LED Tail light - Calling Electronic Techs...

EDIT: 1.2ohms instead of 0.8ohms.
I pulled the XJ750 (5G2) Computer apart tonight and had a look at the circuit and basically it is a voltage divider pulling the voltage between the globe and a 1.2ohm resistor (the 1.2ohm ceramic resistor for the brake circuit is glued to the tail circuit resistor, so I will separate them later to read the actual value for the tail circuit later but for now lets say they are the same.)

The question I have is since we ought to be talking about low current directly into the chip what would the low current way of creating the correct voltage for the sensors be?

This is my tail light warning system bypass logic in a picture, see attached.

 

There is a thread in the DYI section about the required modifications, but I don’t think anyone has made an effort to modify the chipset. Every workaround for the taillight warning has involved adding or subtracting jumpers.

 

did a bunch of searches and found people saying, press the button twice or put the incandescents back in.
Will look further, thanks.

 

oops shoot me. Went to the DIY section and it was the first one by SQL guy! DOH!
Thanks... more!

 

@k-moe any idea what the chips are? I've done car ECU hacking and canbus hacking.

 

Looks like SQLGuy has put diodes in, in place of the ceramic resistors, to drop the voltage enough for the computer to register that the globes are drawing current and removing the voltage divider circuit.
Good thinking! and no cutting wires like my logic.

 

It's been a few years since I've had one open to look at. Might be mentioned in here.
how-to-convert-to-leds-and-still-have-everything-work.14068

 

it is mentioned even has the diode part number for radio shack.
photos are gone but the instructions were straight forward a
never seen inside
when you do this mod maybe snap a few photos

you could add them to the end of the post, His or yours.

 

A bit of added info about the thread:

While adding the diodes does allow for running with LED's or incandescent bulbs without setting the "TAIL" warning, it will defeat the safety circuit so that if the tail or brake lights fail it will no longer report a "TAIL" warning.

I don't believe there are any modifications that can be done to the chip set. It simply reports a warning based on a high or low signal that is created on the mezzanine card by utilizing several transistors and associated components to create a differential detector to monitor the condition of the bulbs / wiring. No voltage drop across the current sense resistor will set a warning, as will no bulb present - the filament check.

 

Kludged.
Pulled the warning light globe out.
Lot easier ignoring the LCD display.

 

I don't believe that's true. Pretty sure I verified it at the time I made the mod. The CPU senses voltage drop over the resistor or rectifier due to current to the lamp. Whether a resistor is used or a rectifier is used, there still has to be current flow to create the voltage drop. A failed lamp or LED will still be detected with rectifiers instead of resistors.

 

I agree with the logic, and without a schematic it is hard to contradict or explain that thought. However, I think what occurs is that there are two fail modes for the tail / brake circuit. With the filament check there is a 1K resistor that feeds 5V to the current sense resistor and with the incandescent bulb installed the low side of the resistor is effectively grounded resulting in zero volts across the differential input detector. When the brake is applied and an open circuit is detected (burnt out bulbs) there is no current flow so 12V is applied to the high and low side of the differential detector, which results in zero volts applied to the two inputs. When a diode is installed in place of the resistor a current path on the low side of the differential detector circuit exists that in either situation (open filament check or no current when 12V applied), will cause the usual diode drop and consequently a differential voltage high enough to indicate a safe condition.