# Help! - Reverse light Shows 12v without bulb & 0V with B



## Double-Entendre (May 19, 2009)

Hi.

So today I decided to see why the reverse sensors are not working on my new ('96) Swift Royale (Ducato based). Switched ignition on and no reverse light! That's the problem, me thinks, so I set about changing the bulb . . . . . new bulb doesn't come on!

Out comes the tester and with no bulb in the holder, it shows 4.5V but put the bulb in the holder and the voltage drops to 0V.

I assume a problem with the reverse sensors, so I disconnect the whole lot. Now the holder shows a healthy 12V but still drops to 0V when I put a known working bulb in.

I can't fathom what the problem is and I'm at the end of my knowledge so can anyone please suggest what the issue can be and how to sort it please.

Oh, and with a bulb in the holder, a "Continuity" check, sets the meter to 0.00 (Which means there is continuity between -ve & +ve via the bulb fillament but no voltage.

Thanks


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## camper69 (Aug 30, 2007)

*Re: HELP! - Reverse light Shows 12v without bulb & 0V wi*



Double-Entendre said:


> Hi.
> 
> So today I decided to see why the reverse sensors are not working on my new ('96) Swift Royale (Ducato based). Switched ignition on and no reverse light! That's the problem, me thinks, so I set about changing the bulb . . . . . new bulb doesn't come on!
> 
> ...


To me it sounds like an earthing fault. Have you tried temporarily by passing the -ve wiring by putting in your own earth to the chassis. On a vehicle of your age the connections may be not as good as when it was built.

Derek


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## Double-Entendre (May 19, 2009)

Thanks for that and I can see where you're going with it but it is a "Common" Earth / -VE for all the light cluster and the tail lights read 12V, as do the indicators and the stop lights, so I'm not sure it is an earth issue.

any other ideas folks?


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## richardjames (Feb 1, 2006)

A poor earth can mean that some bulbs will work and some not. I have followed many a Ford that when the brakes are applied some lights go out 8O 8O


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## thePassants (Feb 9, 2009)

A very usefull tool to help figure out what's going on with this sort of fault is a simple test lamp:
Bulb holder (with working bulb in!); one lead to a croc. clip, and the other to a probe.

By connecting the croc. clip end to the chassis (0v), or to a known 12v, you can reliably test for voltage or earth at each point in the circuit, with / without reverse gear selected.
Because a volt meter has such a high resistance, it can give you 'mis-leading' results, whereas a test lamp will either be 'on' or 'off'.


Also, clean & re-connect any connectors in the circuit, corrosion here can often be the cause of faults.

Good luck.


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## Dukeham (Feb 3, 2009)

*Hi.
Long shot but you are using a single pole bulb (indicator) & not a double pole (stop & tail)*


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## duxdeluxe (Sep 2, 2007)

I have exactly the same problem and have not yet resolved it, but will get it fixed when I get it serviced and MOT'd in August (yes, I know that reverse lights are not part of MOT)

They suggested an earth fault but it is all on the same plate (indicator/brake/rear) so must be a bad connection from the reverse light switch on the gearbox - you need long arms to get at it - through the wiring to the lamp cluster. I can't trace the wiring so will leave it for now. It's just annoying......


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## Double-Entendre (May 19, 2009)

Thanks again folks.

now I'm on this "Reply page" I can't remember all the points to reply to but I'll give it a go:

1. my "Contacts" are like new in the light cluster but I reseated them just for good measure.

2. I was the owner of a ford escort with the disco rear lights - problem being a corroded earth that furs up and grows to met another contact - mine on the MH are all spotless.

3. The stop bulb is a single filament, separate to the tail bulb.

4. If it was a problem with the +VE from the reversing switch, why does it show 12V without the bulb in? and 0V with a bulb? and 4V with the reverse sensors attached?

5. I thought the Reverse Light WAS part of the MOT? (My van passed 10 days ago, hence my panic - can anyone confirm if it is a testable item?)

