Issue with marker lamp wiring

[Faster Behr]

I'm currently chasing down a fault with my lighting. My marker lamp/tail lamp circuit is blowing fuses. Oddly it's only when the instrument dimmer rheostat is turned up that this happens. The tail lights dim when the rheostat is dialled up and the combination switch harness heats up and the fuse blows. I've ruled out issues with the body harness (and all lamp positions) by disconnecting and probing for short circuits there, hence I am currently looking more closely at the dash harness and the rheostat itself. I'm going to have a look at the fuse box rear also when time allows as I understand these are known to cause issues. My progress has been hampered by uncovering previous half witted wiring and rectifying it as I go.

Has anyone got any experience of this issue and know of any other common problems? I have found many similar threads online but they have ultimately been due to short circuits at lamp positions or switch contact point carbonisation.

I could also do with sourcing some wiring loom pins and plugs to make up some test leads and to do some repairs. Has anyone got an RS part number or know of a source for these?

 

[MCBladeRun]

If it blows when the rheostat is turned up, and your loom heats up and the lights dim wouldn't that indicate the rheostat is passing current directly to earth? It sounds like it should be passing through a resistive element such as a lamp, but is instead just flowing through with no resistance, causing an over-current in the circuit and blowing the fuse.

 

[Faster Behr]

I think I’ve cracked it. I’ll attempt to explain - which is always difficult with wiring issues - but documenting it may assist someone in the future who has a similar problem. Without these types of threads and forums then we’d all be starting from square one!

I disconnected the rheostat completely and tested it. It’s open circuit when off and then ranges from 10 ohms to 0.2 ohms when dialled from the first detent (low) to all the way up (high). So no issue there.

The rheostat has two wires. It is earthed via a black wire on one pin. The other pin is a red/blue wire which provides the earth path to dim the instrument lights. This red/blue goes across through the dash harness to each lamp position and also through the body harness on to the console etc.

With the red/blue disconnected from the rheostat I turned the sidelights on and observed current which would have been passing through the rheostat under my fault conditions. Clearly this wasn’t right. The only place this short could happen sensibly is within a plug or bulb holder which carries the “dimmed” red/blue earth for the illumination and the green/white for the marker lamp circuit. So I set about taking out all the clocks and gauges and unplugging everything. I eventually found that disconnecting the console loom from under the heater pipework area dropped the voltage on the red/blue wire and it went open circuit. I refitted the lamps in their holders and reconnected the rheostat and all was well. The console loom plugs were both grotty so they’ll need a good clean up and then if the issue persists once I plug it back together then I’ll strip the loom.

For me it’s not time wasted as I got to learn more about how the car is wired and I worked out a lot of future problems and reverse engineered some bodging - some of which I still have to remedy. Having all the gauges out is ideal too as I’ve got an LED bulb kit on the way. I can also change the speedo over to MPH too.