Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

1976 Vehicles

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The 1976 emission control system is close to the system used in 1975. Catalytic converters are used on all engines, but the 450 series cars have the catalysts installed on the exhaust manifold. Due to Federal regulations, the routing of the fuel evaporation lines has been changed.

230 MODELS



See Figure 1

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Fig. Fig. 1: Emission control component locations1975-76 230 models

Refer to the 1975 230 model Emission Control System testing procedures.

280, 280C & 280S MODELS



See Figures 2 and 3

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Fig. Fig. 2: Emission control component locations1975-76 280 and 280C models



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Fig. Fig. 3: Emission control component locations1975-76 280S models

Refer to the 1975 280, 280C and 280S model Emission Control System testing procedures.

450SE, SEL, SL & SLC MODELS



See Figure 4

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Fig. Fig. 4: Emission control component locations1976 450SE, SEL, SL and SLC models

These cars with the M117 engine use one emission system for the entire U.S. market, including California.

The base color of the vacuum lines is opaque (white). Lines originating at a vacuum source have only one color stripe; lines terminating at a vacuum source have 2-color stripes. Purple is always the second color.

Thermo vacuum valves are used to control the ignition changeover, EGR and air injection. They are color coded:

Black104-F valve

Blue63-F valve

The retard side of the vacuum control unit is only activated when the coolant is below 212-F during deceleration with the A/C off. The advance side of the vacuum control unit is activated when temperatures at the thermo-valve are 104-F or above and the advance is determined by the position of the throttle plate.

The EGR valve works in 2 stages. The first stage (small amount) takes place with coolant temperature above 104-F. There is no EGR with coolant temperature below 86-F.

The second (larger) stage of EGR occurs during acceleration with coolant temperature above 140-F and vacuum less than 7.9 in. Hg.

Air injection takes place above 62-F and is cancelled below 50-F (coolant temperatures).

Testing the System

IGNITION CHANGEOVER SWITCH

Unplug and ground the temperature switch. The engine rpm should increase and the auxiliary fan should operate.

If the engine speed does not increase, check the vacuum lines. The yellow line from the throttle valve housing should go to the center of the yellow switchover valve. The yellow/purple line goes to the outer connection of the switchover valve to the inner chamber of the vacuum unit (retard). Disconnect the plug from the relay and bridge terminals 3 and 4. With the ignition ON, the switchover valve should click. If not, replace the relay. If the switchover valve does not function, replace the valve.

Check the auxiliary fan. Disconnect the relay and bridge terminals 1 and 3. With ignition ON; the fan should run. If not, replace the fan.

Check othe relay. Remove the plug and bridge terminals 1 and 3. With the ignition ON, the fans hould run. If so, replace the relay.

IGNITION RETARD RELAY

Switch on the A/C. The engine rpm should increase slightly. If not, check that the A/C is operating. If the A/C is operating, replace the relay.

IGNITION ADVANCE

Remove the yellow/purple and red/purple vacuum lines from the distributor. The engine speed should increase slightly. Connect the yellow/purple vacuum line to the upper connection at the vacuum diaphragm. The engine speed should increase.

If not, replace the 2-way vacuum diaphragm.

EGR SWITCHOVER VALVE

Remove the red/purple vacuum line at the EGR valve. Connect a vacuum gauge to the red/purple line and run the engine at idle. Increase speed to 2500 rpm. At idle, the gauge should show no vacuum; at 2500 rpm, vacuum should be present.

If no vacuum is indicated, check the line connections. The red line from the red connection at the throttle valve housing should go to the angular connection of the thermovacuum valve. The red/purple line should be attached to the vertical connection at the thermo-vacuum valve, to the red connection of the vacuum control switch and red connection of the EGR valve.

Check the 104-F thermo-vacuum valve. It should open if the surrounding temperature is above 104-F. It should close below 86-F. If the valve is not functioning properly, check the bore of the throttle housing vacuum connection.

VACUUM CONTROL SWITCH

Remove the brown/purple vacuum line from the EGR valve. Connect a vacuum gauge between the line and the bottom of the EGR valve. Increase engine speed to 2500 rpm. At idle, vacuum should not be present. During acceleration, vacuum should be present briefly until the rpm stabilizes. If no vacuum is present, check the vacuum lines. The red/purple line should be connected to the red connection at the vacuum control switch, the white line to the center of the vacuum control switch and the red/purple line to the brown control switch connection. If vacuum still is not present, replace the control switch.

EGR VALVE

Disconnect the yellow/purple line from the distributor diaphragm. Disconnect both vacuum lines at the EGR valve. Connect the yellow/purple line with the upper, then the lower connection of the EGR valve. The engine should run rough or stall in both cases. If not, replace the EGR valve.

DIVERTER VALVE

Run the engine at idle. Connect a CO tester. Note the reading. Remove the blue/purple vacuum line from the blue thermo-valve and note the reading again. It should change noticeably.

If not, check the vacuum lines. The blue line runs to the angular connection on the blue thermo-vacuum valve. The blue/purple line goes to the vertical connection of the same valve.

Check the diverter valve. Remove the muffler on the valve. Disconnect the blue/purple vacuum line at the blue thermo-vacuum valve. Air should flow from the diverter valve. If there is no air flow, replace the diverter valve. If necessary, check the drive belt tension.

CHARCOAL CANISTER

Remove the thin hose from the charcoal canister. Cover the hose opening with your finger or connect a vacuum gauge. Slowly increase rpm to 2,500. At idle, a small amount of vacuum should be present, and vacuum should increase with engine speed.

If no vacuum is present at idle, check the purge line to the intake manifold. Disconnect the charcoal canister hose at the purge valve and clean it by blowing through with compressed air in the direction of the intake manifold. Replace the purge valve if necessary.

If vacuum does not increase at idle, check the vacuum at the purge valve. Disconnect the white vacuum line at the purge valve. Connect a vacuum gauge or close the line with a finger. Increase engine speed. At idle, there should be no vacuum. With increasing engine speed, vacuum should increase. If vacuum is present, replace the purge valve. If no vacuum is present, blow through the line towards the throttle valve housing.

 
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