July 2023 12:46:22 AM
Are We Lost's reply has part of the answer, but it doesn't explain the presence of the relay click you are hearing, so I think a more complete explanation is warranted...
Modern cars send control signals along the same wires that are used to actually illuminate it's lights. They are to detect whether the globes are blown or not by detecting the correct continuity and resistance on the circuit, and alerting the driver to a blown globe at the dash if they don't get the expected results. These signals, sometimes referred to as a "cold check" are of sufficiently short duration and power to check the circuit without actually illuminating the globe itself, so normally we're not even aware of them happening.
When you attach a trailer the car does the same thing on the trailer indicator circuits, but in this case open circuit doesn't indicate a blown globe, but that there's no trailer attached, whereas continuity and a certain level of resistance indicates there is one attached. It uses this knowledge to automatically turn on it's various 'smart towing features' if it detects a trailer is attached. These do things like de-activating the rear fog lights and reversing sensors on the car to prevent them being a nuisance when there's a trailer behind, and alter the car's auto transmission and braking mapping to cope better with towing a heavy load and use the car's ABS circuit to automatically correct trailer sway.
So far so good, but there's two potential issues with this scenario when the trailer lights are LED.
First, LED lights don't create as much resistance as filament globes so often aren't detected by the car, so if the car doesn't know a trailer is attached it won't activate the extra towing features. Now most of these are nice to have convenience and safety features, but aren't essential, and the car will still tow the trailer and the trailer lights operate as normal. But, there's an easy solution as Are We Lost said, put load resistors in parallel with the LED indicators in the trailer to create enough resistance in the circuit for the car to detect it's there and activate the towing features.
Second is that the control pulses though not of sufficient duration or power to illuminate a traditional filament globe, or even an LED unit specifically designed to work on that car, can make the simple LED light units on a trailer flicker annoyingly. The solution to this one is to have the trailer indicator circuit operate a relay which in turn operates the trailer LED lights. The relay being an electro-mechanical device that needs a current of a certain duration and strength to operate will not be triggered by the short weak pulses in the control signal, so effectively filters them out so the LED doesn't flicker, but will still turn the light on when the full operating current is sent down the wire.
So, a complete solution requires both a load resistor in parallel across the indicator circuit, and a relay in series to operate the lights - as in the attached diagram from a LandRover group.
It's worth noting that not all cars and all trailers with LED lights have these issues, some don't need this workaround, and some (like mine) only need load resistors and not relays. However, some auto sparkies will fit them both to a trailer just in case, as they don't know what car you may use the trailer with in the future, and the worst case scenario is that you'll just be using a bit more power in your indicator circuits than LED lights should, and you have one more element to fail - the relays!
You both made the assumption that there were LED "lamps" involved - that is not the case as both the car & caravan & previous boat trailer (sold) have globes for the blinker circuits. The car has LEDs for the tail & stop lights only, as well as the daytime running lights. And the car does not have indicators that a blinker or any other lamp is blown - unlike my MY80 Subaru Leone 4WD where it had reed relays in the stop light ccts, giving indication that a lamp had failed.
When I occasionally borrow a normal 7x5 trailer for green waste disposal, I do check the lights for correct operation - they are LEDs but they work ok every time.
It is plugged into the wiring loom at the back, same compartment as the jack, and the trailer lights have their own fused power supply.
If the fuse is blown then there are no trailer lights.
There is a little black box controller unit and I suspect the lights are turned on via opto-couplers, a form of solid state relay, or maybe just driver transistors, or mechanical relays.
I would imagine that most late model cars would use something similar.
Mamil
"put load resistors in parallel with the LED indicators in the trailer to create enough resistance in the circuit for the car to detect it's there and activate the towing features."
That is completely wrong. Because Rt = 1/R1 + 1/R2
Which means that the total resistance is less than either R1 or R2. Basic Ohms law
The network current will increase as the network resistance is less.
Maybe Mamil meant current. The purpose of creating less resistance is so more current will flow, which the vehicle detects as "no problem with the globes".
Rt = 1/(1/R1 + 1/R2).
I was relying on my memory of schoolwork from 1957.
I would not recommend other people follow that suggestion.
While getting resistors from Jaycar is a great idea if the $15-$20 is too much to justify, the size you have chosen is far from ideal. While it may work for you that 120 ohms will make very little difference to the current drawn. Others may find it is insufficient.
An incandescent globe of 21 watts (the normal wattage for a single indicator globe) has a resistance of 6-8 ohms. The goal of fitting resistors in the circuit is to simulate the power drawn by such a globe instead of the low current drawn by LED.
The Narva load resistors, designed for the purpose, have a 6 ohm resistance and 25 watt capacity. For $18 a twin pack, with terminal fitted I suggest getting the correct ones is better than saving a few dollars.
Narva Load Resistors at Supercheap
-- Edited by Are We Lost on Monday 7th of August 2023 03:43:50 PM