Manual Transmission w113 with Emission Control harness

[lpeterssen]

Dear Friends

Just completed another interesting project on a w113 280SL harness.  The work was completed in record time as per requested, doing an average of 12 intense hours a day for 4.5 days.

I want to share some insight related the EMISSION CONTROL HARNESS which was new to me, and after reading many literature published here at the Technical Manual section, I think it is not clearly expressed there.

This was a manual transmission car, I am not aware what year it is.   The diagram attached below (not exactly a wiring diagram since cable colors, and gauges are not detailed) matches perfectly what I found there.  This page was extracted from Pagoda forum technical EMISSION CONTROL SYSTEM manual, page 3.

Here some insight in order to understand better this harness.

1. Ground loop.  There is a ground signal loop that pases  through different components to tell this early ECU (RELAY BOX) that part of the conditions have been met in order to activate the fuel shut off solenoid.
2. Ground loop route. 
     2.1.  First a ground signal is connected in parallel to 3rd and 4Th gear switches.  These are Normally open
             Switches that close only when the particular gear is engaged.  So if either the 3rd or 4Th gear are
             Engaged, the ground signal passes through them and goes to next element or condition in the
             Route
     2.2.   Clutch pedal switch.  This is the next component in the ground loop.  After passing 3rd or 4th
              Switches, ground signal comes to the clutch pedal.  If the pedal is not pressed, the ground
              Continues its travel to the IDDLE SWITCH
     2.3.    IDDLE switch.  The final component in the conditional ground loop is the IDDLE switch.  So if the
              User is not pressing the accelerator pedal, this IDDLE SWITCH should be closed, and then Ground
               Can pass through it, and finally exits this component through a brown/black cable, which enters
               relay box on PIN number #1.


3. Temperature signals.   As indicated in all the manuals, there are two temperature switches.  Both of them have as output signal a ground pulse.  One is the 100C switch above the thermostat housing, and the other the 17C switch on the cylinder head next to the fuel shutoff solenoid.  I understand these switches are Normally open.  The 17C switch closes giving a ground signal if temperature is below 17C.  The 100C switches closes if temperature is above 100C.

4. Speed RPM signal.  This cable on the case of the harness just rebuilt was a green coaxial cable.  Has two “0” shaped terminals at one end.  This side goes to the COIL.  The inner connector goes to coil terminal 1, which I understand is the one marked as POSITIVE.  The other end which is the external coaxial jacket for electromagnetic interference, goes to chassis.  Same thing at the other end of the cable going to the speed relay.


What is Normally damaged on this harness.
===========================

I found that all cables that were routed above or on engine side were all toasted.  So my advise for all of you will be:

A. Replace leads going to both temperature switches
B. Replace lead going to fuel shutoff solenoid behind injection pump
C. Replace the ground loop signals going to 3rd and 4th gear switches.
D. Replace the ground loop for the clutch switch


That will fix most of the problems.


Any questions, send me a PM.

Best Regards
Eng. Leonardo Peterssen

[roymil]

I just ran across this post since I'm rewiring some things and still searching for the 3rd and 4th gear switches that live on top of manual transmissions.  Mine are frozen up.  No luck at the usual sources and wondering if you might know any details about these switches, their connector types, or brands.   appreciate any tips, thanks.

[lpeterssen]

Dear roy

Check that any of those switches has continuity or is an open circuit all the time

The connector type is standard 2 prong female as the one used on door switches or in the brake pedal switch

Best regards
Lp

[lpeterssen]

Of course check for continuity of those switches when transmission is engaged either on third or fourth gear

[BHap]

I was able to loosen the 3rd & 4th gear switches with some PB Blaster sprayed from under the car.  They sit on top of the transmission and face each other.  When 3rd or 4th is engaged, the end of the shift linkage presses in the switch which and when the throttle is closed, activates the fuel shutoff solenoid.  When a switch sticks, the system thinks you are coasting in 3rd or 4th anytime the clutch is released and the throttle is closed and shuts off the fuel - the car will not idle with the clutch released.  I have my fuel shutoff solenoid disconnected until I can get the switches off the transmission and cleaned up - I don't trust the "easy fix".

[lpeterssen]

Yes do the job right,

Remember the fuel shutoff activates when all this conditions are met at the same time

1. RPM Above a certain range, unknown to me. Should be 1,500 or something like this since EC can not be active when car is on Idle (750-800 rpm). Signal given by speed relay, according to coil readout (rpm)

2. IDDLE SWITCH at intake manifold has a closed butterfly, and therefore gives continuity on the IDDLE SWITCH

3.TRANSMISSION is engaged on 3rd or 4Th gear. (CONTINUITY Among any of those switches)

4. CLUTCH PEDAL is not pressed giving CONTINUITY among the Clutch switch


So check all those components since car can not shutoff by itself if you are idling