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Automotive Warning light help

MarkGSi

Jul 16, 2014
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Hi everyone,

Please could you help in creating a circuit that I need to make. Im looking to replace a failed circuit that monitors brake fluid level in a car. The failed circuit lies within a 30 odd year old check control unit and the brake fluid monitoring section has failed. I have tried to use a 555 timer for this but have become stuck. I need the circuit to light a warning lamp when required. As im going to use wiring / float etc already in the car, the lamp needs to light when the float drops and breaks the path to ground / goes open circuit with ground. Also I need the light to remain on and not bounce the light on and off as the fluid moves around the reservoir tank as the car corners. This is important as I would intend to use the same circuit to replace a screen wash monitor etc.Please excuse my lack of knowledge, maybe there is a better way to do this? Any help would be gratefully received. Many thanks.
 

narkeleptk

Oct 3, 2019
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The best solution for this is already whats in place imo, you cant just replace the float that is already there?
If you want to build your own just copy whats there already. The signal to control module or lamp is just as you described open or grounded.
 

MarkGSi

Jul 16, 2014
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The best solution for this is already whats in place imo, you cant just replace the float that is already there?
If you want to build your own just copy whats there already. The signal to control module or lamp is just as you described open or grounded.

Hi, thanks for your answer. My issue doesn't lie with the float, wiring etc all of that works ok. The issue lies within the check control unit circuitry. The PCB within the check control operates 8 different functions and i cant trouble shoot the PCB. Basically the check control bulb tells me that the car has ran out of brake fluid when it hasn't.( I get an MOT advisory for this) As an electronics project, my aim was to create a circuit that would be stand-alone from the check control unit. (but still use the current physical bulb holder) I was hoping to create a circuit using a 555 timer as described in my initial post. Many thanks, Mark
 

flippineck

Sep 8, 2013
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Could you let us know more details about the vehicle (make model variant year etc), might be able to find a circuit diagram that will help illuminate what's already there. Although it might at first appear more daunting than it really is, it might well be easier to fix what's there. Perhaps bypass the original control box if there really is a completely unsurmountable problem in there but, I'm not sure why you're involving a 555?

Where is the warning lamp, is it a 12V filament type, is it easy to get access to and isolate it's terminals? Is the brake fluid level sensor a simple float-operated make or break switch, and are it's terminals easily accessible?

I always worry with something like this though, that one has to be careful not to mess up something else (maybe the level sensor is feeding something else in the brains somewhere, you don't want to create a situation where you're inadvertently affecting the operation of antilock / auto brake assist / etc).. you could do with in-depth knowledge of the system to ensure you're not creating a modification that affects your insurance etc.
 
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MarkGSi

Jul 16, 2014
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Jul 16, 2014
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Could you let us know more details about the vehicle (make model variant year etc), might be able to find a circuit diagram that will help illuminate what's already there. Although it might at first appear more daunting than it really is, it might well be easier to fix what's there. Perhaps bypass the original control box if there really is a completely unsurmountable problem in there but, I'm not sure why you're involving a 555?

Where is the warning lamp, is it a 12V filament type, is it easy to get access to and isolate it's terminals? Is the brake fluid level sensor a simple float-operated make or break switch, and are it's terminals easily accessible?

I always worry with something like this though, that one has to be careful not to mess up something else (maybe the level sensor is feeding something else in the brains somewhere, you don't want to create a situation where you're inadvertently affecting the operation of antilock / auto brake assist / etc).. you could do with in-depth knowledge of the system to ensure you're not creating a modification that affects your insurance etc.


The car is astra mk2 gte from 1988. The check control circuitry wasn't very good to start with back in the 80's and appears to be problematic in nature. The check control circuitry looks overcomplicated and was improved in 1990, the two systems are not interchangeable. You cant seem to find an early version such as mine that works. They all seem to fail with this same fault and I have never known anyone that has fixed this particular problem, hense why I was looking to adapt something. In my inexperience I was thinking of trying to use a 55 timer as I felt it would keep the physical size of my project down as there is limited remaining space inside the unit. Any suggestions on components would be welcomed! The unit has 12v filament bulbs, terminals are easily accessible. With regards to damaging anything else, there is a headlight, tail-light, brake light unit that works alongside the check control unit which may somehow get damaged but looking into the circuitry of the car I think its a low risk. I have given this considerable thought in the past and don't think its worth the worry from the perspective of modifying the brake fluid bulb. With this said I totally understand and appreciate your concerns about causing further damage, insurance etc. Thanks, Mark
 

Martaine2005

May 12, 2015
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I think you should upload some photos of the PCB. You’ll be surprised at the eagle eyes of some members here. You won’t lose anything by doing so.

Martin
 

Alec_t

Jul 7, 2015
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my aim was to create a circuit that would be stand-alone from the check control unit
How would that prevent the faulty check control unit signalling low fluid level? Would removing the relevant bulb be enough? Or would there still be an error flagged and stored by the unit?
 

MarkGSi

Jul 16, 2014
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How would that prevent the faulty check control unit signalling low fluid level? Would removing the relevant bulb be enough? Or would there still be an error flagged and stored by the unit?

