12S burnout mods...

[rogerthechorister]

This is probably a bit retro for all you modern Volvo owners with modern continental 13 pin caravan electrics (the invention of the devil) but here goes. THIS APPLIES TO pre-'98 WIRING. DO NOT TRY THIS ON a post-98 van without checking the wiring diagram.

My caravan fridge shows 13.8 volts (as it should) being delivered to it until I turn the fridge 12 volt circuit on. When I started the "sort this out" game, when I turned on the 12 volt circuit for the fridge, the available volts at the fridge dropped to under 10. Which was probably why the 12 side of my fridge was about as useful as a chocolate toasting fork.

To avoid pushing the van about I am using a 12S extension lead - kosher toy with thick wires where it should have thick wires.

I have done the standard earth connection mods (cross-link both earths, ie the one for the fridge and the one for the lighting and other circuits) in the car AND inside the van, and also paralleled up the unused brown wire.

I have found that some looney who built the van had not taken a direct earth for the fridge from the black earth wire on the 12S coming into the van, but had taken all the OTHER earths but not that one via the control unit. I have sorted that and there is now a direct 15 amp wire from the black incoming 12S earth (which is cross-linked to the white earth AND the unused brown wire) to the fridge earth.

Wunnerful. I have got up to 10.7 volts at the fridge now.

Next step I think will be to cross-link BOTH of the switched 12V+ wires coming from the 12S connectors, inside the van. There are three 12V+ wires - one unswitched for internal lighting, and two that are only live when the car ignition is on. These are red, blue, and green (I can't remember at the moment which one of these three colours is unswitched but 2 seconds with a voltmeter will tell you.

I am not expecting to gain as much as another half volt this way.

SO - what then???


I have got an 80 amp feed in the boot of my Volvo - it powers the sub amp and comes direct from the battery. Obviously that would be dangerous to cross-link, just like that, inside the car. But I COULD use a relay, triggered by either of the switched 12V feeds in the car, to add that to the other two +12V feeds in the car and cross-link the lot. I think it would be wise to add a thumping great diode as well - something that would stand a forward current of about 20 amps and a reverse voltage of about 15 volts - to avoid whooshing electrons the wrong way back up the normal two 12V+ feeds in the boot.


Does anyone know a good source for such a relay and diode?

[DC850]

Hi rogerthechorister,

You appear (if I follow you right) done a good job of checking the Switched 12V to the fridge and the earths returns within the Caravan. Have a look at the earth connections from the sockets on the car to its earth points. I had a Failure of road lights and etc if I plugged my 12S into the car when trying to leave a site. This turned out to be the cars earths connections to the sockets (wires melted). All three earths 12N and 12S being commoned together and then wire from the 12N (thinner one) connected to earth. Fixed by connecting all indepentantly to earth.
This installation was done by a towbar fitting company which I arranged.

The drop in voltage can be caused by high resistance (thinner wires) in the cable.
Check the voltage drop by measuring from the 0V (earth) of the fridge to the earth point (Negative) of the car. if this reads 2V its the earth line issue. Do the same for the positive side of battery and fridge. if 0V then thats OK but if 2Vs this is the problem. If both are not 0V or nearly 0V then both are an issue.

Walking back along the connections looking for the 0V reading should show where the issue is.

Note during the tests the fridge needs to be on 12V and the engine running.

Hope this helps

[rogerthechorister]

Yes, thanks - the urf in the car I have hooked up now looks good (my 760T) - all the urfs to a big bolt near the tail-light and aerial. All 3 cars (I have a B234F and a Sport as well) are showing the same volts drop which militates against it being a car-based fault. Before I changed the batteries in my voltmeters (yes, two voltmeters as well!) I was getting readings of about 1 ohm to earth on all 3 cars, from the black centre earth in the car's 12S socket to the nice shiny plate that the back door closes onto (which has 5 self-tappers through it into car bodywork). I will try again now I have new batteries in - all my other resistance measurements went down when I put the new batteries in!

I'm also able to (and will) check the voltages at the 12S plug that goes into the car, by taking the cover off it.

Back later.

