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SRS Warning lamp coming on with Headlamps

3K views 20 replies 6 participants last post by  LeftCoastGeek 
#1 Ā·
Hi all, my 1992 500E is putting on the red SRS warning Lamp in the Instrument cluster when the headlamps are turned on. (High beam and Low Beam both) The SRS lamp stays lit constant with the headlamps on and goes out as soon as the headlamps are switched off again.

I have the carā€™s first Northern Ireland MOT test is coming up within a couple of weeks. Itā€™s very strict unlike the English MOT system. Any warning lamps coming on at all during the test is an instant fail ā€“ so I must attend to this problem pretty urgently. They specifically look for the Warning Lamp functions in NI- like this news article

MoT changes come into force in Northern Ireland - BBC News


Has anyone else experienced this before? Where should I start looking for where the problem lies?


The car has the updated Ceramic Fuses and Eco junk replaced with new MB harnesses. No other electrical problems at all- just this SRS light coming on with the headlamps.


It could be linked not just to the headlamps ā€“ but the interior illumination circuit also. Itā€™s bizarre & annoying though! I do have Star but donā€™t know what use that will be for the SRS fault.


Any hints or tips are appreciated
 
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#2 Ā·
One thought - Measure the battery voltage with and without the headlamps on. You may have a problem with your charging system that is allowing the system voltage to drop under the load from the headlamps. Low supply voltage can trigger an SRS error.
 
#4 Ā·
OK so an update- 500e board figured out what was up

I often wondered why the bulb warning lamp came on & went off after a couple of seconds with the ignition at the first stop. Thought it was just the cars way of telling you the ignition was at accessory or something.

But turns out- only the SRS lamp is supposed to come on then go out after a couple of seconds.

So the warning bulbs are mixed up- what I am seeing is NOT SRS!


I have never had the cluster out before -so it turns out someone else in the past has inadvertently swapped the SRS & Failed lamp warning bulbs within my 500E's cluster.


It makes sense- what is actually happening when the headlamps are switched on is the Blown Bulb warning is coming on. (Not SRS)

Then going out again when the Headlamps are switched off. So I need to check / clean all the bulb connections & see what's going on. After I swap the warning lamps in the cluster around of course!

So for the Bulb failure warning it comes on with the sidelights even though there are no blown bulbs.

So now I am going to order up some quality new bulbs such as Osram and renew the Tail lamp bulbs, Licence Plate bulbs & the Sidelamps themselves. And make sure the holders are all clean & corrosion free.

I do hope this makes that warning lamp go out. I would add that my car still has stock w124 headlamps fitted not 500E items. I will be buying new 500E lamps soon but don't know if someone jacked up the wiring to fit the stock lamps- it's a possibility.

However- the headlamps fitted do behave like 500E lamps. They put the driving lamps on for example when the main beam is selected. Whereas on stock w124's these are just fogs. however they have no chrome / clear glass so they ain't E500 lamps.

Flooby- do you know if the interior instrument illumination bulbs would also be able to trigger the Bulb Failure lamp?

Again- I have already replaced any blown bulbs in the interior but maybe I'll need to take a closer look if the new outside lamps don't make the warning lamp extinguish
 
#5 Ā·
Make sure your tails are making good ground. When those plastic tabs tabs break and the grounding bars get loose, it will cause the BOW. Took me a long time to figure that out on mine. I eventually replaced the assemblies, but my short term fix was to drill holes in the interior covers and wrap zip ties around the grounding bars to hold them tight.
 
#6 Ā·
Well I have got nowhere with the fused warning bulb coming on with the sidelights- I have done the following:

Renewed both License plate illumination bulbs. This included very thorough cleaning of all the terminals / holders and extra zip ties to keep really strong connections on the tail lamp cluster bushbars. New High quality exact OE spec bulbs fitted.

Although the bulbs actually ā€˜lookā€™ brighter in there now ā€“ warning lamp still on.

Swapped out the Bulb monitoring Module for one from my E36. Part number of the E500 unit was 126 545 03 32. E36 item was 126 545 00 32. Pins were all the exact same.

No good- warning lamp behaves exactly as before. (Comes on as soon as the sidelamps are switched on)

Moved on to the front headlamps. Found that the driverā€™s side plug was badly corroded. Cleaned that up. Inside both front headlamps the sidelamp bulbs were also corroded. The sidelamps are the only new bulbs that have not arrived yet. So I cleaned the old ones and put them back in until the new sidelamp bulbs get here.

Warning lamp still on.

I'm fairly certain now the car has E500 Headlamp housings - with stock w124 lenses fitted. (To correct the beam pattern for UK roads)

I guess Iā€™ll fit all new bulbs to both headlamps tomorrow night and see how it is after that. The new E500 headlamps will come very soon but I need to limp it through MOT with these ones in 72 hours!
One other thing ā€“ the warning buzzer for the headlamps being left on comes on all the time if the door is opened. Ie; even with the Ignition on and the car running ā€“ if you open the driverā€™s door with the sidelamps or headlamps on it buzzes constant. Hugely annoying! I think itā€™s only supposed to come on if the ignition is off, headlamps on & the door is opened.

I presume the buzzer is relay controlled somewhere? I cannot locate it on wiring diagrams- so not sure where to look?

That could be linked to the warning lamp coming on all the time, if a module / relay has failed -maybe the system is very sensitive and itā€™s detecting that abnormal current somewhere.
 
