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Replacing a Faulty Hall Sensor Cable

(Cause of No-Start)

1987 740 turbo, LH-Jetronic 2.2, EZ 117K Ignition System

One of the causes of a no-start is a faulty Hall sensor. That fault can arise from a short circuit in the Hall cable, due to worn insulation on the hot wires of the Hall plug. A short circuit in the Hall plug will "fry" the Hall sensor, necessitating a new Hall sensor and a fix of the short circuit. These instructions give you step-by-step instructions on replacing the Hall cable.

Figure 1. The exposed hot wires of the Hall plug created a short circuit. [A short circuit is a short unintended path for current to go from a hot wire to another wire or ground, rather than traveling its designed path through the device in question—in this case, the Hall sensor.]

Symptoms: Engine hiccupped, then intermittently died over a period of months. Sometimes it restarted right away, other times not. Eventually engine stayed dead (but cranked). Ran tests. Got 0.25 VDC at ignition “On,” instead of 12 VDC, when testing #3 red wire of Hall plug. Found bare hot wires in Hall plug (Figure 1), which caused a short circuit. What caused worn insulation? Engine heat and lots of oil leaking past large O-ring of distributor into and around Hall plug. Result: The short circuit will have killed the Hall sensor (Figure 2), necessitating a replacement Hall sensor and a fix of the short circuit. One Solution for short circuit: Replace the Hall cable, which runs from the distributor to the Ignition Control Unit (ICU). Wires! Wires! Everywhere! This difficult task involves working in cramped spaces and contorting your body underneath the dash to isolate and free the Hall cable. Not easy! Some people may wish to just splice the wires of the Hall plug cable somewhere along the firewall. It is a more manageable task than replacing the whole Hall plug cable, but I don't know if it will work. I elected not to attempt the splice because the cable is shielded for EMI (electromagnetic interference). Moreover, I'm not sure the integrity of the joints of the splices would hold against EMI. If you do attempt such a splice, you need to splice each of the 3 wires and then insulate each wire splice. Next, I suggest wrapping medium weight tin foil at least twice around the length of the whole splice (all 3 wires) and overlap the stock shielding, say 1 1/2 inches or so, then having heat shrink insulation over and beyond the whole splice. Be sure you insert the heat shrink tubing on each wire and the whole cable and slide it out of the way before doing each splice. Let me know if this works! If it doesn't, you have no recourse but to replace the whole Hall plug cable.

Figure 2. The distributor baseplate with Hall sensor and Hall plug receptacle.

At times you have to go above and beyond the normal pale of the average do-it-yourself mechanic. In fact, sometimes you may go beyond even what a professional mechanic would attempt to do! Because it’s your car and you like a challenge. Such a time would be replacing the Hall cable that runs from the distributor alongside and through the firewall to the ICU under the driver’s side dashboard (above the gas pedal).

If you wish to replace the Hall cable in your car, then download the instructions, "Replacing a Faulty Hall Sensor Cable."