1988 760 Turbo electrical short circuit

[Tom1988]

Hello, I am hoping someone has encountered this issue and knows how to solve it.

I have a 1988 760 Turbo saloon that has developed a possibly unusual electrical problem.

When I turn on the high beam the fog light and blown bulb light come on in the instrument cluster, along with the High beam blue light. The right hand main/dip beam dims and the left hand headlight lens wiper starts wiping. After a few seconds the engine revs up to @2000 RPM and stays there.

I am presuming it is an electrical short circuit. The interesting part is that no fuses blow.

Has anyone come across this issue and how did you fix it please?

Many thanks to anyone that can help me with this, it would be very much appreciated

[TonyS9]

An electrical short circuit would blow a fuse or start a fire, its more likely to be an open circuit, bad common ground or something.

There isn't a simple symptom-cause map, the problem could be anywhere. You need a multimeter and to understand voltage, current and resistance. Crimp connections eventually become resistive from corrosion in damp or moist conditions.

[Tom1988]

Your quick response is very much appreciated

I still have the question of where to start

Since no fuse is blown and there are a number of things going wrong, what would be the most logical place to start checking?

Many thanks

[TonyS9]

Its easiest to start at the alternator, check the voltage and how the voltage drop across the harness towards the battery with the high beam on. Check both + and - connections. You can voltage drop directly across a juntion (should be V small, <0.1V), or measure the voltage across the items like the alternator or the battery. Evenually try to find the power supply for the engine ECU or the wipers and check all the connections to ground are at ground (0V) and not raised.

Obviously be careful of moving parts.

[Tom1988]

Thank you so much for getting back to me so promptly

My search will begin at the alternator, it may take me some time to work my way through all possibilities but I will let you know how I get on in the next few days, I can only do this part time.

Thanks and very best regards

[Laird Scooby]

Originally Posted by Tom1988

Thank you so much for getting back to me so promptly

My search will begin at the alternator, it may take me some time to work my way through all possibilities but I will let you know how I get on in the next few days, I can only do this part time.

Thanks and very best regards

Forget the alternator and check the multiple earth points near the headlamps on the inner wings. If the alternator was playing up, the charge warning light (and others would be on. What you've got is classic back-feed symptoms, if it's not on one of the MEPs near the headlights, you need to check the MEPs behind the kick panel trims on the A pillars.

After that, investigate the flasher/dip/main switch and then the main lighting switch but i'd started with those MEPs.

[Tom1988]

Thank you for this information, very much appreciated

It really looks like I have some work to do

Thanks

[NI_Volvo_Nut]

There's earthing points in the inner wings behind each headlight, I bet they are stinking!

[Tom1988]

Thank you for the feedback

I am still working my way through the wiring

Thanks