#7 Ā·
I went and looked out one of my spare clusters. Connection the wires like this makes the headlamp buzzer sound (Itā€™s in the cluster!)

So since this part looks to match the 500Eā€™s cluster I will swap this part out & see if it makes any difference. It has a relay soldered on the back ā€“ presumably for the headlamp buzzer function. If the signals to the cluster are AOK then it has to be it!
 

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#8 Ā·
Well itā€™s been a while since I posted about this issue - but my 1992 500E still has the bulb failure warning lamp coming on as soon as either the sidelamps or headlamps are turned on. Then goes out as soon as the HL switch is turned off.

The car has had the following fitted-

Brand new E500 Genuine Mercedes Headlamps.
All new bulbs everywhere on the car ā€“ high quality OSRAM items of correct sizes / wattages.

But still this problem persists. Where should I start to look next? I did see green corrosion on the headlamp plug pins before when I had the dull headlamp problem I cleaned those up. But I am wondering now should I also renew those pins in the event of the system sensing a fault in the pins.

Also ā€“ I should mention I did replace a good few blown bulbs in the dash board ā€“ all illumination bulbs are working now. But I presume these are exempt from the Lamp Failure lamp.

Any suggestions on how to resolve this would be gratefully received. I have a multimeter & am willing to work :)
 
#12 Ā·
Ok this is the video just showing how persistent this blown bulb lamp is.

Yes Jayare, the license plate bulbs are new too. And every bulb on the car works as it should. I've also swapped the Bulb Warning Relay but it behaves the same.

I just noticed when doing the video that sometimes the dash illumination drops out on the HL rotary switch. If I jiggle it they flicker on so it's an issue with the actual HL switch regarding that (brand new!) problem.

 
#16 Ā·
The car did pass the MOT that time (This thread started months ago) but the issue is still not resolved with the fused bulb warning lamp coming on with Sidelamps / Headlamps.

It makes sense to first swap out the HL switch when the new one gets here ā€“ then see what happens (But Iā€™m not holding out much hope)

To recap this is the status-

A; Both front headlamps are Genuine Mercedes brand new E500 units
B; The bootlid wire harness was re-built by me as it was faulty. (Boot lamp on constant). This is a new section of loom into the bootlid and works correctly now.
C; Both rear lamps clusters were cleaned & zip ties added to the bush bars for 100% steady connections. And all new high quality bulbs in the whole car.
D; All new copper ceramic fuses fitted,
E; Eco junk is replaced with brand new Genuine MB harnesses (upper & lower).

Note my comments previous that the HL warning buzzer come on when the driverā€™s door is opened even if the ignition is still ON and the car running. It should not do that ā€“ I am sure this is related somehow......

I trawled through WIS & found the information attached in the picture.

So I know the interior light circuit is not included ā€“ this is good. But why to hell is the car thinking there is a blown bulb on the TAILAMP ā€“ SIDELAMP circuits? I know this because the warning lamp comes on as soon as the sidelamps are selected & stays lit. This includes when the headlamps are on but I think the Sidelamp / tailamp circuit is where the car thinks there is an issue.

If the new switch does not cure it this is what I am thinking-

1; Find & clean the earth points relating to the HL circuits.
2; replace the HL plug pins incase it is sensing corrosion there.
3; Pull the warning relay & test the pins to each component ā€“ checking for shorts etc I guess.
4; Check that someone has not added a circuit somewhere on the Taillamp / HL circuits.

Iā€™d like to hear what Floobydust / LCG thinks?
 

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#17 Ā·
OK ā€“ I think I have just stumbled across the fault. I decided to start up the 300E-24 & 500E side by side & turn off the lights to compare tailamps.

BUT ā€“ look at the difference in License Plate illumination. 500E on right is much duller. There is a fault there! The pic is the bulbs I fitted. (They are correct I think) When pulling both lamps on the 500E & using a circuit tester something strange happens....

Applying + with the tester to the RH license lamp it lights up brightly. The left one not so much ā€“ it very much seems like a short when trying to apply power to the + terminal. The bulb will not alight when connected in. It's almost as if when both License plate bulbs in are place they run in parellel circuit hence the duller operation.

Happily ā€“ with this left license plate bulb removed the warning light goes OFF at last! But obviously have work to do. I think I did not go deep enough into the carā€™s trunk casing the defective wire harness. Now I know where the fault lies I will pull the boot lid loom again & re-make as far as I can reach into the car. And get a circuit diagram to compare whatā€™s going on.

I shall keep you posted
 

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#18 Ā·
Do you have and special bulbs that are not mb recomended brands . In this i mean any place on the car ..I had this problem with a Volvo i had after fitting up rated bulbs , it took very little amperage to light the bulbs up as the wattage was that low. And so it lit the bulb out light up on the dash. And as for the dull light you have , can you fit another earth lead to it .
 
#19 Ā·
I did check the earth connections but they were fine. I believe there is a short on the LH lamp power cable. On checking wiring diagrams the 2x power cables feeding the license plate lamps come directly from the bulb failure monitor.

I have to find where the fault is in these license plate wires. If need be I will run 2x new cables the whole way.
 
#21 Ā·
re bulb types, the one thing I've consistently found on european cars (vw, volvo, mercedes at least) is to use CHROME base bulbs, NOT brass based ones for the standard bayonet style turn/brake/reverse/parking lamps.... and always replace both bulbs with the matching new ones.
 
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