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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4971
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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Turned ignition several times finally started but the dash

Customer Question

hey there turned ignition several times finally started but the dash board went hay wire all lights flickering, gauges dead and engine went to only firing on about 2 cylinders ???
JA: Have you checked the fuses? The dashboard bulbs?
Customer: ECM ?
JA: What is the model/year of your Mercedes?
Customer: 91 500SL
JA: What year is your 500SL?
Customer: 91
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: checked all the small fuses, all good
JA: Anything else you think the mechanic should know?
Customer: Is the computer fried ?? why is engine missing and dash going hay wire, is this an indication of anything??
Submitted: 1 year ago.
Category: Mercedes
Expert:  Lou P. master tech replied 1 year ago.

Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

Expert:  Lou P. master tech replied 1 year ago.

ok the first thing i need to know is,

  • what is the battery voltage and are the cables tight
  • was the car recently washed or wet
  • when was the last time you drove the car and how many miles do you average per year?
Customer: replied 1 year ago.
Aloha Lou,the batt V is 12.3, and cables are titecar has been sitting for about a year, not wet under the hoodstarted and drove car just before electrical freak out, everything was good beside the convertible top lock alarm stayed on indicating that the one of the locks for the hard top was not fully lached I assume
Customer: replied 1 year ago.
battery just replaced actually
Customer: replied 1 year ago.
during intial start up and drive, engine seemed to be missing on maybe one cylinder, then I smelled a strong fuel burning smell after car sat at idle for 5 minI shut It down, put fresh gas in it, then after 5 cranks, the electrical freak out happened ?
Customer: replied 1 year ago.
the burning I want to say smelled like old fuel, but maybe electricalI can use a OBD I , I just need a schematic of the hook ups and fuse and relays if u could find for me
Expert:  Lou P. master tech replied 1 year ago.

READING NOW

Customer: replied 1 year ago.
when the ignition is switched to on the dash lights stay in a constant flicking state, not flashing but an erratic flicker
Customer: replied 1 year ago.
thank you for your help
Expert:  Lou P. master tech replied 1 year ago.

ok, so if the car sat for a long time i am worried that maybe mice or similar have chewed wiring. how much fuel was in the tank when it sat for a year?

Customer: replied 1 year ago.
when I first started the fuel tank was close to E and guage worked
Customer: replied 1 year ago.
i was thinking mice too, i found a small nest under air filter during cleaning
Expert:  Lou P. master tech replied 1 year ago.

ouch, and the low fuel level can hurt too. we can have a lot of varnish in the tank. so you may want to pull the spark plugs and make sure they are not wet and fouled.

Customer: replied 1 year ago.
ahh ok makes sense for the misfiring hopefully
Customer: replied 1 year ago.
can i get a wiring schematic from you guys here of this year model ?
Expert:  Lou P. master tech replied 1 year ago.

yes and no, the way the wiring is laid out its by group. not like an american car where its one wiring schematic, we have about 100 lol.

Customer: replied 1 year ago.
oh ok, maybe just the ones that you think apply to my situation then. Also a pic or schematic of where i hook up my OBDI
Expert:  Lou P. master tech replied 1 year ago.

send me a picture of your engine bay from a birds eye view and also of your scanner.

Customer: replied 1 year ago.
I am at work now, and i don't the reader yet have to order one, i just need to know where to hook it up
Customer: replied 1 year ago.
it is a Mercedes OBD ONE jumper wire style reader
Customer: replied 1 year ago.
i think it blinks for the fault codes, but if i could get a location of where to check for flickering dash lights and engine misfire like a relay or fuse location that would be helpful
Customer: replied 1 year ago.
or where the wiring comes from that controls the dashboard
Expert:  Lou P. master tech replied 1 year ago.

well, i need to know if you have the square diagnostic port for the impulse counter or the round diag port.

Expert:  Lou P. master tech replied 1 year ago.

and in this model the fault codes wont be as good as the newer cars so we would be better off with basic electrical checks.

Customer: replied 1 year ago.
ok sounds good.the reader uses wires with pins to go into a connector that is upper engine bay passenger side that's all i knowold style pin plug reader
Customer: replied 1 year ago.
it only shows a sequence of blinks for codes
Customer: replied 1 year ago.
is there a way to reset or test the main computer. like ECM or relay somewhere on this model
Customer: replied 1 year ago.
i disconnected battery and problems remained
Customer: replied 1 year ago.
this is recommended : MB STAR system with HHT connector
Customer: replied 1 year ago.
connects at a rectangular block with (16?) numbered holes near the firewall on the passenger side of the car
Expert:  Lou P. master tech replied 1 year ago.

lol, easy easy, i know what you are talking about, that is why i want to see your engine bay to make sure you have that connector. also some had a button you press on the block to make a led flash the codes right away without a scan tool. or blinker counter.

Customer: replied 1 year ago.
ohh ok, that would be good. Not sure tho i have to get back to you
Customer: replied 1 year ago.
would this explain a flickering dash?
Expert:  Lou P. master tech replied 1 year ago.

i am not hopeful it will, like i said the older ones were more for emissions and engine sensors, not so much for diagnostics of all electrical items so lets see what we can find before getting a scanner.

Customer: replied 1 year ago.
can the electrical plug for the hard top cause a fuse to blow or relay to trip
Customer: replied 1 year ago.
is there a procedure for this 2 prong wire to remain connected and does it affect operation if it is not?
Customer: replied 1 year ago.
the wire is just behind the driver door on the inside of car on the side wall ?
Customer: replied 1 year ago.
can i get a procedure for this
Expert:  Lou P. master tech replied 1 year ago.

that wire is for the defrost for the hard top when installed.

Customer: replied 1 year ago.
ahh ok, if that is disconnected does it cause any errors
Customer: replied 1 year ago.
if i disconnect the ECM will it reset these problems ?
Expert:  Lou P. master tech replied 1 year ago.

no lol, if it would i would have told you.

and no, if you disconnect the ecm it will do nothing.

Customer: replied 1 year ago.
ok cool
Expert:  Lou P. master tech replied 1 year ago.

ok so first we check the spark plugs to see if they are wet and fouled then we may need to flush the fuel system.

also take a sample of the fuel.

does the dash flicker just when turning the key on?

Customer: replied 1 year ago.
awesome! will do
no only 3 red lights on in auxillary, then as soon as i turn to start they go into irradic flicker and stay like that when the car starts
Customer: replied 1 year ago.
car starts every time now right away but dash is irradic
Customer: replied 1 year ago.
i could be confusing this tho, the lights may also be erratic in aux mode as well,?
Expert:  Lou P. master tech replied 1 year ago.

ok so we need to also check the voltage when the car is running. make sure its not over 14.6

Expert:  Lou P. master tech replied 1 year ago.

i have to leave for a bit. i will check back later.

Customer: replied 1 year ago.
ok thanks for the input on my issue
Customer: replied 1 year ago.
apparently fuse 9 is for the instrument cluster/dash
Customer: replied 1 year ago.
is this correct?
Customer: replied 1 year ago.
hey brada you still there ?
Customer: replied 1 year ago.
can i get another tech
Expert:  Lou P. master tech replied 1 year ago.

Sir I will opt out.sorry for the delay. I have small kids I need to tend to for a few hours.

Customer: replied 1 year ago.
thanks lou have a good one