I'll try the "Add a new earth" option tomorrow but I really can't get why the voltage drops to 0V with a bulb in the holder?

thanks folks, thus far

Keep the suggestions coming and I'll keep you posted - seems like I'm not the only one!!!!!


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## chrisjrv (May 7, 2007)

Hi,
If you have a poor connection you can have 12 volts showing but as soon as you try to draw any current e.g. a bulb,the voltage will drop due to the high resistance at the bad connection,my money would be on the switch, with very long arms try bridging it out, best of luck,
Regards,
Chris


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## sallytrafic (Jan 17, 2006)

This is another one of those electrical problems where a digital voltmeter is not your friend. Why? Because it doesn't draw any current through the circuit. (actually it does but it might be a millionth of an Amp) So if you measure an open circuit (bulb removed) it almost doesn't matter how much resistance (badly corroded contacts etc) is between the measuring point and the battery voltage you will still measure the battery voltage.

I know to a non electrically trained person this doesn't sound right but its true.


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## Double-Entendre (May 19, 2009)

stickey said:


> Hi,
> If you have a poor connection you can have 12 volts showing but as soon as you try to draw any current e.g. a bulb,the voltage will drop due to the high resistance at the bad connection,my money would be on the switch, with very long arms try bridging it out, best of luck,
> Regards,
> Chris


Chris, thanks, your suggestion is one I've not tried, though as a total electrical-muppet, I'm not sure just what you mean.

Would you please explain just what I have to do. It's the "Very long arms, try bridging it out" bit that I'm struggling with. Where will i find the switch and what does "Bridge it out" mean (Sorry for being so thick ;-)

Thanks


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## Jezport (Jun 19, 2008)

My bet would be that there is a bad connection or water in the junction box which connects the conversion part of the motorhomes wiring to the base vehicles wiring. Or if towbar electrics have been fitted look there also.

If you enter your location in your profile someone near you may be able to help.


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## Double-Entendre (May 19, 2009)

Jezport said:


> My bet would be that there is a bad connection or water in the junction box which connects the conversion part of the motorhomes wiring to the base vehicles wiring. Or if towbar electrics have been fitted look there also.
> 
> If you enter your location in your profile someone near you may be able to help.


Oh, Man!

Where would I start looking for the junction? Yes, there are towbar electrics (Though not connected to the reverse light)

I'm in Sheffield, S. Yorks, if anyone is near?


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## Jezport (Jun 19, 2008)

You follow the cabling back and I would think the first junction will be the one where the electrics split to supply the towbar. check here first then work further up line.

I am in north Leeds, if you are passing I dont mand having a look.


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## Double-Entendre (May 19, 2009)

Jezport said:


> You follow the cabling back and I would think the first junction will be the one where the electrics split to supply the towbar. check here first then work further up line.
> 
> I am in north Leeds, if you are passing I dont mand having a look.


Jezport, I thank you kindly Sir, for your offer and if I can't make it right over the next week or so, I'll PM you and hopefully take you up on your offer.

The Towbar electrics are all soldered directly to the wires on the rear light cluster on the opposite side to the reverse lamp and all seem ok, connection wise. I'll spend today giving all the suggestions a check but I need to know where the reverse switch is located and how do I access it?

Thanks again y'all


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## chrisjrv (May 7, 2007)

Hi,
This is what I would do....somewhere on your gearbox there should be a switch with two wires connected to it, one will be a supply wire from the fusebox the other will go to the reversing lights, the switch will be in the form of an object screwed into the gearbox casing, the only other thing screwed into the gearbox casing is the speedo sensor(depending on age of vehicle) on my van the switch you want is at the front but it is not easy to see. Hoping that you can get test meter probes to the switch :roll: switch on ignition, select reverse and test VOLTAGE between each switch terminal and earth with a bulb in the reverse lamp and again with no bulb. If you lose the 12 volts on one side of the switch but not the other when a bulb is in the lamp then it has to be the switch, if you lose the 12volts from both sides of the switch with a bulb in then the bad connection has to be from the fusebox to the switch. If you get a healthy 12 volts on both side of the switch it indicates a bad connection between the switch and the lamp. NOW it is possible that the reversing switch operates a relay to turn the reverse light on and off but at this stage you need an auto electrician familiar with your base vehicle,
let me know how you get on,
Regards,
Chris.


