94 e420 check engine light LED issue
 

Hello, I am new to this site and somewhat new to the gas benz world. My check engine light came on last week with no noticable symtoms. I tried to check and reset the code using the built in LED. The instructions I was following was to turn the key to the on position, hold down the button for 2-3 seconds and it is supposed to flash the code. When I turn the key to on, the LED is on continuously, and the button does nothing. I am in the proccess of building the radio shack code scanner, but shouldn't I be able to get the diagnostic module codes from the built in LED? Any help would be greatly appreciated.

thanks,
Will John
94 E420
82 300DT

You do realize the led is next to the push button?????

yes

OK

..so , are you saying the led is ON without pressing the button??

or are you saying the led lights , but does not flash when you hold the button??

the LED is on continuously when the key is turned to the on position. Do I need to be trying this with the key in a different position?
thanks for the replys

NO

The lite only comes ON when the button is pressed and the key is ON

See if you have 12v at pin 16 and ground

Now it is flashing when the key is on, without pressing the button. i will check the voltage on pin 16 at lunch.

the LED is still flashing when the key is in the on position, with or without the engine running.

I do not have a test light or meter with me, but I will check it as soon as I get home. My diagram shows pin 16 to be the AC push button control module. How would this affect the Diagnostic Module?

<<My diagram shows pin 16 to be the AC push button control module. How would this affect the Diagnostic Module?
>>

My mistake.. I thought you had a E320. [ 16 pin socket]

How many miles, and more to the point, have you replaced your wiring harness yet?
No other symptoms besides the check engine light?

I'm interested in the "continuous on" LED part, my friends 88 300E I've been dealing with does this, the LED comes on as soon as the key is turned on, and stays on. I am however able to read out the codes- #'s 4 and 5, o2 sensor and the air mixture housing, but can't seem to reset/erase them. This car is a california model.

The car has 95k miles, it was originally sold in Hawii, then sold to a guy in Las Vegas, who I bought it from. He has dealer receipts for the wireing harness, ECU, and throtle position sensor. All of which was done between 55k and 70k. I bought it in April with 82K. The LED was on constantly one day and now it is flashing with the engine running or not when the key is in the on position. I dont know how to get any codes while it is doing this.

The only symptom is 2 or 3 less MPG. It starts and runs no different.

Reading Fault Codes

- Turn ignition key to “on” position, all dash warning lights will be on.

- Remove cover on 16 plug diagnostic connector near battery.

- To read check engine light codes (DM), hold down built in button next to LED light for 3 to 4 seconds, LED light should be on, release button, count the number of LED flashes that follows. Hold button down for 3 to 4 seconds again to check for another code. Keep doing this until all codes have been read and first code comes up again.

- To clear codes - hold down button for 10 to 12 seconds, turn ignition key off for a few seconds, repeat to clear additional codes. Ignition must be turned off between each code cleared.

To check other modules using code tester – insert black plug into socket #1 (ground). Insert red plug into socket #16 (12V power). Insert yellow (third color) plug into socket to be tested, for example #8 or #14. Repeat steps above for LED button using code tester button and LED. Same process for deleting codes as with LED button.

Check appropriate fault code list to determine fault.

Checks codes as quickly as possible at the first sign of a problem. Codes spread from one module to another and will often show up twice or cause other codes. The sooner you check the codes the more likely you are to identify the source problem.

OK so I finally built the home brew LED switch. I got some codes for the ABS module, Base Module, and SRS. I was able to reset them all except for the SRS which I have read is digital so that would make sense. CEL is still on and the built in LED/SW is still blinking.

Heres the wierd thing. when i pluged the yellow test lead into pin # 16 (O2S1) the LED lights up faintly. What does this mean? Posibly some short circut? Once again thanks for all the help.

Will
94 E420
82 300D