The early Mercedes 450SL was fitted with the Bosch D-Jetronic, EFI or Electronic Fuel Injection system. While this blog post will not cover in detail the entirety of the D-Jet system, it will cover all the steps for a full removal in preparation for an inspection and refurbishing of the harness. This will be covered in a future article.
It is a good idea if you own or are in the market for a Mercedes 450SL or any classic Mercedes, to either regularly inspect or throughly inspect the car respectively. As our cars age, much of the rubber, plastic, and wiring is in poor condition and needs to be either replaced or repaired. Don't risk a fire!
Removing the EFI - D-Jet wiring, is not complicated, does not require any special tools, but will require some physical contortion. As is indicated throughout the car used for this blog is a 1975 450SL. The wiring will vary depending on chassis series.
STEP 1
DISCONNECT BATTERY!
STEP 2
Remove air cleaner and the plug to the air temp sensor. While in the process, check the seal on the underside of the air cleaner housing. This is the seal between the air cleaner and throttle body. This seal is still available from Mercedes-Benz. TIP: When purchasing a new seal, make sure it is for the D-Jet. The later K-Jet will not fit.
STEP 3
Label every wire and take photos before removing any thing. I use masking tape looped around the wire and stuck to itself with a piece long enough to write on with a Sharpie.
STEP 4
Unplug the wires from their respective terminals but leave harness in place. Check all terminals for damage as you unplug each wire
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| Throttle Position Sensor (Shown: 1975 450SL) |
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| Intake minifold sensor—passenger side (Shown: 1975 450SL) |
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| Harness ground wires (Shown: 1975 450SL) |
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| Thermo Time Switch (Shown: 1975 450SL) |
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| Water Temp. Sensor and Auxiliary Fan Sensor (Shown: 1975 450SL) NOTE: Depending on the chassis series, the Aux. Fan Sensor and driver side manifold temp sensor were wired independently from the main harness. |
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| Cold Start Fuel Injector (Shown: 1975 450SL) |
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| Plug to distributor (Shown: 1975 450SL) |
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| Remove bracket and receptacle (lower half of photo above) (Shown: 1975 450SL) |
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| Manifold Pressure Sensor (MPS) under master cylinder (Shown: 1975 450SL) |
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| Unplug all 8 fuel injectors and be sure to label (Shown: 1975 450SL) |
STEP 5
Remove the lower passenger side footwell dash panels and locate the ECU under the dash as shown in photo below.
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| ECU under passenger side dash area (Shown: 1975 450SL) |
Remove the lower spring clip by pulling down on it. The hinge of the clip is attached to the firewall. Once the lower clip is removed, gently wiggle the ECU back and forth while pulling down to release the flanged retaining peg from the upper bracket.
When the ECU is free, remove the plastic harness plug guard (not shown) and the wiring harness retention collar (illustrated below) to release the harness wire and unplug the harness.
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| Illustration above shows how the ECU is held in place. (Image curtesy of Mercedes-Benz |
STEP 7
Unplug both relay connectors and single line plug. Should one of your plug housings break, they are available from Mercedes.
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| Harness plugs under dash (Shown: 1975 450SL) |
STEP 8
Release the rubber grommet from the firewall. It will more than likely not survive the removal and will require renewal. They are are still available through Mercedes for $80. NOTE: It is not necessary but will make the job easier to remove the valve cover.
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| Harness firewall grommet (Shown: 1975 450SL) |
STEP 9
With help, pull the wire through the firewall. It is best to have one person in the car guiding while the other pulls from the engine bay side.
STEP 10
Remove entire wiring harness from engine compartment and your done, at least with the first part.
In my next blog, we will cover the inspection and restoration of the harness.
