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  #1  
Old 02-06-2015, 05:52 PM
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Join Date: Sep 2006
Location: Charleston, WV
Posts: 61
1987 420 SEL no current at EHA

Hi All,
The car I am working on is a 1987 420 SEL as the thread title suggests. Recently, I had the fuel distributor professionally rebuilt. It came with instructions saying to adjust the fuel mixture with the 3mm hex screw with the engine fully warm. I hooked up my digital meter set to read mA (was looking for a reading of around 1-3 mA as indicated in the instructions sent with the rebuilt FD). Upon doing this I cannot get any reading on my meter with the car idling and the leads connected to the EHA harness (connecting into the wire closest to the FD). Why would this be? I already tried replacing the OVP relay with a new one thinking that would power the EHA, but that didn't make a difference. Where does the EHA get power from?

Thanks in advance,
Dave
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  #2  
Old 02-06-2015, 07:46 PM
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Join Date: Feb 2011
Location: Modesto CA
Posts: 2,617
Quote:
Originally Posted by Benz124s View Post
Hi All,
The car I am working on is a 1987 420 SEL as the thread title suggests. Recently, I had the fuel distributor professionally rebuilt. It came with instructions saying to adjust the fuel mixture with the 3mm hex screw with the engine fully warm. I hooked up my digital meter set to read mA (was looking for a reading of around 1-3 mA as indicated in the instructions sent with the rebuilt FD). Upon doing this I cannot get any reading on my meter with the car idling and the leads connected to the EHA harness (connecting into the wire closest to the FD). Why would this be? I already tried replacing the OVP relay with a new one thinking that would power the EHA, but that didn't make a difference. Where does the EHA get power from?

Thanks in advance,
Dave
To answer your second question first; the EHA is driven by the engine control unit (ECU).

Re: Test lead connections
The leads must be in series with one of the two wires connected to the EHA, rather than across both of the wires, which would be a parallel connection. To do this, a jumper is placed between harness and EHA on one leg, and the meter leads connected to the remaining pin and socket. If connected in series, good. In that case, the absence of a current points to the ECU, or the wiring itself.
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  #3  
Old 02-07-2015, 10:44 AM
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Join Date: Sep 2006
Location: Charleston, WV
Posts: 61
Hi Frank,
I wired the meter just as your said in the second setup, in series. I guess now the only thing to do is go through a wiring diagram to see which ones go from the ECU to the EHA and check continuity. I may be able to borrow another ECU to try as well.

Thanks for the reply.
Dave
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  #4  
Old 02-07-2015, 11:16 AM
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Join Date: Feb 2011
Location: Modesto CA
Posts: 2,617
The wiring colors for the EHA are black (to pin 12 on ECU), and brown/black (to pin 10).

The ECU is powered through the OVP relay, so yes, eventually the EHA is also powered through the OVP.
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  #5  
Old 02-07-2015, 11:18 AM
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Join Date: Jun 2013
Location: PNW
Posts: 242
There should be two connections.

EHA 1 ---------------Connector going to CIS-E engine computer.
EHA 2 ----Meter-----Connector going to CIS-E engine computer.

Does not matter which wire the meter is in series with, you would be able to see "amperage" changing. Also the car needs to be warmed up to 80C. (running temperature) for the system to be in (closed loop) and so you will see the meter changing.

The CIS-E gets its power from the Overvoltage Protection Relay (sort of like a surge protector).
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