E24 auto check system

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635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

E24 auto check system

Post by 635Oxford »

I am trying to set up LED bulbs in my 1983 E24 euro, but I am having problems with the rear lights and Brake lights. I always get the warning signal on the dash and the red lights in the check panel whenever I try to use LEDs for those lights. The headlights, backup light and fog lights work fine with LEDs. I have tried different bulbs and putting resistors in parallel all to no avail. The system works fine with the old style bulbs , no hitches at all.
Please Help
marc79euro645
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Location: pensacola

Re: E24 auto check system

Post by marc79euro645 »

I added an extra bulb holder in the rear lights, and use an original bulb there as a pull up, for the leds. They work fine, and I like the brighter lights, but my dash indicator glows all the time, so not a perfect solution
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

Thanks,I don't really want to add extra holders. Do you know if the check system is a current measuring system, or resistance measuring or something totally different. Are there any auto electricians who know how it is set up and how to get round it. I would like to travel without any warning lights if possible.
Jlc
Posts: 287
Joined: Sun Sep 27, 2020 5:27 pm
Location: Atl

Re: E24 auto check system

Post by Jlc »

marc79euro645 wrote: Fri Dec 10, 2021 1:37 am I added an extra bulb holder in the rear lights, and use an original bulb there as a pull up, for the leds. They work fine, and I like the brighter lights, but my dash indicator glows all the time, so not a perfect solution
I am looking at a similar solution. Did you wire it in series or parallel? Parallel would essentially bypass the stock bulb for the check control.
85 635CSi Bronzit
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

Thank jlc. You have given me an idea. I shall try it and get back, fingers crossed I don't blow too many fuses.
633_Dream
Posts: 16
Joined: Thu Dec 16, 2021 9:29 pm
Location: EU

Re: E24 auto check system

Post by 633_Dream »

CHECK system measures the current flowing through "Check Relay" to respective bulbs.

It provides feedback to the Check Display.

If the current is not big enough it will not activate the reed relay and thus report bulb error (too less current => no bulb)
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

I said I would get back to you after trying a few things. I assumed that the check system was measuring current or (I), so I measured the current in the system for a normal filament bulb which for me was 1.7 amps. I then swapped to the LED bulb which gave me a measurement of 0.2 amp ( quite a big difference ). It was then time for me to go back to my old basic electricity books to find out how to get the same current which is of course through two resistors in parallel. With V=IR( volts= current times resistance ) and the resultant current of a parallel circuit is the sum of the currents through each part of the parallel circuit I total = I 1 (through LED ) plus I 2 (through the load resistor) . The current through the load resistor had to be 1.7 -0.2 or around 1.5 and as V=IR then R=V/I or 12/1.7, which is 7 ohms. This means I should have a parallel load resistor of 7 ohms and I need to make sure it is a true parallel circuit without other routes to earth from the bulb holder. I also need a resistor which is capable of handling the power through the resistor expressed as Watts = Current x Volts or 1.5 x 12 = 18 Watts. Thus my Load resistor should be rated above 18 watts. I found load resistors for 6.8 ohm rated at 50w and decided to try them. after setting it up I turned on the ignition had the warning amber light on the dash and the red brake light glowing which disappeared as soon as I depressed the brake pedal. SORTED!!. Just got to do the same with the rear side lights now with new measurements and calculations. Thanks for directing me to the right assumptions and remember if you are doing the same you need to check the ohms on the load resistor and then see if the wattage is man enough.
CHEERS
Chris
633_Dream
Posts: 16
Joined: Thu Dec 16, 2021 9:29 pm
Location: EU

Re: E24 auto check system

Post by 633_Dream »

Thanks for a feedback.

Great you was able to fix it.

If you put a parallel resistance (e.g. resistor) to the LED bulb, you lost CHECK functionality.

Maybe it would be better to change the "trigger level" by modifying the CHECK RELAY.
Each of the circuits being under diagnostic is having a coil with the reed relay inside it.

I was not testing this, but it might be possible to add more wire turns onto respective "relay" and thus lowering the threshold current per single channel of detection (e.g. STOP, license plates etc). Might be - due to big difference in load/bulb current, much more turns has to be added. Not sure but might be that such reed relay with the coil can be bought on specific current.

Such modification would allow than to keep CHECK functionality and (what is more) lower the heat spreading in the parallel resistors which is not a good rule to have it. Hi temperature might cause faster wire isolation deterioration, melting plastic parts being near-by etc. Also soldering (especially for the parts working with big temperature changes) is not long-lasting solution for electric loads in the car.

I hope this will make potential users more sensitive to such modifications. Each and every solution have some pro's and con's.

Take care!
Jlc
Posts: 287
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Location: Atl

Re: E24 auto check system

Post by Jlc »

Glad you fixed it. One thing that would be interesting is how hot do the resistors get, if the check control keeps current through them whenever the ignition is on.
85 635CSi Bronzit
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

Thanks 633-dream and jlc. I had previously fitted a tow bar so I have a big chunk of steel running across the back panel of the car, an ideal place to screw the resistors to, but I agree take care and position your resistors to clear any heat safely.
I haven't started on the rear side lights yet, but with the brake lights, I had them on for a long time and the 6.8 ohm 50w load resistor was barely warm. With my check system there is no current flowing in the circuit after start up (depressing brake pedal after ignition switched on). Current will only flow when the brake light switch is activated so there is no residual flow through the resistor. When the rear lights are on there will be current through the associated resistors, but the measured current is only 0.36 amp which I doubt is going to create much heat. It is probably sensible to use quite a high power rated load resistor so there is no danger of overheating.
Cheers
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

Sorry I forgot to answer 633-dream regarding check functionality. My system is working with full check functionality. It is a question of finding the lowest total amperage that will satisfy the check system, but when the current from the led side fails the resultant current is reduced enough to activate the check function. It is a bit fiddly and I am not sure I will manage it with the rear lights as the current through LED is only 0.11 amps.
633_Dream
Posts: 16
Joined: Thu Dec 16, 2021 9:29 pm
Location: EU

Re: E24 auto check system

Post by 633_Dream »

Thanks for info.

It is good that the system is close to the switching/detection level and lack of LED lamp contact is triggering CHECK. Good.

True that power resistors can be purchased for cheap in China now. In the past they're quite expensive.

Modification of the CHECK relay would be much more expected for concourse cars and much more tidy. Less wires means also smaller probability any point will fail in the future. It would be interesting to check that option by Someone but it requires some experimenting which could (but not must be) supported with some electrical calculations (magnetic flux created by current flowing across coil of reed relay).

Maybe once I will spent some time on it. So far I'm using standard bulbs in my car. But could be in some time it will not be easy to buy a good quality ones as it is already with home bulbs.
635Oxford
Posts: 16
Joined: Sat Mar 10, 2018 10:41 am
Location: Oxford UK

Re: E24 auto check system

Post by 635Oxford »

I have ordered an adjustable resistor so that I can try and find the amp flow which triggers the check function for the rear lights and see if I can get it working the same as the brake lights. My 635 is quite smart being a total rebuild, but not concours and I suppose that quality of car would stay with original bulbs. Good luck with trying modification to the check system. I think that might be beyond me. Have a good Christmas.
633_Dream
Posts: 16
Joined: Thu Dec 16, 2021 9:29 pm
Location: EU

Re: E24 auto check system

Post by 633_Dream »

It would be interesting to see results with variable resistor but it probably needs to be wirewound type.

Might be someone on this forum will check it. It is far on my To Do list unfortunately. :-?

Merry Christmas and Happy 2022!
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