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TR8 Lower Engine Harness Replacement

by Tim Buja

Kent Clovis has previously reported on repairs he made to a failing wiring harness on his TR8 that affected his fuel pump. This article describes how to completely replace the wiring in the same harness. You may also want to review Dan Master's Making or Repairing Wiring Harnesses article for more information on the details on how to build your own harness.

The harness in question is for 1980 Carbed TR8s with Air Conditioning. This harness (p/n TKC 5913) connects the alternator, oil pressure switch (3 terminal), and starter solenoid to the fuse & relay panel. Loads serviced by this harness include the starter solenoid pull-in and hold-in coils, fuel pump, oil pressure light, alternator output (up to 65 amps), alternator voltage sensing, and alternator regulator excitation power from the IGN light on the dashboard. This harness runs just above the catalytic converter and is subject to premature failure due to the high heat in this area. The failure of this harness can result in an intermittent loss of voltage to the fuel pump. The correct schematic diagram for the fuel pump circuit (shown below ) does not appear in the Owners Manual and is well hidden in the Repair Operations Manual, making it difficult to diagnose the problem, especially under duress when you're broken down on the side of the road.

Since the harness is No Longer Available from any of the Triumph parts vendors, you'll have to make your own replacement harness if your existing harness is too damaged to repair. The wire, connectors, harness tape, lugs, etc. will cost about $35 - $40. This figure pales in comparison to the expense and inconvenience you might face if your TR8 were to suddenly stop running due to what might be erroneously diagnosed as a "failed fuel pump", since replacing the "failed" pump involves draining the fuel tank to remove the pump from the tank. Then, when you're all done, your expensive brand new pump will still fail at the most inopportune time since you've only repaired a "symptom" instead of addressing the root cause of the problem. As you may have suspected, this has happened to me...

A modification to this harness was authorized in a Lucas Service Bulletin (JRT #59, dated September 1981). This bulletin contained instructions to cut the voltage sensing lead (thin brown wire) in this harness and reconnect it to the battery cable terminal at the starter solenoid. This modification was made to prevent battery failure due to overcharging that would occur should a high resistance connection develop somewhere in the lead. The high resistance connection would cause the alternator voltage regulator to sense a low battery voltage even when the voltage was normal. The regulator would then boost the alternator output voltage in an attempt to raise the sensed battery voltage, resulting in an inadvertent overcharge of the battery. This modification is not shown in the harness drawing below, although instructions are given below on how to implement the Lucas Service Bulletin modification as you assemble the harness.

Wire Colors:
 N - Brown          W - White         NY - Brown/Yellow
WY - White/Yellow  WN - White/Brown   WO - White/Orange

Wire Size & Service:
  9 strand:  N - Alternator remote battery voltage sensing
            WY - IGN light to alternator regulator excitation
            WN - Oil pressure switch to oil pressure light & fuel
                   pump inertia switch
 14 strand: WY - Ignition coil ballast resistor bypass during cranking
 28 strand:  W - Fuel pump supply from ignition switch to oil pressure
                   switch (supplies fuel pump when engine is running)
            WO - Start relay to starter solenoid pull-in/hold-in coils
                   and oil pressure switch switch (supplies fuel pump
                   while cranking)
120 strand:  N - Alternator output to starter solenoid battery terminal

  /  \     This is the firewall multipin connector for this harness, looking
 / WO \    into the male pin end of the connector block.
/      \
|x    W|   WO - Starter relay to starter solenoid & oil pressure switch
|      |    x - blank
|N   NY|    W - Ignition "on" feed to oil pressure switch
|      |    N - Battery sense lead
|WY  WN|   NY - IGN lamp to alternator regulator excitation input
|______|   WY - ignition coil ballast resistor bypass from starter solenoid
           WN - fuel pump feed from oil pressure switch to inertia switch

                                     molded oil
                                   pressure switch
                        to starter    connector
                         solenoid       _____
                                 N     /     \
                        WY       O   WN||   || W                 to
                         o       |     |  _  |                alternator
                         | WO    |     \ WO  /
          PVC tape       |  o    |       |||                      remote
        wrap to 5/8"     | /|    |       |||                      battery
          diameter    5" | || 3" | 6"    ||| 4"                   voltage
_________     ___        | ||    |       |||         /-------E  N sensing 
Multi-   |___/   \       / /\    \       ///        /   6"
pin       ___     __________________________________---------O  N Alternator
Connector|   \___/         |     |        |        |\             output
|            |   |         |     |        |        | \-------o NY Alternator  
0"        5.5"  8.25"     26"   27"      43"      47"             excitation

o - small ring tongue lug - starter solenoid & alternator excitation leads
O - large ring tongue lug - both ends of the alternator output lead
E - female spade (Lucar) connector - battery voltage sense lead




Comparable AWG wire size






















To implement the Lucas Service Bulletin modification as you build the harness, cut the thin brown (N) wire at approximately 20" from the mating face of the multi-pin connector. Tape the end going toward the multi-pin connector, and then cover it with PVC harness tape overwrap. Crimp a large ring tongue lug to the remaining end that goes toward the alternator, then bring this lead out of the harness at 27". Continue the PVC harness tape overwrap to 47". The small N (voltage sense) lead will then be terminated along with the large N (alternator output) lead and the battery cable at the starter solenoid battery terminal.

