Pin #3 Voltage Readings? - PeachParts Mercedes-Benz Forum

87-300E_in_NC · #1 12-29-2003, 11:50 PM

Does anyone know what pin #3 at the service connection should read with ignition on and a fully charged battery? Battery voltage is at 12.41 with radio playing, at 65 degrees.

My readings at service connection:
Pin #1>11.48 volts
Pin #2> 0.00 volts
Pin #3> 3.76 volts >>seems low from what I've read in other posts...
Pin #4> 12.33 volts
Pin #5> 12.33 volts
Pin #6> 12.37 volts

Readings at TDC (crank) sensor plug: 794 ohms

Voltage reading at EHA plug (disconnected)> 4.14 volts

Fuel Pump Relay:
Pin # 1> 2.18 volts
Pin # 8> 12.40 volts
Pin # 9> 12.31 volts
Pin # 10> 11.47 volts

When the fuel dist. was checked under pressure, it was damp, and wet each time when depressed by thumb. Actually the stem/seal fit is loose...very loose, where the stem travels through the adjustment/locking cap. Maybe someone rearranged this fuel dist. before I purchased the automobile?.....the locking washer showed evidence. If I need another fuel dist. for parts (to obtain correct fitting to the stem locking cap), would someone have a bad fuel dist. for parts? Will make more tests tomorrow and post them here. Thanks for everyone sharing info, especially to Steve!

pesuazo · #2 12-30-2003, 12:12 AM

The ideal is to measure duty cycle, but you can take an average of voltage to know your duty cycle. This information I copied from some article, I am not to be credited for it:

"Mercedes has chosen the less common definition of "duty cycle" in this case. They are referring to the percentage of the entire pulse period during which the voltage is zero, not the time when it is near battery voltage (mine was +13.6 V when the battery was +14.0 V). In other words, if the pulse rests at ground for 7 milliseconds and then rises to +13.6 V for 3 ms, the duty cycle is considered to be 70%.

If you have an oscilloscope to measure this timing, fine. If not, it can still be estimated with a voltmeter. Since continuous "0 volts" would be considered 100% and continuous 13.6 V would count as 0%, just measure the voltage between pins 2 and 3 of X11 and divide that by 13.6. Next subtract that ratio from one, and convert the result to percent.

For example, if the meter reads 4.0 V, first divide 4.0/13.6 = 0.294. Subtract 1.0 - 0.294 = 0.706, or 71%. In equation form:

Duty Cycle = [1 - (V{pin 3}/V{max})] x 100%

Remember that if the oxygen sensor is doing its job and the system is operating closed-loop, the reading will jump around, so you might see readings from 5 V to 7 V and have guess at an average"

Sorry we could not meet on Sunday, @ what time will you be working on the car?
I may be able to meet with you on Tuesday. (I am on vacation)

pesuazo · #3 12-30-2003, 12:14 AM

And here is the rest of the pins on the diagnostic socket:

Pin Function
1 RPM signal, 0 to 12 V, 4 pulses per revolution. A frequency of 43.3 pulses per second corresponds to the desired idle speed of 650 RPM. (To check your tachometer you could measure the frequency at this pin and multiply by 15 to get RPM.) This signal is derived from the ignition amplifier and is synchronized with the low side of the ignition coil.
2 GROUND
3 Lambda on/off ratio, 0 to 13.6 V, 100 Hz. This is not a replica of the output of the oxygen sensor. The "duty cycle" in Mercedes literature refers to the percentage of time this signal is low. Longer duty cycle (lower average voltage) = lean, shorter = rich.
4 Low (negative) side of the ignition coil. This is where to monitor the dwell angle of the electronic ignition amplifier, and is also a good place to view a reflected and attenuated image of the spark. This is about 50 VPP, safe for an ordinary oscilloscope.
5 Hot (battery voltage) in Run or Start, NOT FUSED! This is actually the positive side of the ignition coil.
6 Hot at all times, through fuse 2
7 Shielded cable from TDC sensor. This is not the same sensor which triggers the spark control unit, but it could be used in a clever circuit with pin 1 to measure spark advance.
8 Shielded cable from TDC sensor.
9 Shield for pins 7 and 8.

87-300E_in_NC · #4 12-30-2003, 12:38 AM

Hey pesuazo,

Thanks for the reply, and the valuable info!

It's been a while since I had a good working oscilloscope, but I am in the market now, huh? LOL! Well, I have a DMM so I can make the best of it.

I would like to check your voltage readings and compare them with mine for a start, and then move on to more tests. It would be great if we could get together for a comparison, and then I would post the info here for others as well.

My fuel dist. is currently sitting on my desk, as I have had it out for cleaning and examination, noticing the remarkable free-play in the stem and locking nut area (where fuel was present and would leak while depressing it with my thumb).

I'll send you an IM...in a minute

Thanks again for the info!

#5 12-30-2003, 01:56 AM

If 70% is your key on, engine off value, it's correct if your car is a non-Calif car.

If it's 70% with the engine running, you're running lean (but I doubt since you're experiencing flooding).

My $0.02.

Cheers.

pesuazo · #6 12-30-2003, 10:21 AM

87-300E_in_NC:
I have been calling you for the last hour, phone always busy.
Give me a call with the details to get there.

87-300E_in_NC · #7 12-30-2003, 11:06 PM

Hi pesuazo,

I'm back online now...I sent you a few messages (PM & Email). Would like to get together tomorrow, morning if that's good for you...my schedule is flexible right now.