This might sound a bit stupid but while removing the instrument cluster from my 1986 740 some connectors just popped out in the back before I was able to mark them. Most of them would only fit one socket so no problem there, except a small one with two cables only (red/white) which can fit multiple places so not sure where this one should be connected.

The cluster diagram in the manual I have only shows the bigger multi connectors and not this smaller one, I hope someone can help me identify where this one goes (and eventually what it is for would be nice as well)

added photo of the connector here:
https://volvoforums.com/forum/attach...1&d=1497297052

 

I only have diagrams for 240's, and 1991+ 740/940's, and I know 740 changed the instrument panel in there somewhere so its not the same

 

I think this is a ground wire, (but verify), and goes on the back of the cluster approximately where the tach is. The only other possible male connector is on the back of the speedo.

 

a red/white wire is usuallythe timing signal right off the ignition coil pin 1, and goes to the tach. brown or brown-white is ground.

on the 1991+ 740/940, that signal is on the "A" 8 pin connector, on pin 6. but I bet a early 740 is completely different

ok, google found this. 1984-1986 volvo 740 wiring diagram, the instrument panel connections.... yeah, white-red or red-white, there's two wires to one connection, and its the tach ('r/min' aka RPM) connection. '85' next to the r/min connector is the speedometer.

 

and, here's two versions of the 740 instrument panel, yours is probably the bottom one, VDO...

spade lug (1) is the tach connection, red/white wires

spade (4) is +12V power

spade (5) is the speedometer output for the cruise control.

 

Many thanks pierce this is great help. I have the top panel in fact (1986 EU model) and I confirm the missing red/white was indeed the tach connection on number 1.

The panel is back on and everything works (except that now I know I need a new engine harness )