No power to rear lights (tails brakes or running) on 75 M500

[Vogue75]

Just some background I had my rig running great and taking surf trips for about 4 years after I finally put it on the road. Unfortunately I parked it for a year at our ranch and now I have potential corrosion or pest issues causing me problems. Good news is I had everything working before I parked it so I'm hoping I just need to find some spot problem.

Only one front turn signal and flasher was working, I found a lot of problems with the brown L10 wire and grounds in the front and when I fixed some spots I now have both fronts working and flashing in turn and hazard setting (so I'm assuming it's not an issue with the signal switches).

I ran battery voltage to the (L10?) brown wire coming down from above in the back as a test and the tail lights and running lights come on in the back but no turn signals flashers or brake lights.

Tomorrow I'm going to dig into the brake switch and see if there's corrosion there or at the connection but I have a few questions.

1: If I want to splice into that brown wire permanently where should I attach it at the front? Just anywhere I have voltage with the headlight switch on? Trying to understand how the flasher circuits work and if there's more wires to chase that run front to back.

Under my dash is a rats nest of aftermarket jobs which is positive and negative, previous owner installed a TBI among other upgrades, and it looks like they removed the bulkhead connector and now the wire loom just runs straight through the firewall into the underneath of the vehicle. Some of the colors are not correct but the positive is these wires are in good shape on the interior side as a result.

2: Would a bad brake switch or connection affect the rear lighting or flashers other than just the brake lights? I think it's weird they all went out at once but it may help me narrow down the problem spot. I'm in California so my rig has the electronic speed switch shown in the wiring diagram, not sure what this is for (cruise control? No controls on the dash if so). I plan to test there tomorrow as well.

3: front running lights are out as well, probably more L10 brown wire corrosion just thought I'd mention in case that wire typically goes through there before heading to the rear of the vehicle.

Sorry for the long post but I've been chasing this all for a few days so my brain is becoming mush even though I'm starting to figure things out. Tough to work on a vehicle where most every wire is 50 years old and 30+ feet and disappear under the body or into the house portion for most of the run haha.

[RockwoodMike]

I am looking at the 1973 wire diagram and there is a harness connection that hooks up to the steering column..I see 8 wires..involving all the things you are having problems with..

Check that connection..Take it apart and make sure it is clean..And then all these concerns also involve the turn signal switch..wiggle it to see if it intermittently might cause things to start happening again..   

[Vogue75]

I am looking at the 1973 wire diagram and there is a harness connection that hooks up to the steering column..I see 8 wires..involving all the things you are having problems with..

Check that connection..Take it apart and make sure it is clean..And then all these concerns also involve the turn signal switch..wiggle it to see if it intermittently might cause things to start happening again..   

The turn signal switch is definitely flimsy so my guess is that it's the original part, but the front turn signals and hazards work just fine so I was looking elsewhere in the wiring. Is it possible the switch is only negatively affecting the rear lights?

I'll start there in the morning thank you!

Edit: yeah I see on the schematic that the rear lights and stop lamp are actually three straight pins on one end of this "light blue" connector. Fingers crossed I just have some corrosion in that area 🙏

I will report back tomorrow

[Vogue75]

So this morning I checked the 8 point connector and cleaned it up a bit but it looked ok.

Come to find out though when the plug is disconnected I'm not getting voltage at any of the pins on the vehicle side of the connector except the dark blue hazard flasher, which causes the test light to blink when probed.

I'm ordering a new turn signal switch to replace the flimsy one but shouldn't I have 12v to these pins with the headlight switch activated?

[RockwoodMike]

Again looking at the wire diagram, there is a 20 amp fuse at the fuse box that feeds a flasher..that flasher then feeds to the turn signal switch in the column..

Turn left and it feeds the rear-front lights..same with right.

There is 4 contacts in the turn switch..one for each corner of the motorhome..

Then when you use the hazard 4way switch, it by passes the column switch..blinking all four corners..
But it gets power from another fuse and blinker at the fuse box..