[DC850]

Originally Posted by rogerthechorister

Yes, thanks - the urf in the car I have hooked up now looks good (my 760T) - all the urfs to a big bolt near the tail-light and aerial. All 3 cars (I have a B234F and a Sport as well) are showing the same volts drop which militates against it being a car-based fault. Before I changed the batteries in my voltmeters (yes, two voltmeters as well!) I was getting readings of about 1 ohm to earth on all 3 cars, from the black centre earth in the car's 12S socket to the nice shiny plate that the back door closes onto (which has 5 self-tappers through it into car bodywork). I will try again now I have new batteries in - all my other resistance measurements went down when I put the new batteries in!

I'm also able to (and will) check the voltages at the 12S plug that goes into the car, by taking the cover off it.

Back later.

From this information, I would measurethe voltage from 12S pin 6 to pin 7 at the while plugged into the car and car running and fridge set to 12Volts.

I expect to see 12V to 13Vish

Move the Pin 7 connection to the Fridge Negative terminal (extend wire if needed) Take voltage measurement.

If = 10Vish the problem is poor earth return if 13V to 14V then the earth is Ok

Repeat the measurement from pin 7 to Positive connection for the fridge

If = 10Vish then the problem lies in the pin 6 to Positive cable.

If both equal the same (11Vish) then both cables have the issue.

Replace the cable that is showing the issue with large enough gauge cable, I would use 30amp cable to replace them

[rogerthechorister]

Sound theory. The problem is finding long enough leads for my voltmeter!

Anyway, today's progress so far is that I do in fact have good urfs in both cars that I have tried (- the 760T with the B230ET lump and the B234F - the newer Sport has not had 12S urf mods done yet).

I have rescued a scotchlok connector and AFTER checking which is the constant +12V into the van and which the other one that is controlled by the ignition switch (I am bad on telling blue from green under artificial light, bit of a snag when doing wiring) I will scotchlok the two switched +12Vs in the van together. Then I am going to check how the +12Vs on the 12S on the car behave with load on.

I am showing no resistance from one end of the extension lead to the other (both on urf and feeds) but of course that is on no load.

Back in half an hour.

[rogerthechorister]

13.16 volts at the fridge - under no-load conditions.

Turn the fridge on - 10.2 V at the fridge

10.96V at the fusebox just inside the van. This points to a significant resistance somewhere in the van, and the earth (that I just installed) is continuous bar one choc-block from fusebox to fridge and AFAIK the +12V does not go through the van control box but direct from fusebox to fridge. JUST possibly a previous auto-electrician might have shared a terminal at the van battery charger, but I doubt it in that (a) before he died he was a very good problem-solver (b) at the time he was fitting a new van battery charger (one of the clever modern Italian ones) to replace the van's original that had died (c) there would have been no need and (d) the Italian thing came with totally weird plugs that were all that would fit it so nobody in their right minds would have added a connection there.

But it's still not enough to explain the problem.

Next stop the two ends of the extension lead...

[rogerthechorister]

Using a different voltmeter (they read slightly differently)

9.96V at the fridge
10.83V at the fusebox
11.38V at the van end of the extension lead
11.97 at the car end of the extension lead
12.73 at the car fusebox (so THAT must be where the autoelectrician took the 25A feed for the trailer box)
13.13 at battery.

I think I'll try a different car now!

[rogerthechorister]

Doh! I had the headlights on.

Now 12.27V at the back of the car.

10.2V at the fridge.

So I'm losing a volt in the car and a volt in the caravan - and half a volt in the extension lead which I will not be losing in actual towing.

What I need now is a FLAMING GREAT feed to the little switchbox in the car - that's easy in one car but not in the other - and some really hench wiring in the van. Next question - how to get it from the front of the van to the fridge without ripping the shower out. I REALLY didn't want to have to drill through the draining board to get it down from above, and I also didn't want to have to add conduit under the van! I suppose some conduit at floor level just inside the 'van door is going to have to be essential...

Alternatively, I might give up.

[rogerthechorister]

In the other car

13.6V at the car fusebox
12.9V at the back of the car
10.7V at the fridge

I give up for the time being.

When I can find some HUGE 12V cable like for sub amps I will run that brom battery in car to the little switchbox thing in the car, and from the fusebox in the caravan to the fridge although fox know how to do that tidily.

[mikealder]

From the figures you recorded the most significant volt drop is in the tin tent, address that by fitting some 6MM or even 10MM diameter cable (the same type you use for a cooker or shower in a house (twin and earth but you won't need the earth wire) this cable should go all the way from the van plug to the fridge - Mike