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  #1  
Old 04-27-2021, 08:43 AM
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1987 560SL Duty Cycle

My 17 year old son and I have been working on a 1987 560SL that we bought not running. We have been at it for almost a year. The car has not been running since 2004.

We basically replaced almost all the fuel system, timing chain, water pump, amongst several other parts and we have the correct fuel pressures and the car now starts and runs fine.

Although it takes a little while to start and seems to be running rich.

When I go take take the Duty Cycle with the ignition on and the car not running I'm getting 0 when it should be 70%.

Following the formula (duty cycle [%] = [1 - (Vp3 / Vp6)] * 100) the voltage at the X11 between 2 & 3 and 2 & 6, we are basically getting the same voltage on both. I also get same reading when the car is at idle.

If I use a meter with Duty Cycle I also get 0

I've been looking and looking and can't seem to find why. Does anybody know what it could possibly be the cause(s)?

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1987 560SL Duty Cycle-img_0629.jpg   1987 560SL Duty Cycle-img_0626.jpg  
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  #2  
Old 04-27-2021, 11:46 AM
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Location: Modesto CA
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Use the illustrated "Fluke 78 Automotive Meter" set on "dwell" scale between pins 2 & 3.
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  #3  
Old 04-27-2021, 01:08 PM
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I get 0 on everything. From my experience and what I have read, its supposed to be around 7V but i'm getting battery voltage instead between the 2 & 3
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1987 560SL Duty Cycle-img_0630.jpg   1987 560SL Duty Cycle-img_0631.jpg  
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  #4  
Old 04-27-2021, 02:17 PM
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CIS-E Fault Codes:
https://www.benzworld.org/threads/duty-cycle-error-codes.3017918/#post-17697988
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  #5  
Old 04-27-2021, 04:13 PM
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Quote:
Originally Posted by amilian21 View Post
Although it takes a little while to start and seems to be running rich.
When I go take take the Duty Cycle with the ignition on and the car not running I'm getting 0 when it should be 70%.
With the engine running (idle), is there oscillation of the meter reading when on the dwell scale?
[If the mixture is too rich to be compensated by the EHA the meter reading at idle will be "0".]
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Old 05-02-2021, 11:05 AM
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Ok so the problem was the ECU. now we have to do some more tests and see if we can get it running right since now it runs super rich
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  #7  
Old 05-04-2021, 11:42 AM
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Is there a pod where meter probes can be inserted to measure the duty cycle?
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  #8  
Old 05-04-2021, 11:46 AM
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on the x11 at ports 2 &3
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  #9  
Old 05-05-2021, 09:41 PM
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So we replaced the ECU. Now the ingtion voltage at port 3 is correct when we started and ran the car we got a 50% code which indicated a bad O2 sensor.

We replaced the O2 sensor and now the Duty Cycle oscillating between 3.9V to 6.58V with Battery Voltage at 13.33V.

Which, If I calculate it correctly, its bouncing between 50% to 70%.

Vp3 3.9 70.74%
Vp3 6.58 50.64%
Vp6 13.33
Duty Cycle Mean Value 60.69%

Any Ideas?
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  #10  
Old 05-06-2021, 12:36 AM
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Quote:
Originally Posted by amilian21 View Post
So we replaced the ECU. Now the ingtion voltage at port 3 is correct when we started and ran the car we got a 50% code which indicated a bad O2 sensor.
We replaced the O2 sensor and now the Duty Cycle oscillating between 3.9V to 6.58V with Battery Voltage at 13.33V.
Which, If I calculate it correctly, its bouncing between 50% to 70%.

Vp3 3.9 70.74%
Vp3 6.58 50.64%
Vp6 13.33
Duty Cycle Mean Value 60.69%

Any Ideas?
Basic mixture is a bit lean. Begin with 1/8th of a turn CW of the main mixture screw.
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  #11  
Old 05-06-2021, 09:52 AM
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Quote:
Originally Posted by amilian21 View Post
on the x11 at ports 2 &3
That's what I thought.
I think the reading there should be about half the full voltage from the battery. Probably if you put a scope in there, one might see square waves and might visually adjust the duty cycle.

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