Below is the schematic for how the starter, oil pressure switch, fuel pump, and ignition coil are tied together with the harness you've constructed and installed in your 1980 TR8 (post 200000 VIN, with carbs). This schematic is drawn assuming that the Lucas Service Bulleting modification has been made to bring the small brown (n) lead out of the harness so it senses the battery voltage at the starter solenoid battery terminal. Per British Standards, any unfused battery positive connection is supposed to have brown insulation, but on my TR8, the heavy cable from the battery to the starter solenoid has red insulation instead of brown (N).

 Battery           ign switch   ign switch
 Unfused            "start"     "on, start"
    |    Battery  (through          |
    |    voltage    start           |
    |    sensing    relay)        W |
  N |    lead (to     |             |
    |   alternator)   |             |_____________
    |  n/             |             |             | W
    |  /    WO _______|________     |          Low Oil
 . .|./. . . .|. . . . .    WO |    | W        Pressure
.   |/     ___o___      .     .|. . |.          Light
.|  O     |       |     .    . o\   o .           |
.|~~~~~Pull-in~~Hold-in .    .   \    . Oil       |  
.|  o    coil    coil   .    .    o   . Pressure  |
.|   \    |       |     .     . . |. .  Switch,   |
.|  O_\ __|      Gnd    .         |     shown at  |
 . .|. \ . . . . . . . .          |     zero psi  |
    |   \           Starter     WN|_______________| WN
    |    \          solenoid     .|.     
 Starter  \WY                   .|o .  fuel pump
  motor    \                    .|  .  inertia
    |    Ballast                .|o .  switch
   Gnd   Resistor                .|.
         bypass to        Wht/Grn |
         ignition coil           Fuel
         (provides full          Pump
         battery voltage    Black |
         to ignition              |
         coil during             Gnd

Sorry for the crude ASCII graphics. When the ignition switch is "on", the W line is energized. With the engine not running, the oil pressure light is lit since there are two ground paths - one through the inertia switch and fuel pump, the other through the oil pressure switch to the starter solenoid pull-in & hold-in coils.

When the ignition switch is moved to the "start" position, it energizes the start relay (behind the glovebox) which energizes the WO line to the starter solenoid and oil pressure switch. The starter solenoid contact shorts across its three terminals, applying battery voltage to both the starter motor and the WY ballast resistor bypass lead. This starts the motor and increases the voltage on the ignition coil from 6 volts to full battery voltage. Since the engine is not yet turning fast enough to develop any oil pressure, the WO line also energizes the WN line through the oil pressure switch. This starts the fuel pump and extinguishes the oil pressure light. (12V applied to both sides of the bulb turns it off.) Voltage at the starter motor terminal also turns off the high current pull-in coil in the starter solenoid, leaving the low current hold-in coil to maintain the solenoid contacts in position.

As the oil pressure comes up, the WO/WN connection is broken and the W/WN connection is made in the oil pressure switch. The oil pressure light remains off since 12V is still applied to both sides of the bulb. The fuel pump remains running as long as the ignition is on, the oil pressure remains above the switch setpoint, and the inertia switch contact is not jarred open.

Releasing the ignition switch back to the "on" position deenergizes the start relay and starter solenoid. The solenoid contact drops out, the starter motor stops, and the ignition coil voltage is reduced back to 6 volts by the ballast resistor.

If the oil pressure drops below the switch setpoint (5psi or so), the W/WN connection is broken and the WO/WN connection is made. This inserts the oil pressure light in series with the fuel pump. The fuel pump motor stalls with the added resistance of the small lamp bulb in the circuit, and the bulb lights up to indicate a loss of oil pressure. Once the fuel pump stops, the engine stalls shortly thereafter as the carb fuel bowls go dry.

While this circuit does not appear in the wiring diagram for 1980 USA market carb TR8s with A/C in the TR8 Repair Operations Manual (page 310), it does show up in the wiring diagram for 1981 UK market TR8s (page 28 of the 1981 ROM supplement). The only other place I've found it documented is in issue 47 of The Vintage Triumph magazine



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