We are hoping for some real data here!

Can you get your hands on a multi-meter of some sort? If you can:

1. Set function to DC Volts on a scale where you can read to nearest tenth of a volt.

2. After driving car (it needs to be good and warm), and with the car idling, measure the voltage across the + and - terminals of the battery. Remember the reading.

3. With the car idling, put the positive lead of the meter in Pin 3 of the diagnostic socket, and the negative lead in Pin 2. Remember the voltage reading.

4. Have someone bring the RPM up to 2500 and observe the voltage reading.

5. You can now compute your duty cycle (DC) like this:

DC in % = (Battery Voltage - Pin 3/2 Reading) / Battery Voltage * 100

Example:

Battery Voltage = 13.5
Pin 3/2 Reading = 7.5

DC = (13.5 - 7.5) / 13.5 * 100

Duty cycle = 44%

6. With engine at idle, oberve the Pin 3/2 voltage for a few minutes and see if it changes in a pattern corresponding to what the engine is doing.

Go for it! Let us know the results.

Note - if the meter has a Duty Cycle function, you can read the numbers directly. Every meter has a voltage function, though, so I decided to use this method.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I will have to see if I can find someone with a multimeter that good. the old analog one I have barely works under the best of circumstances. I have a buddy who has one if I can catch him over the weekend when I have daylight time to play with it.

I see from some of the other posts that you're somewhat of an expert on these FI systems.
Thank you again for your help.
John

Ok, I'm beginning to feel like a real idiot here (which may not be far from the truth). Where's the diagnostic socket? I looked on here for it, and my post is the only one that comes back with "diagnostic socket" for a 380.
Google wasn't much more help.
Thank you
John

It certainly is. That explains much.

Still can't figure the car out. regular troubleshooting is useless since it's inconsistent. This morning it surged all 16 miles to work. around lunch I drove it 8 miles and it purred like a kitten the whole way with nary a problem. 2 hours later, it died trying to go into a driveway. This evening, it drove home surging almost the whole way. After it had come up to temp, for almost exactly 1/2 mile, the surging stopped.

weird.

its the round capped thing on the left side inner fender well.

__________________


1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350