4 replies to this topic

[Jayse]

Started rebuilding our 75 1275gt.

Starting with the electrics to the back, loom all appears to be connected correctly. Linking the brake pressure switch at the offside front I can see 12v on the green and purple wire at the back. However when the bulb is inserted (to the offside rear cluster), it doesn't light and the voltage reads near zero. This is when measured from the joint near the near side rear.

Tested the lights directly connected to the battery and it lights up.
Also since tested the tail light (connected on the same bulb) and that works.

Any thoughts on whas going on would be appreciated.

Thanks.

[weef]

I presume that you are using a digital multimeter ( DVM ) to measure the voltage at the rear connection.

A  DVM does not draw much current and the readings can be misleading as it does not "load" the circuit, whereas a bulb will.

It may be the front to rear Purple/Green wire is damaged and a quick check is to run a temporary wire from the front to the rear.

Connect the temporary wire to the Green brake switch feed and connect it to the joint that you are using to measure the voltage disconnecting the Green/Purple wire that feeds this joint.

See what happens now, if the brake lamps work as designed then the front to rear cable may be faulty, if the lamps do not work as designed look for a fault in the lampholders.

I hope this helps you.

[Jayse]

Thanks weef

Had a quick look and tried what you suggested today, but it appears there are other gremlins in the system…

Initially I couldn’t trace 12v between the green wire at the brake switch and the joint in the loom on the passenger side of he engine bay. Only had a little time today, so eventually gave up on that for the day

I also ran a direct cable and no luck…. Finally used the tail light wire in the loom and this works for all bulbs in each cluster, so I’m assuming I can trust that the light units work, are earthed properly and the bulbs are also fine.

Look likes my next task is checking each part of the loom.

[sonscar]

Connect a bulb at the switch end after and before the switch and see if it lights.As weef mentions there is a high resistance in the circuit not allowing enough power.Is there a fuse?they often make poor contact in the holders of corrode underneath the fusebox.Steve..

[Steam]

If the brake switch is a pressure switch version in the brake line, these are prone to having intermittent poor connections inside them and also prone to failing intermittently. While new ones are available these seem to be worse than the originals. I changes mine to a pedal mechanical switch. No more hassles.