Have you tried the hazard switch?? the column switch is not involved at all in blinking the hazard lights

Looking at the diagram, that 8 wire plug has the 4 wires that feed the 4 corners of the motorhome..With a jumper wire to each of the 4 wires, one at at time, should just light up that one corner..One wire to one corner..If it doesn't, then you need to look at that light or the wire feeding it

[Vogue75]

The front turn signals and hazards both work, headlights work as well as the front corner lights. Just no brake lights or anything else to the back of the rig. No upper running lights on front or back.

I forgot to mention I couldn't get voltage at the brake switch connector this morning either. It looks like I have cruise control installed so that harness has some splices to check just behind the connector.

All the fuses are good at the box I'm gonna pull it off and check the back of the box tomorrow since we've got rain and I'll be inside.

[RockwoodMike]

Look at the picture showing the harness in red box..Applying 12 volts to each wire one at a time should make that light come on..

If you have nothing at all to the back end, it sounds like a cut or burnt harness feeding the back..

[Vogue75]

Look at the picture showing the harness in red box..Applying 12 volts to each wire one at a time should make that light come on..

If you have nothing at all to the back end, it sounds like a cut or burnt harness feeding the back..

Yeah it feels like the harness to the back or a connector to it in the front is broken or degraded. I temporarily bypassed the harness directly from the battery to the rear and got rear tail lights so it's disconnected somewhere in the middle or front.

Trying to figure out where's a good spot to splice into the front harness where everything will work. If I need to just run new wires to the back it's not the end of the world as long as I have full function.

I'll power the pins on that connector tomorrow and see if the voltage gets through to the back end.

[Vogue75]

So when I jump 12v at the steering column connector only the light green and yellow pins illuminate the test light (right front and left front which both work normally)

The horn has been disconnected ever since I've had the rig not sure if this is tied into the rest of this stuff somehow and degraded.

No completed circuit to any of the top 3 pins (right rear left rear and stop lamp which makes sense since that's where the problem is. Unfortunately there's miles of wires in this section to examine.

I'm going to make a longer jumper and try to jump power at the yellow and red molds where these wires extend and see if I have more luck there tomorrow and maybe narrow down the breaks.

[RockwoodMike]

Have you opened the lenses at the rear lights? maybe just a bunch of crudded up light sockets..

It doesn't show much on the diagram but does the rear lighting system have a common ground?? If the ground is bad, nothing would work..

[Vogue75]

Have you opened the lenses at the rear lights? maybe just a bunch of crudded up light sockets..

It doesn't show much on the diagram but does the rear lighting system have a common ground?? If the ground is bad, nothing would work..

I cleaned the sockets and ran a long hot wire to the back outside the rig and the running lights and tails work fine on this, so the rear ground seems ok. I've been going end to end ruling things out trying to narrow down the problem.

Part of the problem is at least in the front the original connectors for the turn signal lamps have been bypassed long ago so I'm dealing with random wires that don't match the diagram in a lot of cases, and they left the unused ones just hanging there so when tracing bundles there's a lot of defunct wires.

[RockwoodMike]

Great! The "Rats Nest" left by the PO..The ultimate puzzle..

Looks like you have narrowed it down to the harness leading to the back..

Do the backup lights work??

I hope you have a nice mechanic's creeper to roll yourself around underneath ..I think that harness is strapped to the left inside frame rail..
A good spot light to inspect the full length too!!

[Vogue75]

So I had to wait for my turn signal switch to be delivered and in the meantime I pulled the fuse box and check out the back, which some of the wire ends look bad so I'm repairing a few one at a time but they are other circuits from the one I'm troubleshooting.

I had a friend over and we were able to do a little more testing since someone could be inside and I'm definitely not getting any power to the pink wire down by the brake switch. I see where this comes out of the fuse box so next thing I'm planning is to just replace the pink wire from the box and see if I can get at least the brake lights working.

