740 electrical gremlins

ChiefSOSA

I've got a few questions regarding some wiring problems I've encountered on my 1987 744. After installing a new head unit awhile back (successfully), I decided recently to tidy up the centre console and remove the eq/caught lighter which is no longer wired in. I wired a usb power adapter into the cigarette wiring and testing everything and it was fine. When rearranging the head unit I noticed the sound of a spark and lost power to my stereo/usb port/automatic shifter lights/roof car light. I'm not sure where to start looking for the problem.

Also the odometer plug in the differential constantly comes undone and if anyone has a flx for me if love to hear it. I've used duct tape which helps but isn't a permanent fix.

Thanks in advance.

tbro

Check your fuses first, if one is blown your on the right track. Grab a test lamp and put the earth to one side of the fuse holder and the probe to the other side, if it lights up you have a dead short and now the fun begins.
To be continued if needed....................................

Major Ledfoot

ChiefSOSA wroteWhen rearranging the head unit I noticed the sound of a spark and lost power to my stereo/usb port/automatic shifter lights/roof car light.

Caution - you may have lost power to your brake lights as well. Check the condition of fuses 4, 5, and 14, and make sure the green/yellow, green/red, or orange wires aren't tapping out to the car body anywhere.

As Tezza suggested, get a test lamp and see if you've got juice -
- on the green/yellow wire from fuse 4 and green/red wire from fuse 5 = all the time
- on the orange wire from fuse 14 = with ignition in accessories and on

This is the link you need for an '87 740's wiring diagram. (26.2 Mb pdf)

http://www.volvowiringdiagrams.com/volvo/740%20Wiring%20Diagrams/TP31058%3A1%201987%20740%20760%20Wiring%20Diagrams.pdf

These diagrams make fault finding very easy, unless some previous owner has been ... creative, shall we say... with the wiring.

familyman

Yeah, difficult to diagnose via the internet. It's the kind of thing you have to look in the area you were working in, think about what you moved, what might have touched together... Look slowly at each wire for what could have touched, and/or tiny signs of a 'weld mark' where something touched... you either just see it with your eyes - or, you have to begin tracing fuses/wires and/or looking at a circuit diagram.

i.e. Obviously something has touched together that shouldn't have. But whatever that was may no longer be touching. (You might have only brushed against something while putting the dash together, but now it's back together, is nowhere near touching.) Or it could be touching constantly now. Or maybe only touches if you for instance go around a corner at a certain speed causing wires to move enough to touch, if you get me.

* If it was a momentary touch and is no longer that way, replacing the blown fuse solves it.
* If it's permanently touching, replacing the fuse will blow it again.
* But if it's something that brushes together only at certain times, it may only blow the fuse again. But it could also start a fire inside the dash that you might not be able to extinguish in time (especially if the dash has since been screwed back together).

I've had this happen on a steep mountain with an 18 wheeler up my rear, and nowhere to pull over to put it out. It ain't nice.

Don't know anyone that can look it over? Or live near anyone on here? Maybe post where you are, and someone might be able to have a poke around if you're not confident?

Vee_Que

Start with the fuse in that area, remove the box where the eq was and see if anything is touching. I assume you used joiners or soldered and at the very least electrical taped the wires too? There isn't much in that area that is exposed in my extensive experience with 740s.