Forcing the fuel pumps to run by "fuse jumpering" (or jumpering 2 relay terminals) as a trouble-shooting aid does not activate the complete FI system. Even though the engine may start, it won't run well (K-Jet) — and may not start at all on LH 2.2 and 2.4

This is because both K-jet and LH Fuel relays have two (2) separate "outputs", only one of which powers the pumps. (The LH relay is actually two separate relays under that white cover.) This second output is especially important in LH systems–less so with K-Jet. Its terminal number and functions are as follows:

LH 2.2, Terminal 87-1 controls:
• AMM pin 5
• ECU pin 9

LH 2.4 (and presumably 3.1)
• AMM pin 5
• ECU pin 9
• IAC Valve voltage
• Injector voltage (RS relay on 700/900, and '93 240?)

K-Jet, Terminal 87b controls:
• The complete Lambda (O2 Sensor) System (if so equipped)
• Heaters for Aux. Air Valve and Warm Up Regulator (not essential to starting)

For a complete diagnostic relay bypass, a 3-legged jumper having flat male terminals is needed—and plugged into the relay harness socket, as follows:

Leg 1 from socket terminal 30 (heavy Red wire, fused battery +12V)
Leg 2 to socket 87-1 (or 87 for K-jet)
• Yellow-Red wire (to K-jet pumps)
• Orange, Brown wires (LH 2.2 AMM etc.)
• Red-Black wires (LH 2.4 MM etc.)

Leg 3 to socket 87-2 (87b for K-jet)
• Blue wire (K-jet Lambda sytem)
• Yellow-Red (LH 2.2 pumps)
• Yellow-Red (LH 2.4 pumps, O2 heater)

As with other jumpering schemes, the pumps run immediately. For more than a quick test, a control switch can be wired into the common leg connecting terminal 30 to the other two legs.

Comments and questions welcome...

--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Hello Lucid,

I agree that a fuel relay jumper can be a very useful tool to troubleshoot one's inoperative vehicle. The fault can even be isolated, or even completely eliminated, by using this technique. You are right in saying that bypassing only one leg of the relay will not operate the engine, as the other electronics are left unpowered.

This is why I propose the manufacture of "relay socket testers". These devices would plug right into the factory wiring with factory connectors, this would eliminate the risk of improper jumping which can damage the LH control unit as has been noted by the factory. The device would consist of a salvaged relay with the internals removed, after which a double pole toggle switch is installed into the case. The connections are then appropriately soldered. When installed in place of a suspect relay, the engine can be operated as easily as flipping a switch.

I understand that many would simply replace the relay in the event of a fault. Replacement however would not give the control that a relay socket tester would give the troubleshooter. Also it might become standard glovebox equipment, in order to run the automobile to safety, e.g off the interstate.

I am seeking feedback in order to determine if the construction of these devices should begin, in order to offer them to fellow Brickboarders.

Goatman

This was very helpful.