I finally received the turn switch and tried plugging it into the harness (outside of the column) but there was no change in the lack of rear lights.

It looks like the brake light power goes down the pink wire to the brake switch then back up to the turn signal switch, I'm starting to think this pink wire is a large part of my problem.

[DaveVA78Chieftain]

Here is the complete wiring diagram if you do not have it: 74/75 Wiring Diagram

To view larger version just select the page.

[Vogue75]

Here is the complete wiring diagram if you do not have it: 74/75 Wiring Diagram

To view larger version just select the page.

Yeah I have the book with the diagrams in the back problem was finding where the break is so I bought one of those $20 harbor freight tone tools. 

Found the break in the harness in 5 minutes, in a wrapped section along the engine. Best tool I never knew existed I'm about to buy extra ones just in case this thing is priceless for a rig like these with miles of wiring!

I'm gonna repair this section and hopefully I got the lights all back 🙏

[Vogue75]

By the way speaking of priceless thank you sir for your website I'm planning to do a full brake job later this year and it'd be extremely hard to source stuff without your resources.

We need to keep as many of these things as we can on the road, new RVs and vehicles in general are stale, it's people like those on this site that keep all the good stuff running.

Thanks y'all

🫡🫡🫡🫡

[DaveVA78Chieftain]

Hope my site helps.  Be aware though I put that together around 15 years ago and things change. Some parts are no longer available.  Make sure you know what chassis you have (M300, M400, etc.).  Also, 73/74 M350 was redesignated as M400.  73/74 M400 was redesignated as M500.  Many aftermarket suppliers set the system up under M300/M350/M400 but did not update their parts info.

[RockwoodMike]

Found the break in the harness in 5 minutes, in a wrapped section along the engine

Just curious what caused the damage?? Hungry rat?..Hot exhaust??

[Vogue75]

Found the break in the harness in 5 minutes, in a wrapped section along the engine

Just curious what caused the damage?? Hungry rat?..Hot exhaust??

Hahaha back for an update I have multiple breaks here, I got the rear turn signals and flashers working and the reverse lights after two repairs but still no hot to the brake switch.

Up in the front the three wires going to the back had gotten raw inside the bundle (after the yellow and red connectors) and melted together so I repaired that area but still no lights. I followed the sound toner tool and found another area internally corroded all the way at the back right where these wires go back up to connect to the rear lights which got some of it working again.

The white wire for the brake switch tones clear from the turn signal switch to the brake switch but I have no 12v at the pink wire so I may just have to run a replacement and see if that does it.

Ultimately I should replace the entire lengths of wire to the back but for now I'm just patching the bad spots. There were previous repairs back there already which I also redid but I'm making progress and hopefully I'll have it wrapped up after a couple more days of suffering 🤣

[DaveVA78Chieftain]

I assume you are aware that the pink wire (circuit F48), powered from the main 20 Amp EXTLPS (External Lamps) fuse, supplies power for the Hazard Warning Flasher, Stop Light Switch and the B2 terminal of the Headlight switch. So, several things might not be working.  Attach your toner at the fuse (option 1 of toner manual) and work down to the stop light from there.  Can even verify the Hazard light Flasher and headlight switch from there.

[mytoolman]

Guys IMHO the Power Probe 3 is a remarkable tool for this type of work. Of course there are related tools that also do the job but this one tool handles all of what several do by itself.


https://www.amazon.com/Power-Probe-III-Case-Acc/dp/B006P5E87K/ref=asc_df_B006P5E87K/?tag=hyprod-20&linkCode=df0&hvadid=312280085431&hvpos=&hvnetw=g&hvrand=12440881966732950264&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9061189&hvtargid=pla-450195997104&mcid=ea5f919ea061349e805393c4a3f08aea&gclid=EAIaIQobChMI1e-85629hQMVdkp_AB02IQVFEAQYASABEgLez_D_BwE&th=1