Looking for Ignition On 12v signal in Rear Curb Bay

[DaKevster]

Have a 2008 Holiday Rambler 40PLQ and am looking for way to get a 12v signal in the rear passenger (curb) side bay where the Rear Distribution Box is, that is on when the ignition key is in the run or start positions (any position other than Off position). I’m looking to tap the signal to engage a relay just before the starter is engaged or while the engine is running. Looking at the Rear Distribution Box, which is in the rear bay, there are a few fuses that are tantalizing, but difficult to test without repeated engine starting and a second person. I know it'd be easy to get from the front bay distribution, but I really don't want to have to run a wire all the way from front to back. Anyone have thoughts? Some that seem possible are:

F1 – DIAG FUSE

F2 – TRLR CHRG FUSE

F4-ALT SENSE FUSE

F6 – ALT RLY FUSE

[jacwjames]

Can you post a picture of you rear electrical bay?

[jacwjames]

I could not find a 2007 wiring diagram but I found a 2006 in the downloads sections. 

There is a page showing the rear run bay.  There are two relays that have an ignition control wire. 

Suggest downloading the wiring diagram and confirm it matches what you have, then you should be able tap into on of the wires. 

Pictures would help.

[Happycarz]

Kevi,

What model Holiday Rambler do you have? Scepter? Or, other model. 
Pictures of rear run bay would help. 
Also, what are you adding to the electrical system?

[MyronTruex]

The trlr fuse is likely a keyed function. You could test the 7 pin towing socket to confirm. 

[Tom Cherry]

1 hour ago, MyronTruex said:

The trlr fuse is likely a keyed function. You could test the 7 pin towing socket to confirm. 

The “charging” pin on the 7 pin is IGNITION SWITCH controlled.  There is a relay with “hot - fused” power that is turned on.  No charging voltage….find relay and test pins 85 & 86.  They are interchangeable.  One will have switched 12 VDC IGNITION and other is GROUND.  NO 12 VDC to either….NO IGNITION (SWITCHED) POWER…

[DaKevster]

On 3/25/2024 at 7:17 PM, jacwjames said:

Can you post a picture of you rear electrical bay?

This is pic of it. The fuse/relay labels graphic is from the manual. I don't have an actual pic of the inside cover at the moment. 

22 hours ago, Happycarz said:

Kevi,

What model Holiday Rambler do you have? Scepter? Or, other model. 
Pictures of rear run bay would help. 
Also, what are you adding to the electrical system?

It's a 2008 Ambassador 40PLQ. Pics added to another reply.

I'm adding a lot 🙂 1800W of Solar, 3KVA Victron Inverter, MPPT Charge Controllers, 14kWh of 24v LiFePo4 batteries. This particular need is for the two Chassis 12v FLA batteries, which I want to trickle/maintain charge the chassis batteries from the 24v LiFePo4 batteries, but only when the engine is off, and not have connected during the engine start due to the large current draw on the charger. Simple solution is a NC relay that opens when the ignition is turned on disabling the charger. 

Edited March 27, 2024 by DaKevster

[MyronTruex]

If F2 has voltage controlled by the ignition switch, pull it and find out which end is hot when the ignition is on.

Then use a Fox and Hound to insert a tone on the (cold) side of that fuse. Then sniff the wiring bundles on the output side of the fuse holder to chase down which is the trlr wire.

You may have to resort to then sharpening the positive lead or your voltmeter or test light to find the exact wire by inserting the fuse and taking it out while finding which wire is the actual one.

If you look very closely at the  wires they may actually have writing on them.

You may want to use the 7 pin trailer socket while removing each of the connectors on the right side of the fuse panel, one at a time to get you to the right bundle to start. 

Edited March 27, 2024 by MyronTruex

[amphi_sc]

On 3/26/2024 at 9:44 AM, Tom Cherry said:

The “charging” pin on the 7 pin is IGNITION SWITCH controlled.  There is a relay with “hot - fused” power that is turned on.  No charging voltage….find relay and test pins 85 & 86.  They are interchangeable.  One will have switched 12 VDC IGNITION and other is GROUND.  NO 12 VDC to either….NO IGNITION (SWITCHED) POWER…

Just FYI, my '07 Patriot Thunder wiring diagram shows the Trlr Charge line as ALWAYS hot and fused with a 20a fuse F7-3.  I can verify the charge line is indeed hot all the time the chassis battery disconnect is on.  By the way I've been debating cutting that line (as I can't find that fuse in case it ever blows) and using a spare relay socket on the relay board in the RR with signal from the nearby ign relay (which I've already tied into for switching the TPMS booster) and a 15a or 20a CB and a standard 85a diode like is used for toad lights.  Anyway just my thoughts as I was desiring a charge line to my toad and don't really like how my coach is presently wired.