The 560SL Throttle Valve Switch Replacement Narrative

[BabyBlueBenz]

In pursuit of reducing the high idle speed my 1986 560SL had been experiencing, I determined that the Throttle Valve Switch (TVS) was bad. The test procedure is outlined in Service Manual document 07.3-152 "Checking, renewing throttle valve switch". The experience detailed below was guided by 07.3-230 "Removal, reinstallation of throttle assembly" and 07.3-225 "Removal, reinstallation of mixture control unit". Besides the new TVS, you will need a new Throttle Housing Gasket and perhaps a handful of new vacuum line rubber elbows. For additional cleanliness, buy a few cans of carburetor cleaner or your favorite degreaser. A shop vacuum would be practical as well.

For my preceding high-idle adventure, see the thread "Idle Speed Air/Control/Slide Valve Confidence Check" at Idle Speed Air/Control/Slide Valve Confidence Check.

In a nutshell, this job is not as bad as I thought it would be. In fact, I'll be doing it again soon, because of reasons mentioned below.

Break out the toolbox, 'cause here we go...

[BabyBlueBenz]

Here's what we're dealing with...the topside of the M117 engine with its CIS-E fuel injection system. This view is front-to-rear, with the distributor at the bottom of the photo and the firewall at the top. Pardon the filth and rust...I'll get to those tasks someday. The TVS is located in a hole as pointed to by the arrow. You can't reach it until the Mixture Control Unit (MCU) & Air Guide is removed.



Here's a close-up with the partially-visible yet dirt-encrusted TVS highlighted in red.

[BabyBlueBenz]

First, open the fuel filler cap to release the pressure on the fuel system.



Unplug the wiring plug to Idle Speed Air Valve "Y6". Loosen and remove the clamp securing Y6. Remove Y6 along with the two black rubber elbows connected to it by twisting it to the left to slip off the first elbow and then pull upward on the second elbow, which is held snug by a barbed plastic fitting atop the black Air Guide. Set these parts aside.

Unplug the Air Flow Sensor Position Indicator "B2".

Carefully disconnect the vacuum line at the back of the fuel Diaphragm Pressure Regulator "40". I pulled mine too hard and the rubber mushroom fitting came out of my regulator. A little soap and a small screwdriver convinced it to go back in.

Unplug the Electrohydraulic Actuator (EHA) "Y1" at the back of the MCU.

[BabyBlueBenz]

Find the large fuel line leading to the MCU. About midway in the exposed metal portion of the line is a welded-on bracket, which is fixed to the base of the MCU with a 10mm bolt. Remove this fastener and set it aside.

Get a hold of some paper towels or discardable rags to absorb the upcoming spillage of fuel.

With a 17mm wrench, slowly loosen the large nut retaining the Fuel In line at the MCU. Prepare for some minor squirting of fuel onto your rag/towel. Should the amount of gas overwhelm your catch rag/towel, simply tighten the fitting again and get some more rags. Once free of the MCU move the loose end of the fuel line aside and place a fresh rag/towel beneath it to catch any more gas that might leak out.

Using the same rag/towel catch scheme, loosen the fuel return line at regulator "40". Now my car has had its fuel cooler removed, so your fuel line arrangement will likely differ from my photo. Once freed, swing the loose ends of the fuel lines over the valve cover.

Remove the two flat-head screws holding the EHA onto the Mixture Control Unit and set these three parts aside. A little fuel may drip out of the EHA and MCU.

[BabyBlueBenz]

With a 12mm wrench, loosen the first of nine injector connections, starting with the Cold Start Valve "Y8". A small amount of fuel may drip from these lines. Make sure that the retaining nuts are completed unthreaded from the injectors.

Continue loosening the remaining fuel injector lines. They were fairly tight, so I applied a bit of a counter-hold on the rigid fuel lines themselves.

[BabyBlueBenz]

The Breather Hose from the driver's side valve cover is held in place by a small tab fastened by a 6mm Allen-headed bolt. Loosen (no need to remove) this bolt and the tab will swing aside. Pull this end of the breather hose up and off, then swing it toward the front of the car.

Hiding beneath the breather hose is the plug connection known as "S29/2x1"...unplug it. UPDATE: This junction is also known as "X56" in the service manual. The bottom/male portion of this connection is fixed with two 10mm bolts. Remove them and set the bolts aside. The male connector you just freed is the pigtail end of the TVS.

Nearby is the Coolant Temperature Sensor "B11/2" with its two largish barrel-shaped pin connectors. Unplug these two and route the wire so that it will not tangle with the fuel injector feed lines.

On the passenger side, disconnect the forward end of the long throttle rod linkage. The socket end will pull off with a mild tug. Swing the rod aside so that it doesn't lie atop the injector fuel feed lines. If your car is equipped with a Cruise Control Actuator, the aft link will need to be disconnected as well.