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## duxdeluxe (Sep 2, 2007)

The reversing light is not mentioned in the *MOT Test Manual* so it should not be a failure item.

Thanks for the above explanation on voltages - I just need long arms to get at it


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## Double-Entendre (May 19, 2009)

duxdeluxe said:


> The reversing light is not mentioned in the *MOT Test Manual* so it should not be a failure item.
> 
> Thanks for the above explanation on voltages - I just need long arms to get at it


Where is it? Where is it??????????

Where is that reversing light switch located PLEASE!!!!

My latest testing shows the following:

1. Connect a test light - does not light
2. take a +VE feed from the sidelight to the reverse light . . . . hey presto, it comes on!
3. The Reverse light +VE still shows 12V until you connect any sort of bulb

What does this all mean?


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## rayc (Jun 3, 2008)

Hi, you may be better of combining your two topics or at least just use one of them, because they are basically the same problem.

The answer to: 
3. The Reverse light +VE still shows 12V until you connect any sort of bulb What does this mean?

As has been explained in previous replies it means there is a high resistance in the circuit i.e the fuse, the switch or the wiring connections.
Without the bulb you are not drawing current and you are measuring the open circuit voltage. When you put the bulb back in it completes the circuit BUT the volts is dropped across the high resistance leaving none at the bulb. Say the reversing switch has poor contacts inside it. The voltage will pass through the contacts without the bulb but be "dropped" across the contacts with the bulb inserted.


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## 115430 (Aug 11, 2008)

good tests and makes things clearer.

You did say you were electrically challenged so I'll talk down to you if that's OK ?:twisted: 

What it means is the cable feeding the 12V to your reverse light has some corrosion somewhere and the energy (current) needed to light the bulb is being used up in the corrosion and not in the bulb. The meter you are using will still read the 12V at the lamp fitting as it doesn't need much energy to work.

As a 12V feed from the tail lamp in your test does light the reverse bulb you have proved the earthing is OK from the lamp fitting to the chassis. So this leaves the cable TO the lamp fitting and the first culprit will be the reverse switch.

This will be located by the gearbox so it'll get activated by the action of selecting reverse, look for a sticky out thing with 2 wires on it !. 

Sadsack


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## duxdeluxe (Sep 2, 2007)

That was so simple, even I could understand it.... cheers!

Open the bonnet of thevan and look down into the engine bay to the right of the engine as you look into it. The gearbox is sort of near the bottom and down there. You should see the reverse light switch (2 wires) but you need very long arms to get at it. I'm leaving it to the garage to deal with........

P.S. that's a 2.0 JTD - others might be different but maybe not - I did ask on a fiat forum HERE

Hope that helps


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## Double-Entendre (May 19, 2009)

sadsack said:


> good tests and makes things clearer.
> 
> You did say you were electrically challenged so I'll talk down to you if that's OK ?:twisted:
> 
> ...


Muchas G Sadsak

That's just the kind of response I was looking for - an explaination / answer to my question.

FANTASTIC & Thank you . . . . . Now I understand


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## Double-Entendre (May 19, 2009)

duxdeluxe said:


> That was so simple, even I could understand it.... cheers!
> 
> Open the bonnet of thevan and look down into the engine bay to the right of the engine as you look into it. The gearbox is sort of near the bottom and down there. You should see the reverse light switch (2 wires) but you need very long arms to get at it. I'm leaving it to the garage to deal with........
> 
> ...


thanks to you too, a descrioption of where to find and what I'm looking for.

Brilliant!

Much simpler than all the techno stuff and dead links and everyone can understand.

Thanks again


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## sallytrafic (Jan 17, 2006)

That was a nice put down for all those that tried to help you earlier3.

