Backup Lamp Wiring
Thread Starter
Rennlist Member
Rennlist Site Sponsor
Joined: Jun 2001
Posts: 6,469
Likes: 13
From: Ball Ground, GA
Backup Lamp Wiring
I had a customer who was having problems with his backup lamps, and thought that a brief tutorial on the wiring diagram might help others.
On the earlier cars, the backup lamp circuit for the automatic transmission cars simply isn't shown, or is shown on a separate page.
On the later cars, the circuit for the auto cars and the five-speed cars are both shown on the wiring diagram.
The wiring diagram is made more complex by the need to show both five-speed (M481) and auto (M249) wiring on the same diagram. It is important that you understand that you will only have one of the wiring set-ups, not both.
Power for the backup lights starts on the 15 bus (ignition-switched 12 vdc) and runs behind the Central Electric Panel to the backup lamp fuse (Fuse #12 or Fuse #40 on most) on both systems.
For the auto cars, the power from the fuse runs to terminal 30 (power) of the Backup Lamp Relay. The relay is switched by the transmission switch (which is also the neutral start switch and which also operates the gear indicator lights when used) which applies power to terminal 86 (one side of the relay coil). The other side of the coil is grounded thru terminal 85 to the 31 bus (ground). When the relay operates, it connects 30 (power) and 87 (output). Power flows behind the panel to a plug on the bottom of the panel (U21 or U12 on most). (The bridge shown is NOT on the auto cars, neither is the wiring that is shown for it. The bridge is just a little brass piece that bridges from terminal 30 to terminal 87 in the empty relay socket.)
For the five-speed cars, the power from the fuse runs to terminal 30 (power) of the Backup Lamp Relay socket. The relay is not installed. The power is transferred from terminal 30 (power) directly to terminal 87 (output) by a small piece of brass called the bridge. The bridge is shown on separate wires - there are no separate wires, that is only an attempt to show the circuit on the same diagram as the auto cars. From terminal 87, the power flows to a plug on the bottom of the panel (U21 or U12 on most) on one of the multitude of gray wires behind the panel.
So, whether you have the relay or the bridge, power flows to the rear of the car on a single wire, blue on the five-speeds, blue/black on the autos. The diagram shows two wires on the plug - there is only one wire. Again, it is only to show both auto and five-speed circuits on one diagram.
The backup lamps are NOT monitored by the light monitoring system.
On the auto cars, the power is switched by the relay, so that wire runs directly to a connector, usually near the right lamp unit, and from there to the right and left backup lamps.
On the five-speed cars, the power is present anytime that the ignition switch is on, due to the bridge. That blue wire (remember, there is only one wire, not the two shown in the diagram) runs to the transmission switch. The switched power then flows from the switch on a blue/black wire to the same connector near the right lamp unit.
I hope that this will help if you ever have backup lamp problems.
On the earlier cars, the backup lamp circuit for the automatic transmission cars simply isn't shown, or is shown on a separate page.
On the later cars, the circuit for the auto cars and the five-speed cars are both shown on the wiring diagram.
The wiring diagram is made more complex by the need to show both five-speed (M481) and auto (M249) wiring on the same diagram. It is important that you understand that you will only have one of the wiring set-ups, not both.
Power for the backup lights starts on the 15 bus (ignition-switched 12 vdc) and runs behind the Central Electric Panel to the backup lamp fuse (Fuse #12 or Fuse #40 on most) on both systems.
For the auto cars, the power from the fuse runs to terminal 30 (power) of the Backup Lamp Relay. The relay is switched by the transmission switch (which is also the neutral start switch and which also operates the gear indicator lights when used) which applies power to terminal 86 (one side of the relay coil). The other side of the coil is grounded thru terminal 85 to the 31 bus (ground). When the relay operates, it connects 30 (power) and 87 (output). Power flows behind the panel to a plug on the bottom of the panel (U21 or U12 on most). (The bridge shown is NOT on the auto cars, neither is the wiring that is shown for it. The bridge is just a little brass piece that bridges from terminal 30 to terminal 87 in the empty relay socket.)
For the five-speed cars, the power from the fuse runs to terminal 30 (power) of the Backup Lamp Relay socket. The relay is not installed. The power is transferred from terminal 30 (power) directly to terminal 87 (output) by a small piece of brass called the bridge. The bridge is shown on separate wires - there are no separate wires, that is only an attempt to show the circuit on the same diagram as the auto cars. From terminal 87, the power flows to a plug on the bottom of the panel (U21 or U12 on most) on one of the multitude of gray wires behind the panel.
So, whether you have the relay or the bridge, power flows to the rear of the car on a single wire, blue on the five-speeds, blue/black on the autos. The diagram shows two wires on the plug - there is only one wire. Again, it is only to show both auto and five-speed circuits on one diagram.
The backup lamps are NOT monitored by the light monitoring system.
On the auto cars, the power is switched by the relay, so that wire runs directly to a connector, usually near the right lamp unit, and from there to the right and left backup lamps.
On the five-speed cars, the power is present anytime that the ignition switch is on, due to the bridge. That blue wire (remember, there is only one wire, not the two shown in the diagram) runs to the transmission switch. The switched power then flows from the switch on a blue/black wire to the same connector near the right lamp unit.
I hope that this will help if you ever have backup lamp problems.
Addict
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Jul 2007
Posts: 4,539
Likes: 4
From: Germany
This thread should have been for my PO. He installed a bridge for a 5-speed instead of a auto relay en then taped over the bulb contacts because apparently the backup lamps stayed on all the time. Weird people..
Addict
Rennlist Member
Rennlist Member
Joined: Jul 2001
Posts: 1,319
Likes: 2
From: Irving, TX
My '87AT b/u lights are on in "Park" sometimes. Also, I got pulled over once by the police because my backup lights were on (while driving forward). An intermittant problem. Transmission switch faulty?
Trending Topics
Thread Starter
Rennlist Member
Rennlist Site Sponsor
Joined: Jun 2001
Posts: 6,469
Likes: 13
From: Ball Ground, GA
The switch is the most likely possibility - but if it is loose, I would expect the neutral start function to be intermittent as well. It is possible that the adjustment is just a little bit off, I suppose. That, added to a little play due to wear, might make the reverse portion of the switch make improper contact on an intermittent basis.
It is also possible, but pretty unlikely, that the relay is sticking intermittently.
An easy check would be to have a helper behind the car watching the lights. Turn the ignition switch on and slowly move the shift lever into and out of reverse from both directions. The helper should yell instantly when the lights come on and go off. You might be able to judge the adjustment and the sloppiness from that.
It is also possible, but pretty unlikely, that the relay is sticking intermittently.
An easy check would be to have a helper behind the car watching the lights. Turn the ignition switch on and slowly move the shift lever into and out of reverse from both directions. The helper should yell instantly when the lights come on and go off. You might be able to judge the adjustment and the sloppiness from that.