TBP · Jan 28, 2024

Hi again all,

Its been a while since ive chatted here.

I am still in the process of putting my 1991 (300TDI) truck cab back together and even with a new main loom (autosparks) I'm struggling to get power on the instrument lighting circuit (red/white).

I've checked the fuses, checked power at the light switch and all good.

I have the instrument cluster out at the mo, would that make a difference?

I just cant seem to find where this circuit gets its power from!

I have the old loom making a large mess on the kitchen table with the multitude of wiring diagrams but still cant see a straight forward way how this circuit is powered.

Any direction much appreciated.

tottot · Jan 28, 2024

What are you using to test the circuit? Have you got the switch in correct position?

v8250 · Jan 28, 2024

Did you pre-test the wiring loom before installation? If not, you should have.

Electrical fault finding is very straight forward, you either have 12v, or you don't. Check as follows noting you'll need to use the +Ve of your DVM to earth - this provides your closed 12v cct :. you have 12v...

1. you have 12v from the fused output :. check for 12v output across your dashboard lighting switch [direct from ign+light switch ON] - if 12v ON, move to
2. check for 12v on red/white cct at loom input [disconnected from rear of dash] If no 12v you know you've a fault in the wiring/loom :. track back and by-pass with a new wire. If 12v, connect to dash pcb and check for 12v with all bulbs out. If no 12v [unlikely] it's most likely an earthing issue with your dash pcb. Check this by running a new direct earth from the pcb/dash connector to earth. Then re-check.
3. If 12v tests okay but then fails with bulbs in you simply have a bulb failure :. change accordingly with new.

ps make sure you've set your DVM correctly, set to 12v DC at 20V range.

TBP · Jan 31, 2024

tottot said:
What are you using to test the circuit? Have you got the switch in correct position?

Multi meter set on 12v DC. Test from earth point across instrument (Red/White) +tve feed.

TBP · Jan 31, 2024

v8250 said:
Did you pre-test the wiring loom before installation? If not, you should have.

Electrical fault finding is very straight forward, you either have 12v, or you don't. Check as follows noting you'll need to use the +Ve of your DVM to earth - this provides your closed 12v cct :. you have 12v...

1. you have 12v from the fused output :. check for 12v output across your dashboard lighting switch [direct from ign+light switch ON] - if 12v ON, move to
2. check for 12v on red/white cct at loom input [disconnected from rear of dash] If no 12v you know you've a fault in the wiring/loom :. track back and by-pass with a new wire. If 12v, connect to dash pcb and check for 12v with all bulbs out. If no 12v [unlikely] it's most likely an earthing issue with your dash pcb. Check this by running a new direct earth from the pcb/dash connector to earth. Then re-check.
3. If 12v tests okay but then fails with bulbs in you simply have a bulb failure :. change accordingly with new.

ps make sure you've set your DVM correctly, set to 12v DC at 20V range.

Thanks for the reply, I was testing the red/white circuit without the gauges etc attached, I believe this was the problem as this circuit is fed from the instrument loom (as far as i can work out).
So I will fit the dash and test again, fingers crossed it doesn't release the smoke and I have instrument lights!!
PS i have changed all my bulbs for LED so they should last a bit longer.

tottot · Jan 31, 2024

If your 300 is like my 200 there is a switch down right next to hazard that is 3 way, interior lamp, nothing and power to instrument lights.

v8250 · Jan 31, 2024

TBP said:
Thanks for the reply, I was testing the red/white circuit without the gauges etc attached, I believe this was the problem as this circuit is fed from the instrument loom (as far as i can work out).
So I will fit the dash and test again, fingers crossed it doesn't release the smoke and I have instrument lights!!
PS i have changed all my bulbs for LED so they should last a bit longer.

1. You need a wiring diag to work this through.
2. Hope your LED's are not the over bright cheapo Chinesium jobbies - if so, you'll be wearing sunglasses at night.
3. Remember LED's are diodes :. have polarity. Fit the wrong way and they won't work.

TBP

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tottot

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v8250

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TBP

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TBP

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tottot

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v8250

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