I would be disabling the faulty part of the circuit and replacing with the new stand alone circuit. The bulb is removed at the moment, the car is over 30 years old and in daily use doing whatever I need it to. Many years ago, the car developed a leaky brake cylinder meaning I lost fluid over several weeks and I had no idea the fluid was running low.
 

MarkGSi

Jul 16, 2014
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I think you should upload some photos of the PCB. You’ll be surprised at the eagle eyes of some members here. You won’t lose anything by doing so.

Martin
photo-jpg.14177
 

narkeleptk

Oct 3, 2019
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can you show other side and mark the pin for the signal from float switch. I work on auto modules, may can help you fix it. but souncs like its possibly a I/O failure on the tms from your other post.
 
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flippineck

Sep 8, 2013
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float drops and breaks the path to ground / goes open circuit with ground

On the float switch.. what is present on the opposite terminal to it's 0V/ground/chassis terminal, with ignition on, and reservoir empty?

On a non computerised car it would probably be around +12V same as the battery + post

It might be different if the sense signal is being generated by the board above.. but if there's always something stable there with the reservoir empty, then with it full, whatever voltage was present will collapse to zero assuming the float switch is a straight switch and involves no trick circuitry of it's own.

Aim would be to read that change in such a manner as not to disturb anything already connected, then generate a switched +12V based on that, wired to one side of the low brake fluid lamp, after disconnecting it from the existing board.

I'm visualising a resistor connected to the non-ground terminal of the float switch, feeding into the base of a suitable PNP transistor. Collector of said PNP transistor connected to ground, emitter to warning lamp, other side of warning lamp via a suitable low value fuse to +12V.

A suitable capacitor could be connected between ground and the transistor's base to reduce bounce.

Hopefully you'd be able to fit the capacitor, transistor and resistor on the back of the warning lamp holder itself, they wouldn't take up much space.
 

MarkGSi

Jul 16, 2014
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can you show other side and mark the pin for the signal from float switch. I work on auto modules, may can help you fix it. but souncs like its possibly a I/O failure on the tms from your other post.
 

narkeleptk

Oct 3, 2019
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can you show other side and mark the pin for the signal from float switch. I work on auto modules, may can help you fix it. but its possibly a I/O failure on the tms chip which you'd prob be stuck scavenging from another module to fix
post is blank
 

MarkGSi

Jul 16, 2014
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can you show other side and mark the pin for the signal from float switch. I work on auto modules, may can help you fix it. but souncs like its possibly a I/O failure on the tms from your other post.

upload_2022-1-21_20-15-35.jpeg
 

MarkGSi

Jul 16, 2014
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On the float switch.. what is present on the opposite terminal to it's 0V/ground/chassis terminal, with ignition on, and reservoir empty?

On a non computerised car it would probably be around +12V same as the battery + post

It might be different if the sense signal is being generated by the board above.. but if there's always something stable there with the reservoir empty, then with it full, whatever voltage was present will collapse to zero assuming the float switch is a straight switch and involves no trick circuitry of it's own.

Aim would be to read that change in such a manner as not to disturb anything already connected, then generate a switched +12V based on that, wired to one side of the low brake fluid lamp, after disconnecting it from the existing board.

I'm visualising a resistor connected to the non-ground terminal of the float switch, feeding into the base of a suitable PNP transistor. Collector of said PNP transistor connected to ground, emitter to warning lamp, other side of warning lamp via a suitable low value fuse to +12V.

A suitable capacitor could be connected between ground and the transistor's base to reduce bounce.

Hopefully you'd be able to fit the capacitor, transistor and resistor on the back of the warning lamp holder itself, they wouldn't take up much space.
 

flippineck

Sep 8, 2013
358
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Is that bit of yellow (rubber band?) showing the pin that goes to the float sensor?

How that pin behaves when the float moves (voltage with respect to ground) would be useful info.

Which one is the bulb that warns low brake fluid?

Those bulbs are all turned on and off by the row of transistors top right. You could possibly bridge a resistor and capacitor straight across from the float sensor pin, to the relevant transistor base & a convenient ground pad. You'd want to be careful to follow the tracks & make disconnections where necessary by cutting the relevant transistor base track to make sure you don't end up feeding anything back into the chip.

At least then you'd know you were using the appropriate spec of transistor to suit the bulbs used..
 
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MarkGSi

Jul 16, 2014
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Is that bit of yellow (rubber band?) showing the pin that goes to the float sensor?

How that pin behaves when the float moves (voltage with respect to ground) would be useful info.

Which one is the bulb that warns low brake fluid?

Those bulbs are all turned on and off by the row of transistors top right. You could possibly bridge a resistor and capacitor straight across from the float sensor pin, to the relevant transistor base & a convenient ground pad. You'd want to be careful to follow the tracks & make disconnections where necessary by cutting the relevant transistor base track to make sure you don't end up feeding anything back into the chip.

At least then you'd know you were using the appropriate spec of transistor to suit the bulbs used..

yes the yellow identifies the pin, all mod cons! the pin gains a ground the loses the ground as the float activates. the bulb is the third in from the right. also just to recap, its the loss of the ground from the float that makes the bulb light up. presence of the ground makes the bulb go out.
 
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