85 740 Dash Panel Illumination Lamps Out
#1
85 740 Dash Panel Illumination Lamps Out
Hi,
After replacing a leaky heater core in my 85 740, all the dashboard nighttime illumination lamps are dead. No instrument illumination, nor heater control, etc. Brightness rheostat has no effect. Gauges work, clock works. Guessing I either accidentally cut a wire, or didn't hook one up correctly. The wiring diagrams I have found so far have not been a help.
Does anyone know what the feed/ground wires for this lamp circuit look like, and where they connect? Thanks!
After replacing a leaky heater core in my 85 740, all the dashboard nighttime illumination lamps are dead. No instrument illumination, nor heater control, etc. Brightness rheostat has no effect. Gauges work, clock works. Guessing I either accidentally cut a wire, or didn't hook one up correctly. The wiring diagrams I have found so far have not been a help.
Does anyone know what the feed/ground wires for this lamp circuit look like, and where they connect? Thanks!
#2
Do your headlights work OK?
I only have diagrams for a 1992, not a 1985. On a 1992, the power goes from the battery to the positive terminal which is under the driver side dashboard, and from there to pin 30 on the main headlight relay and to the ignition switch. ignition switch pin 151 has a blue-yellow wire that goes to the headlight switch pin 1. a white wire comes off the headlight relay pin 87 and goes to pin 1 (top) of fuses 21 and 22. the other side of fuse 21 goes to all the dashboard control lights via another white wire which branches out to white wires to all said lights. this also goes to the instrument panel lights via pin A:5. instrument panel pin B:1 and the other side of all the discrete control lights have blue wires that are all connected together and go to pin 4 of the dimmer module next to the headlight switch. pin 6 of this dimmer module has a black wire to the main ground point near the lower right side door hinge.
so, if your headlights work, I think I'd be finding this fuse 21 and seeing if its hot, and if so, then check the wiring to the dimmer module. and of course, your 1985 is fairly early and may very well be different than my 92, which is for all practical purposes a 940.
I only have diagrams for a 1992, not a 1985. On a 1992, the power goes from the battery to the positive terminal which is under the driver side dashboard, and from there to pin 30 on the main headlight relay and to the ignition switch. ignition switch pin 151 has a blue-yellow wire that goes to the headlight switch pin 1. a white wire comes off the headlight relay pin 87 and goes to pin 1 (top) of fuses 21 and 22. the other side of fuse 21 goes to all the dashboard control lights via another white wire which branches out to white wires to all said lights. this also goes to the instrument panel lights via pin A:5. instrument panel pin B:1 and the other side of all the discrete control lights have blue wires that are all connected together and go to pin 4 of the dimmer module next to the headlight switch. pin 6 of this dimmer module has a black wire to the main ground point near the lower right side door hinge.
so, if your headlights work, I think I'd be finding this fuse 21 and seeing if its hot, and if so, then check the wiring to the dimmer module. and of course, your 1985 is fairly early and may very well be different than my 92, which is for all practical purposes a 940.
#3
Headlights work OK. Everything seems to work OK except for the instrument panel and heater control illumination lamps.
I will inspect the fuse 21 again and its wiring, but the dimmer is set up differently than your description. It is integrated into the instrument panel and that is the main reason I cannot figure out its feed/ground wires. They are probably part of one of the bulk connectors that connect to the instrument panel. Was that what your callouts to A:5 and B:1 were?
Swapping in a spare instrument panel gives same symptoms.
I will inspect the fuse 21 again and its wiring, but the dimmer is set up differently than your description. It is integrated into the instrument panel and that is the main reason I cannot figure out its feed/ground wires. They are probably part of one of the bulk connectors that connect to the instrument panel. Was that what your callouts to A:5 and B:1 were?
Swapping in a spare instrument panel gives same symptoms.
Last edited by tintintin; 09-14-2012 at 02:16 AM.
#4
Was able to do more troubleshooting on this today. Fuse 21 gets +12V with headlight switch on. Both sides of the fuse.
At the 8-pin connector to the instrument panel, pin 2 white wire and pin 1 blue wire each get +12V also with headlight switch on.
The dimmer module is actually a 1.25in. x 2.5in box screwed onto the back of the instrument panel with 3 wires coming out of it which are screwed or soldered into different points on the circuitry on the back of the instrument panel.
Anyway, I guess I did find out a little more, but am still confused as to where to concentrate next.
At the 8-pin connector to the instrument panel, pin 2 white wire and pin 1 blue wire each get +12V also with headlight switch on.
The dimmer module is actually a 1.25in. x 2.5in box screwed onto the back of the instrument panel with 3 wires coming out of it which are screwed or soldered into different points on the circuitry on the back of the instrument panel.
Anyway, I guess I did find out a little more, but am still confused as to where to concentrate next.
Last edited by tintintin; 09-20-2012 at 11:43 PM.
#5
well, that dimmer is /probably/ a potentiometer, which is like a resistor with an adjustable center tap. one wire probably goes to +12 (when the lights are on), one probably goes to ground, and the third goes to the dashboard lights, and would vary in voltage between 12V and a much smaller value (it probably doesn't go all the way to zero)
catch-22, I don't have any wiring diagrams for 740's before 1991. my 92 has the dimmer down by the headlight switch, next to the electric clock adjust buttons.
catch-22, I don't have any wiring diagrams for 740's before 1991. my 92 has the dimmer down by the headlight switch, next to the electric clock adjust buttons.
#6
Poked around there some more today w/ a test lamp. I disconnected the connector w/ the white (from fuse 21) and blue (other side of lamps) wires from the instrument panel.
At the free connector, both those wires show +12VDC when the headlight switch set to pkg. lamps on.
My guess is that the blue wire should not be showing current, with the dimmer circuitry disconnected, correct?
At the free connector, both those wires show +12VDC when the headlight switch set to pkg. lamps on.
My guess is that the blue wire should not be showing current, with the dimmer circuitry disconnected, correct?
#7
Problem Solved
This turned out to be a bad dimmer rheostat. What confused me is that the spare instrument panel I swapped in had the same problem, but I figured, what were the odds of that? Of course, by the time I revisited the possibility of (2) bad dimmer rheostat, I had already taken apart the dash and console looking for shorted, broken, or misconnected wires.
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