My problems are minor just enough to be aggravating. My 1991 740 Turbo wagon has a clock that you press two buttons to adjust time. It isn't operating correctly. At the same time when I turn on the parking lights or the headlights my dash lights will first come on at full bright then go out and ramp up again slowly over an eight minute span and the control that slides up and down which I believe is the adjuster doesn't work. Add to these things I want to know where the horn relay is located. I looked in vain for a Haynes manual (in the past with my previous Volvos' Haynes manuals were THE most comprehensive with the electrical schematics. I looked for a Haynes manual for my car and there isn't one in print so I bought a Chilton manual. A total waste of my money !!!! No scheme for the horn circuit, not even a scheme for the power windows and switches !!!
Anyone have an idea where I can begin to look for a comprehensive schematic for the entire electrical system for my wagon ?

 

Volvo "Greenbooks" were the shop manuals used at the dealership when the cars were new...come up quite often on eBay. They are awesome...sounds to me like your rheostat is toast on the instrument lights...

 

Hi swiftjustice44
If the dash lights didn't work at all I would also think the rheostat is bad but the dash and panel light up brightly. Also on that same switch panel is the control that operates the clock so I'm thinking there's an outside chance that there may be a disconnection or crossed wire. My biggest concern is the fact that I can't find the horn relay (I need the horn to pass state inspection) and when I called a Volvo dealership the parts counter guy couldn't find a horn relay listed for a 1991 740 Turbo Wagon. I can't believe the horns would be hard wired to the switches. I'm still doing a net search for a wiring diagram.

 

There is no horn relay. As relay happy as the Swedes were with the Volvo, there are two glaring omissions. The horn circuit and the auxiliary fan. On the 91, power goes first to the turn signal switch and from there to the horns, it is a yellow wire when leaving the column. There is a connector at the base of the driver's side A pillar. With the key on, power is continuously supplied to the horns; pressing the horn buttons grounds and completes the circuit.
I should still have the left side panel w/ headlight switch, dimmer, etc from a 91 turbo sedan. If you think it might help, drop me a p/m.

 

Thanks again swiftjustice44 for posting the block diagram. I was looking at that earlier on a different site (k-jet) then I noticed it was for a 760...I suppose it could be identical but like you alluded to the Swedes' have been relay happy since the first Volvo rolled off the assembly line. I'll get busy with my multi-tester and see if there's a constant 12V at the horn and troubleshoot backwards from there. PM on the way.

 

The Haynes book number for Volvo 740 years 82-91 is 1258