Dometic 3 way fridge and Thule omnistep connection

[Mickthemin]

Hi All, I am new here and have searched existing topics but can't find the problem I am having. I have a 2005 Bessacarr E765 MH and recently the step alarm has started coming on when the fridge is turned to 12v with the engine running. If I extend the step, the alarm stops and the green power light goes out on the fridge. The step used to retract automatically on starting the engine but no longer does. I have read that they share the same relay. I can't work out if it's my fridge at fault or the step. Tried to find the step position switch but it is fully enclosed and I would have to remove the step to get to it (not a problem but if it's the fridge at fault I don't want to waste time and effort). The fridge works well on gas and 240v.

Any ideas of what would cause this set of circumstances would be much appreciated?

[Labby]

The problem may be with the relays , or their connections , including earths ( Split charge and fridge/step ) ,that you should be able to locate under the bonnet near the starter battery (they may be behind a cover) . There should also be a small fuse box there which would also be worth checking for possible faults .

[rayc]

Start by finding out why the step does not retract on engine start up. Leave the fridge switched off. https://www.motts.org/Omnistep.htm

If you can extend the step with the engine running there is obviously a fault. Is the D+ signal present at the step?

[Robinhood]

At that vintage, the electrics may well have been supplied by Nordelettronica. If so, the following wiring diagram might be useful (it's for the 400 series, but the circuitry supplying the fridge and step are likely to be similar).

https://www.motorhomefacts.com/attachments/van_wiring_true_col_300_174-jpg.10708/

I'm struggling to think of any switch or relay failure that could give your symptoms, but I've only briefly considered it.

[onecal]

Hi

Is your leisure battery is been charged when the engine is running ?

Have you checked the fuses to see have any blown, relays  and wiring for any disconnections/break or corrasion ?

Also the step limit switches for corrosion? 

Also why not supply a photo of your electronic unit for us to advise?

Regards

[Alanb]

Is the fridge working on 12V?  A failure may not be obvious.

As suggested by @Labby check the fridge 12V element supply fuse.

I can postulate a mechanism by which the problem can occur, but with the lack of actual wiring details, I have to make the following assumptions.

1. Fridge relay coil, and step alarm are both connected to D+.

2. The step retract relay coil is connected to the fridge side (87?) of the fridge relay contact.

3. The fridge element supply fuse has failed. This may be due to corrosion.

With the step retracted, and the engine running, the step alarm will receive a supply from D+.  The limit switch is open, but with the fridge element fuse blown, and the fridge selected to 12V, the fridge 12V element will provide a return path for the step alarm via the step relay coil in reverse to normal direction.

@Micktheminit takes time to research and answer these queries, so please engage with the forum by posting what you find, on this thread.

Thanks,

Alan

[Mickthemin]

Many thanks for your suggestions, much appreciated. Sorry it has taken some time but I now have some additional information. On the step relay in the engine compartment, I have no connection to terminal 87a. The red and white wire at the control switch is live and the step works both ways with this wire connected. If I unplug this wire the step doesn't work. I tried changing the relay for a new one but this didn't alter anything.

Correction on the fridge working, sorry. The fridge igniter works (sparking for 30 secs) but I don't seem to be getting gas to the burner (plenty in the bottle, and it's turned on and the isolator valve is also turned on). The gas valve would not appear to be opening, so would that be the D+ signal absent?

How would I measure the D+ signal at the step?

I have checked all the fuses I can find and they are all OK according to my multimeter.

I think my van is wired slightly differently to the wiring diagrams you have kindly supplied. The only time I get power to my 12v fridge is when the engine is running, step fully retracted and alarm buzzer sounding.

At the step relay with engine off and step in, I get 13.7v at terminal 86 when measured to earth. All other terminals read 0v and with step out all read 0v. With engine running I get 14.7v at terminals 85 and 86, with 0v at 30 and 87 with step in. With step out I get 15.6v at 85, 86 and 87 and 0v at 30

I'll write to Bessacarr to see if they have a wiring diagram they are willing to send me (I won't hold my breath).

[Mickthemin]

Hi Alan

Your reply has just popped up. That would make sense, I just couldn't think how a connection would work in reverse. I don't know where the fridge element supply fuse is, presumably at the rear of the fridge?

 

[Derek Uzzell]

This wiring diagram is taken from a 2010 thread on another forum, but it may not be appropriate for a 2005 Bessacarr E765. (I've rotated it to (hopefully) make it more readable.) I notice that the electrical system is based on Sargent equipment.

[Keithl]

That wiring diagram looks very much like our AutoTrail with Sargent electrics and our fuses are located next to the starter battery under the bonnet alongside the fridge and split charge relays. I think on Fiats these relays and fuses may be under a plastic cover centrally on the scuttle above the engine.

[Robinhood]

Having searched again, it would appear that the 2005 700 series might have been fitted with Sargent electrics (the E400 probably not).

Anyway, I suspect the most appropraite wiring diagram you'll get is that posted by Derek above, see the manual here:

https://www.bessacarr-owners-club.org/info-centre/manuals/motorhome/Bess-MH-Service-Book-2005.pdf

[Labby]

Is your fridge a fridge/freezer , if so there is an extra power relay and 30 amp fuse fitted , if just a fridge its fuse is one of the ones near the relays under the bonnet. (15 amp )

[Alanb]

4 hours ago, Mickthemin said:

Many thanks for your suggestions, much appreciated. Sorry it has taken some time but I now have some additional information. On the step relay in the engine compartment, I have no connection to terminal 87a. The red and white wire at the control switch is live and the step works both ways with this wire connected. If I unplug this wire the step doesn't work. I tried changing the relay for a new one but this didn't alter anything.

Correction on the fridge working, sorry. The fridge igniter works (sparking for 30 secs) but I don't seem to be getting gas to the burner (plenty in the bottle, and it's turned on and the isolator valve is also turned on). The gas valve would not appear to be opening, so would that be the D+ signal absent?

How would I measure the D+ signal at the step?

I have checked all the fuses I can find and they are all OK according to my multimeter.

I think my van is wired slightly differently to the wiring diagrams you have kindly supplied. The only time I get power to my 12v fridge is when the engine is running, step fully retracted and alarm buzzer sounding.

At the step relay with engine off and step in, I get 13.7v at terminal 86 when measured to earth. All other terminals read 0v and with step out all read 0v. With engine running I get 14.7v at terminals 85 and 86, with 0v at 30 and 87 with step in. With step out I get 15.6v at 85, 86 and 87 and 0v at 30

I'll write to Bessacarr to see if they have a wiring diagram they are willing to send me (I won't hold my breath).

If there is no connection to 87a (NC) contact on the step auto retract relay, somthing is wrong, as it would normally be connected to the manual operating switch/button.

I have previously dowloaded a copy of a Sargent 700 wiring which is probably similar, and wiring methods usually evolve slowly. 

Unfortunately my hypothesis does not fit the diagram, but in view of the missing connection on step relay 87a above, what else could have been changed?

More thinking required.

Alan

[Alanb]

Following from my previous post, I have now extracted the step wiring details from the downloaded diagram, and sketched out a schematic for the step connections, but using the simplified step switch symbol used by Sargent, as this switch is working correctly.

The step is powered solely from the starter battery, with the auto retract function being via the fridge relay. The 15A fridge fuse seems rather closely rated as it has to carry the fridge element current, and briefly, the step motor stalling current.

The wire colours on the step relay should be as follows.

86  Rd/Ye 12V supply from fuse 2 (under bonnet) 15A

85 Bl/Wh earth via step limit switch when step not fully retracted.

87 Rd/Ye as above, fridge supply.

30 Rd/Bl to step motor. (The step retracts when this wire is +ve.)

87a Rd.Gn from push button.

The warning buzzer is connected in parallel with terminals 86 & 85 of the step relay, and should have Rd/Ye and Bl/Wh (limit switch) connected.

Thule/Omnisep diagrams show a 2200 mfd capacitor in parallel with the relay coil. This is to slightly delay the release of the step relay, and allow the step to drive fully home. This is not shown on the Sargent diagram, where the capacitor would also discharge into the warning buzzer, reucing the intended delay.

Alan

 

I am not familiar with the Sargent/Bessacar installation. Is the manual step control via repeated pushes on a single button, or separate linked buttons for extend and retract?

 

[Mickthemin]

Thanks again for your suggestions. The fridge is just a fridge and not a fridge/freezer.

Relay connections are as follows:

30 is fused battery +ve red wire

85 is battery -ve black wire

86 is a red wire going to split charger relay

87 is a red wire in a black sheafed cable running with a black wire (which is a fused battery +ve). No idea where these run to.

The step switch is a rocker switch, press one end for extend and the other end for retract.

I think this relay has been totally re-wired at some point to accommodate some additional fitting(s) in the past.

Looks like my options are to get an auto-electrician in, who might be able to identify which wires run where, or I just re-wire to the original wiring diagram. (Third option is to leave it, push the step out a few mm to cancel the alarm and put freezer packs in the fridge for travelling.)

Thanks again for all your help

Chers

Mick

[Alanb]

Hi Mick,

The connections you describe are suggesting it is the fridge relay that you are looking at.

If  I understand correctly we are discussing relays located under the bonnet.  For clarification how many relays 2 or 3?

I would expect to find split charge and fridge relays, the step auto retract relay could be elsewhere.

You mention a red wire going to the split charge relay, but to which terminal please?

You mention a black sheathed cable  which has red core as relay switched positive, and black core is fused battery.  At a guess this could go to the fridge?  Usually you can see some of the wiring if the grille is removed.

Further investigation could be if possible to ease out the step switch, and record the the wire colours.

Sorry for all the questions, but I am trying to understand what you have.  I agree that it does not seem to be original, but who knows?

If you are happy to make minor wiring corrections if necessary, let's see if option two is possible.

Alan

[Alanb]

In view of the lack of a further reply from @Mickthemin, I can only conclude that he does not wish to proceed with option 2, which was to rewire to the original diagram.

Alan