You didn't even acknowledge me telling you exactly where the switch was on your other thread


> You will need to give your exact make and model number for a definitive answer for example on a 2006 Ducato 2.8jtd it is grey in colour and on top of the gearbox but hidden by an air pipe and clutch mechanism (from Fiat forum)


 :evil:


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## Hymie (May 9, 2005)

*Reverse Light*

I had the same problem on our 2.8jtd- Fiat dealer said "we replace loads of switches" - about £100 fitted.

But when i checkerd under the bonnet i could see a connector at the back that was obviously damp but very difficult to reach.
A quick spray with Corosuon Block and within 24 hrs all was well until we sold the vehicle a year later.

Happy Travels


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## Double-Entendre (May 19, 2009)

sallytrafic said:


> That was a nice put down for all those that tried to help you earlier3.
> 
> You didn't even acknowledge me telling you exactly where the switch was on your other thread
> 
> ...


Put down?????!!!!! NOT

I think you'll find it was a THANK YOU for answering the question I asked in a simple & concice way. (Much how I asked in the first place)

Sorry I didn't get round to thanking you in my lunch break today, for telling me where it is on a different vehicle to mine. If you take the time to read the first post that I started the thread with, I tell what the make & year of the van is and if you read further, you'll see your post that in no way answers what I was asking. I appreciate that you can give me a lesson in the technicalities of electrical systems but that's not what I asked for.

I always thought that forum threads that ask for help were only answered by people who could help but if you read this one through, there are many posts with frustrating "Half answers", which is again why I had to ask again and start another thread.

Yes, it's great that other folks added to the thread but no one answered, which is why I had to keep asking dumb sounding questions.

Eventually, the answer came, in a way I could understand (And also someone else commented on how they could now follow it - did you bitch at them too?) So thanks were due, in my book.

Was I wrong to say thanks for helping me then?

If it doesn't help, should you also say thanks?

Thanks for bitching then, you can put your dummy back in now :roll:


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## chrisjrv (May 7, 2007)

Err. oh it's not worth it!!!!!


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## AberdeenAngus (Jul 26, 2008)

stickey said:


> Err. oh it's not worth it!!!!!


heh heh heh


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## sallytrafic (Jan 17, 2006)

stickey said:


> Err. oh it's not worth it!!!!!


Yes but he's still ungrateful, I have just re-read both threads nowhere does he give the engine gearbox type so we were still in the dark trying to help.

anyway 'whatever'


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## duxdeluxe (Sep 2, 2007)

I think there's been a misunderstanding of some kind. 

Actually the best bit of advice was to spray the connector........ let's keep it cool, it's not worth anyone getting upset, which is very easy when emailing and texting etc.

P.S. I just ordered some of the corrosion spray off ebay - it's worth a try and will have other uses anyway. Cheaper than a new switch plus labour if it works.

Cheers everyone!


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## BlackScorpion (Apr 30, 2008)

Thank you all for your reports, it does show that electrics are a specialist thing, and that you can't always go by what the test meter tells you. 

PS Anybody found the location of the fuse controlling this light ??


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## rayc (Jun 3, 2008)

BlackScorpion said:


> Thank you all for your reports, it does show that electrics are a specialist thing, and that you can't always go by what the test meter tells you.
> 
> DC is not particularly difficult to understand at the level required to find out why a bulb doesn't light when it should. I disagree that you can't go by what the test meter is telling you. The test meter is a tool and it is your brain that interprets what it is measuring. If you don't understand the difference between open circuit and closed circuit or load - no load readings then you have only a limited chance of finding the faults that do not have a blindingly obvious reason for them.
> 
> PS, I think I remember reading somewhere that the revering lights on the Ducato have the live and ground circuits made via the reversing light switch in the gearbox.


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## loddy (Feb 12, 2007)

I love these threads

You don't need to be an electrician to understand that a bad connection cannot be found wiithout loading the cable, try sticking the bulb in and feeling for warmth ( resistance causes heat ) at all the connections.

The reverse light is NOT part of a MOT test, it is in Spain but we won't complicate things

Loddy


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