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1990 500SL
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Discussion Starter · #1 ·
I have a 90 500SL and my alarm isnt working at all. i can reach in the window with the doors locked via transmitter and unlock the doors and open it. no alarm will sound. any one have any idea where i should start looking for problems? i have already checked all fuses.

Thanks,
Devon
 

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1990 500SL
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Discussion Starter · #3 ·
i waited a hour lol
 

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2005 SL600, (2) 2003 SL500
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Have you tried opening both left and right doors? And have you tried opening the hood? If "yes" to these questions and still no alarm, try locking the car with the key at the driver's door and then check for an alarm when you open. Repeat for the passenger door and the trunk locks.

Finally, check fuse #5 in the trunk very carefully, since it powers the ATA module...It's behind a black plastic cover by the right trunk lid hinge.
 

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Strange this. My '90 300 doesn't have an alarm as far as I know. Was it an option, or only standard on 500's?
 

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All North American cars have a security system, and I think it is standard on all others as well. If you have a module in your trunk like the one pictured (N26), then you have the system. If you have one that does not work, it is simple to troubleshoot. For help I would suggest starting a new thread.
 

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1990 500SL
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Discussion Starter · #7 ·
tried it from both doors and hood. no alarm..../?
 

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Next step I would try is to check the Anti-theft Alarm system for diagnostic codes. Read codes, erase all of them, repeat several of the actions which ought to trigger an alarm, and then go back and read codes once more. Report back what you find.

For this test, the diagnostic socket is #11, battery voltage is #16, and chassis ground is #1. I have attached a picture of X11/4 on your car. Refer to the general procedure which follows.

The code retrieval system generally works up to model-year 1996. After this time it is necessary to have an advanced scan tool to read most systems.

The diagnostics are wired to sockets of the car's main "diagnostic connector", X11/4. Early cars with continuous-fuel injection have 16 sockets, while later models have 38 sockets. The sockets are numbered, and each system has a single, dedicated socket for its diagnostics. For example: on a 1990 SL, socket #3 is for the fuel injection, #7 is for the roll bar controller, and #8 is the ignition.

Codes must be read one at a time. That is, you ask the system to issue a code, you read the code, you ask for another code, and so on. You ask a particular system for a code by electrically shorting its diagnostic socket to ground for 2 to 4 seconds. A second or two after the short is removed, a series of electrical pulses will appear at the socket. The number of pulses corresponds to the number of a code. The ignition must be switched-on for the code reading process.

Electrically, the voltage on a socket is normally high. Of course, with the short to ground in place it is forced low. Once the short is removed, the voltage momentarily returns high. The pulses which follow are transitions from high voltage to low voltage.

There are many ways to view the pulses for counting and reading codes. The simplest and recommended method is to connect an LED in series with a resistor between battery voltage and the diagnostic socket. I like to use an LED from Radio Shack (part 276-209) because it has the resistor built-in and costs less than $2. If you use a different LED and an external resistor, the value of the resistor ought to be between 470 and 2.2K ohms.

For convenience, the X11/4 connector has a socket wired to battery voltage and another wired to chassis ground. Also, you may wish to get some banana plugs for making connections.

Once you have read all of the codes, you may wish to go back and erase them. They are erased individually one at a time. To do this, read a code, wait a few seconds, and then short the diagnostic socket to ground for 6 to 8 seconds. It is my experience that when a code is erased, it is acknowledged by a single pulse on the diagnostic socket -- though this may not be the case for all systems.
 

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1990 500SL
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146 Posts
Discussion Starter · #9 ·
i tried the procedure for retrieving codes, and i get nothing at all! i checked for power at the module in the trunk, and there is power.... could the module be bad?
 

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2005 SL600, (2) 2003 SL500
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Could be a bad module...Is it possible your method for code reading was wrong? I should have mentioned that you have to observe correct polarity when connecting that LED (one of the two leads is probably longer than the other...It would be the "+" that goes to battery voltage). To double-check, are you able to read anything from the fuel injection (socket #3)?
 

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1990 500SL
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Discussion Starter · #11 ·
yea i retrieved codes from all the other ones. when i plug in the led it has a steady low glow and thats it. it wont communicate at all.
 

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I have a '91 with the same symptoms, and the problem is a bad controller..[:(]
 

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1990 500SL
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Discussion Starter · #13 ·
checked with dealer yesterday, the computer is $389.99.... now i have two bad computers...... bad AMP and bad alarm controller.... (amp loses left channel periodically)
up to 94 would have the same controller correct?

Devon
 

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'93 SL320 & others
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bobterry - maybe this question isn't relating to this thread, but I've noticed from reading every new thread that you know a lot of about diagnostic codes.. does every R129 have the ability to hook up one of those code finders? I have a '93, someone told me at another forum that before '94 they weren't equipped to handle diagnostic code readings?
 

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Devon: I'm going to try to repair my alarm controller. I won't get to it any time soon though, and if I make any progress I'll send you an email.

Regarding the amplifier cutting-out, you may know that it is a small box located behind a plastic panel in the passenger footwell area. Also, it only drives the woofers, as the radio amplifier drives the tweeters and midranges. If your amplifier is the problem, it should be an easy fix. Email me for help.

SwishaSwede: I believe the information on the other forum is incorrect and that a $.99 LED will read codes on all available systems on every R129 through model-year 1995. What is more, for the system which requires diagnosing the most -- the convertible top -- you don't even need the LED, because the lamp inside the convertible top switch blinks out the codes (up to Jan. of '94). I think it is unfortunate that many owners are not aware of this. Maybe there should be a "sticky" at the top of the forum about it.
 

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1990 500SL
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146 Posts
Discussion Starter · #16 ·
bobterry- that would be great!

regarding the amp, yes i found it, but theres somethign wrong with either the harness or the plug into the amp, becuase if you jiggle the harness it works again but it seems to be making connection everwhere i see. im guessing a broken wire inside the harness
 

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RE radio amp erratic

unlikely to have wire broken in harness. much more probable to have bad connection at connector or cracked circuit board.
 
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