a couple of observations…has the back of your instrument panel been messed with ?
The terminals are very close, they can touch.
Inspect it very closely…
one way to check a fairly simple circuit like fuse 8, (if its blowing all the time ?) is to disconnect each component, and run rolls of (fused) wire external to vehicle…if you are at your wits end
with regard to the earthing interior lights.
There are connectors in the pillars…very time consuming to access though.
You probably need to isolate 12V from travelling down that wiring branch, or isolate the ground at each wire/door switch
If you isolate 12V from each side, at the light switch, that will be a clue
You also need to do continuity tests
unless you remove the rear light door switches, nothing can be conclusive
The test light I have can take the place of a fuse. If it lights up, there is a short and the fuse would have blown.
I have no idea if is has been messed with. Doesn’t look like it, but I am not familiar with what it should look like.
The greens are the three green wires going into fuse 8. If I connect only the one, it is OK, only the next one makes the test light glow faintly, and if I connect only the third, it lights up the test light brightly, so that is where the issue lies. But I don’t know what is connected to that particular green wire… however, it does look like that is the one going to the hisser as with that one disconnected and the other two connected, with a fuse in, there is no power at the hisser solenoid. Having said that, it is also connected to the brake switch.
But I can’t see any issues with the wiring. But maybe this is the opportunity to rewire that to a manual toggle switch in the cabin…
I did not know about the connectors in the pillars and I will have a go at checking the rear door switches, but I have seen a connector block behind the speedo as well… some wires there are live, some are not.
It think it is technically possible…good point… and easier to check than most other things
The lights can be unscrewed and the wiring inspected, the wires are also attached by screws, so can be disconnected at that point, which will aid your fault finding efforts
identify which wire goes to the light switch +ve, and which to door switch -ve, via a continuity test
Check the bullet connectors, they may have deteriorated and earthing where they are clamped to the body. This happened on the ‘S’ that I purchased and I only found out when I realised that a PO had run a new wire to the fog lights.
McBoney:
Could one of the lights themselves be grounding?
It think it is technically possible…good point… and easier to check than most other things
The lights can be unscrewed and the wiring inspected, the wires are also attached by screws, so can be disconnected at that point, which will aid your fault finding efforts
identify which wire goes to the light switch +ve, and which to door switch -ve, via a continuity test
Btw, the pillar vinyl comes off by taking off the door edge trim, then there are two screws holding the carton underneath to the pillar. Underneath that is the hole to get access to the area where indeed the switch wire was happily grounding away. Once back on the switch, everything hunky-dory.
The interior lights come on when one of the doors is open or if I switch the bright/dim/panel light switch to position 1. What does that switch do in position 2? It says ‘bright’, but everything goes off…?
Instrument lights also work again, although something loose behind the speedo as wiggling the wires flickers the lights in speedo and rev counter. Heated screen light does not work, but glovebox is also back again.
On fuse 8 I will first rewire the Hisser solenoid through a manual switch with an inline fuse. Then I’ll see if I can identify which green wire that is to the fuse and disconnect that and isolate all the wires to/from the Hisser solenoid so they don’t continue to short.
Next I will see what the brake switch is like and see if that is grounding. If I can’t trust the wiring, then that will have to be rewired with a new wire, to fuse 8.
Lastly I need to check the instrument voltage regulator… how?
disconnect it at its green power wire…is it getting power ?
The FSM and archives decribe how to check it
I PM’d you the correct 420G circuit diagram… you can magnify or print out the exploded view
you need to double check, there is a few more things on fuse 8…power windows ?
Also goes to the auto trans neutral switch
You have not indicated the nature of fuse blowing…continual, intermiitent, ?
However my first target would be the Reverse Light Assembly
an art nouveau looking piece, but the internal phenolic light holder part can crumble
which would mean the bare ended wires inside (same as internal lights) could very easily touch the metal frame
2 screws to remove…dont break the phenolic, its a nuisance to repair
one reason I am trying to be of assistance is partially self-interest, in that I havent sorted my glovebox and “map” lighting yet, they havent ever worked, some loose P&Y wires in there
its on my “to do” list, there are just always so many other priorities, all my other lights are nearly sorted, many LEDs now.
I do intend to “de-electrify” my in-cabin wiring as much as possible, with the addition of relays, in the (hopefully) near future, headlights first
There is something wrong here. The bright/dim switch should only control the instrument lights. The interior/map switch 1st position up is the map light, 2nd position up turns on the interior lights even if the doors are all closed. That is the way it works on my 3.8S. Hope this helps.
I switched one wire across from one switch to the next and that sorted it. However, there is no difference between bright and dim… and only one wire going into that switch… and none loose (that I can see).
On fuse 8 I made some progress…
Of the three greens going in to fuse 8 I now know which are which. The reverse lights one is fine and the lights work when fuse in and ignition on. I also deduced through continuance testing which one goes to the Instrument voltage stabiliser. This one also works with the fuse in and ignition on, and fuel, water and oil gauges work.
That leaves the third green wire, which therefore must operate the cold start solenoid and brake switch. There is definitely a short in that circuit.
I tested continuance between the cold start green, the otter switch and the green going to the fuse and … nothing. I had expected continuance testing to show that these were connected to that green going into the fuse, but no. Not sure what to make of that, but if I’m going to re-wire that separately with a manual, fused switch, not that big a deal.
That then leaves the brake light switch. I couldn’t find it! Does anyone know where that is located so I can check it and rewire if necessary?
Check my diagram above showing the panel light switch. One red (R) wire feeds the switch and a red white (RW) leaves the switch and branches to the panel lights (the instruments). A red blue (RU) leaves the switch and branches to the three panel edge lights. It appears these do not dim. Also appears there is a resistor in the switch that creates the bright/dim feature feeding the red white (RW) wire.that branches and goes to each instrument light bulb. Sounds like you are making progress. Keep at it.
Hi john, that is exactly how it’s wired right now, but no dimming, it switches it off altogether. I guess the resistor is fubar. Never mind, not important. But thank you for the confirmation!
McBoney: