I fitted some parking sensors this weekend to my 2005 motorhome, a straight forward job, or so I thought.....

I bought a wireless set off e-bay so sensors and control box in the rear of the van, with power to the unit coming straight across the reverse bulb, and in the front of the van the receiver wired up to some aux power point. However, having wired them up in this way they didn't work After many hours of trying to work out why they don't work, I've given up for now to try and find some direction from somebody.

When I connect the control box directly to the battery it works fine, maybe an earth problem I thought but I've checked the earth on the reverse light to an earth at the front of the van an only have 0.7 ohms, so that looks OK. I also connected the earth of the control unit to the earth of the reverse lamp and the 12v to the fuse box and still worked OK. Maybe insufficient current getting to the control unit, so I connected the unit back across the reversing light and measured the current (without the bulb) and that was 32mA, so again looks OK.

Now only thing I did notice was that across the bulb I'm only measuring 10V when the engine isn't running, suggesting the battery voltage is low, but at the engine end I'm getting 12V. When the engine is running I get 12V across the bulb, and the van starts on the button every time, so I don't suspect the battery. Now I know the parking sensor run down to about 8V as I tested them on the bench so it's not that either.

So what am I missing? The parking sensors don't work when connected to 12V across the lamp, but do work when I power them directly from the battery. I know I could connect them to a battery and put a switch in line to turn them off when driving forward, but that means running wires the length of the van which I don't want to do. Can anybody put some new light on where I'm going wrong.

Mike

Mike n Helen said:

So what am I missing?

Click to expand...

Distance from transmitter to receiver? remember wireless signals will attenuated by the body of the van and even in free air signal strength decreases proportional to the *cube* of the distance.

We tried wireless on a previous van (only a van conversion) and these were a pain in the proverbial very rarely worked, despite having replacement rear boxes and receiver units.

Installed dolphin ones in the new van spot on every time.

Sorry to be the doom and gloom one.

Regards
Stewart

moodrater said:

Distance from transmitter to receiver? remember wireless signals will attenuated by the body of the van and even in free air signal strength decreases proportional to the *cube* of the distance.

Click to expand...

No it's not the distance, when I tried powering the control box from the fuse box, the control box was mounted in the back of the van and it worked fine. It's something to do with the feed from the 12v reversing lamp.....

Your voltages all seem suspicious, with the engine running you should be seeing some 13.8 volts between positive and earth, engine off 12.4 to 12.8 some hours after engine was run or charger used. 12 volts shows a problem on the circuit and 8 volts disaster.

Sounds like lots of poor contacts all over the place, my van has problems with poor conductivity on rear light units due to corrosion in the light fittings. If you are really seeing only 12v on the battery with the engine running you have some charging problems and probably a dead battery

PS I have a wireless camera on the back of the van with the rx under the dash bin in the passenger side and absolutely no problems.

If I've understood you correctly you have effectively connected the control unit across the terminals of the reversing bulb. The filament of this bulb has a resistance and so will the internal electronics of the control unit and these two resistances will now be in series.

Thinking back to my college days, two resistances in series will add up to a different resistance (though I can no longer remember the formula) and this will affect voltage and current draw and this might affect the operation of the control unit.

This is only a theory and I'm ready to be corrected but you could test this by taking the bulb out; then the control unit would have the benefit of all the circuit voltage to itself!

Crossingkeeper you're getting confused. Connecting the unit across the reversing lamp puts the resistances in parallel and means that the current taken by the unit is simply added to that taken by the bulb. (The formula for resistances in parallel is 1/R = 1/R1 + 1/R2).

Mike n Helen, the only thing that comes to mind is that the filament of an incandescent lamp has a very low initial resistance until it has reached operating temperature which means it momentarily draws a much higher current. This causes a high initial voltage drop in the cabling. The result is that the voltage at the lamp may, for a few tens of milliseconds, be less than the 8V minimum you quoted for the controller and maybe prevents it from initialising properly. Have you tried operating the unit without the reversing bulb?

Have you taken the reversing bulb(s) out and measured a voltage from the feed side of the reversing bulbs to earth when the gearbox is in reverse and the ignition switch on (but engined not running)? The voltage should be very close to the starter battery voltage (12 point something volts minimum).
If it's substantially lower then suspect the wiring from the reversing switch on the gearbox, the feed to the reversing switch or inadequate earthing somewhere.
The voltage across the reversing bulb(s) when illuminated via the gearbox reversing switch cannot be 12v as your are measuring the potential difference (V) across the bulb and not between the feed to earth which should be about 12v. (hence removing the bulb(s) before checking the voltage).
If your setup works when you apply a 'clean' 12v to it then you need to find out where the voltage is being lost (or where the resistance is!).
If all else fails connect a voltage sensing relay from the reversing lamp feed to earth and switch 12v from that. This should provide you with a 'clean' signal and get your reversing sensors working.
Alternatively install a reversing camera (but preferably not a wireless one!).
Good luck hope you get them working

fenman said:

Crossingkeeper you're getting confused. Connecting the unit across the reversing lamp puts the resistances in parallel and means that the current taken by the unit is simply added to that taken by the bulb. (The formula for resistances in parallel is 1/R = 1/R1 + 1/R2).

Click to expand...

you're quite right of course, however I still think it would be a useful check to try the operation of the control unit with the bulb taken out.

That's definitely the thing to try - taking out both bulbs if there are two lamps.

Thanks for the input, so here's an update.

I've tried using the 12V from the rear tail light and still doesn't work. I've confirmed the system works if I connect the 12V of the receiver to 12V in the fuse box and use the earth at the rear of the van in the plug. I've also confirmed the system will work if I use the 12V from the plug at the rear of the van and an earth from the front of the van. So independently the electrical connections work, but when I try both 12V and gnd at the rear it doesn't work. :bang:

I didn't want to run a wire from the front to the back, which is why I went wireless in the first place, so I thought I would be able to run a short wire at the back of the van to the vehicle chassis. However, poking around with my multi-metre, it doesn't seen as if the rear chassis of the van is earthed, surely that can't be right????

Mike

Do you mean that you have measured the resistance between the earth at the back and the earth at the front and found no continuity?

What happens if you measure the voltage difference between the +12V in the fusebox and the +12V at the rear socket?

I haven't measured the volts drop across the two 12 V supplies, but the earth resistance from to back is only 0.7 ohms

Mike

That sounds perfectly ok, so are you saying that the earth at the socket appears not to be connected to the van chassis at the rear? If you are trying to take the receiver supplies from across the socket that surely wouldn't matter.

Yes, that's exactly what I'm saying and it doesn't make sense as to why the thing doesn't work. I wasn't sure about the chassis earth, the coach builder may have isolated it for some reason but I can't think why....

It might help to get a clearer picture if you could let us know what kind of motorhome you have. Who is the builder? Is it a coachbuilt body on a Ducato chassis-cab or is the rear chassis an addition by Alko or some such? Where are you trying to find the chassis earth? I would expect the floor of the coachbuilt body, probably just ply, to be bolted to the chassis in which case the aluminium skin would likely have no electrical connection to the chassis members unless explicitly bonded to it. This wouldn't be necessary for the rear lighting as it's clear that it has an earth running in the loom from the front of the vehicle.

It's a 2005 coach built MH by CI. The floor is wood as you say, but the chassis is obviously steel with an extension on the rear of the chassis to support the 21ft body of the MH.

I've tried to find an electric connection between the cab earth and the chassis earth but there doesn't seem to be one which I find very strange when I look under the MH.

I'll try again tomorrow and see what unfolds, thanks for bouncing ideas around..

The negative terminal of the battery will be linked to the van chassis near to the battery position this giving the ground; this -ve should be present throughout all the metal parts of the original chassis cab that the motorhome was built on; not sure why it's not present in the chassis extensions which are simply bolted on the original chassis.

Just an idle thought: some of these imported aftermarket gadgets can be set for either +ve earth or -ve earth, could be worth checking this?

If I was doing this normally I would take the supply from the reversing light and then establish an independent earth point somewhere convenient. There do seem to be continuity problems here.

If there are earth continuity problems between the base vehicle and chassis extensions it's probably down to corrosion. However that shouldn't be an issue if the supply for the control box is being taken from across the reversing lamp socket since, as already discussed, there is an earth running through the loom.