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Emission Control System

Installed in model years 1970/71


Emission Control System


Reduces exhaust gases via timing change-over and fuel shut-off under deceleration conditions...

The following download shows how all devices interact. It was produced by Francisco Pereira do Valle who notes:

"I am attaching a diagram of the US version Emission Control System that I have built based on all the info I found on the subject. Please note that I am not an engineer, so I have used logical arrows to document the flows. Original drawings (electrical system) and pictures of components are also in the diagram so people can locate them in different environments."


29 – Working Current Relay

This relay controls the Fuel Shut-off Solenoid on the back of the Fuel Injection Pump

9 – Two-way Valve

This device is controlled by the Speed Relay, 17 degree and 100 degree switch. This device has 2 vacuum connections. The inner connection routes to the throttle body (vacuum). The outer vacuum connection routes to the distributor. This device controls vacuum advance/retard on the distributor under varying conditions (rpms, temps, etc…). When coasting above 2400 Rpms the 2-way valve influences the distributor timing which burns fuel more effectively and reduces emissions

4- Speed Relay & 5- Relay Box

These relays control the electrical “on/off” switching of the 2-Way Valve

17 Degree Switch

The 17 Degree Switch operates via the Speed Relay & Relay Box. The 17 degree switch (screwed into the lower section of the engine block by the ip pump) helps to warm a cold engine by increasing the idle (increases rpms) via the 2-way valve. This switch operates via the emission control devices (Speed Relay & 2-Way Valve) and thus rendered useless if the emission control devices are removed.

100 Degree Switch

The 100 Degree Switch operates via the Speed Relay & Relay Box. The 100 degree switch (screwed into the top of the radiator thermostat housing) increases the idle (via the 2-way valve) when engine gets hot. Basically, it makes the radiator fan spin faster (via increased rpms) increasing air flow and thus cooling of the engine. This switch operates via the emission control devices (Speed Relay & 2-Way Valve) and thus rendered useless if the emission control devices are removed.

4th Relay

A 4th relay in this part of the engine bay is found during a production interval of the 1969 280sl. It is wired to one of the other relays by means of a connector that is piggybacked onto the other relay's connector. The sequence of the relays may vary from car to car. The wiper, FIP, and CSV relays have been thoroughly discussed on the forum. But for a 4th relay of this shape, in this location, it was hard to track down the function.

Joe Alexander finally writes: Here is the scoop on the #4 relay. I had the information in my notes. If your car is one of the later USA emission versions that has the decelleration solenoid (lower one of two on the injection pump), then it was equipped with this fourth relay, along with the large RPM relay over by the coil.

The car should also have a micro switch mounted on the firewall. There is a separate wiring harness which is added to the car to run all these devices. This add-on wiring harness is a bit complex but one end ties into the neutral safety switch on the firewall and meanders to that fourth relay which is a five prong relay instead of a four prong (as are the other three).

  • After leaving the neutral safety switch on the firewall two wires (green/black) and (green/white) branch-off and connect to the micro switch on the firewall.
  • Next a wire from inside the car is feed into this harness (I suspect that this is the wire coming from the fuse box).
  • Next along the way a brown ground wire emerges and is attached to the ground lug on the relay brackets with the some other grounds.
  • Next a four-wire bundle branches-off and plugs into the large deceleration relay (near the coil). The four wires here are brown, black, black/red, green/blk.
  • Next down the harness a green/black single wire branches off and connects to the ignition coil.
  • Another single wire branches off next and connects to the fuel shut-off solenoid on the injection pump.
  • Lastly a six terminal plug attaches to the bottom of that #4 fourth relay.

The fourth relay is a five prong relay as mentioned and two sockets of the six prong plug are empty. Other wire colors in this plug are black/red, green, green/red, and black. So that's as much as I know on the #4 relay. Hope this helps

I suspect that the short circuit in your system might be the lower solenoid on the injection pump. You may consider simply just disconnecting it as is done without consequence on many cars.
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