I have just spent several hours working on sorting out the wiring for the
oil failure light and the brake line failure light for my 76 TR6. Not sure
if all 76's are done like this, but this is what I found.
First, the owners manual says both the oil light and brake failure light
will shine dimly when the car is started. This may have been true of
earlier years, but not the 76. Both shine bright. Also, Bentley manul says
both shine dimly if you lose oil pressure when running. Again not true,
just the oil light which will be bright. Also the wiring diagram in the
owners manul (and reprinted in the Bentley manual) is wrong.
What I have found is as follows:
Oil Pressure warning light.
1) White feed from the voltmeter to the oil warning bulbholder.
2) White/red wire from bulbholder to oil pressure switch. Connection is to
the side that is closed to the common lead with NO oil pressure present.
3) Common lead to ground. (It turns out the problem I was tracking was a
poor connection between the ground wire the the connector the switch)
Brake failure light
1) White lead from the voltmeter to the brake failure light bulbholder.
2) Black/purple wire from the bulbholder to the connector at the PDWA
3) Purple wire from the second lead of the connector to the C2 terminal of
the starter relay. BTW, these two wires are connected together in the
When the car is started, the white/red wire on terminal W1 of the starter
relay will be energized and cause the relay to close. This connects C1
(from the PDWA) to C2 (connected to ground) and causes the brake failue
light to shine brightly, verifing it is good. Also, the EGR blub will light
since it is connected to terminal C4 of the same relay. Once the key is out
of the 'start position', the voltage to terminal W1 drops and the relay
opens, extinguishing both lights.
When the PDWA senses a loss of pressure in either side of the brake system,
the shuttle within the PDWA moves. This causes the plunger to move upward,
which in turn grounds the Black/purple wire, causing the brake failure light
to shine. This can only be extinguished by re-centering the shuttle or
disconnecting the connector at the PDWA.
This same bulb also indicates if you handbrake is still on. Since it is
obvious with the handbrake sitting next to you, this was a requirement laid
on by the US government. There is simply a second black/purple wire running
from the bulbholder to a switch on the handbrake lever. If the lever is
pulled up a certain amount, the switch will provide a ground.
Seems a strange way to wire these lights, but after several days of
troubleshooting and tracing wires, this is what I found.
'76 TR6 CF57005U
(Home of the airlineless airport - Mid America)