as a by product of my 'quite possibly' incorrectly wired tow-bar electrics, I blew a fuse on the front and rear off-side side lights (and later discovered an airbag fault light). I replaced the fuse for the lights, and skoda are going to have a look at the airbag light.... but one of the things I noticed was that the voltage at the fuse for the side lights was 17 volts, is this correct for my 1.9td roomster ?

I've towed my caravan before using a work van and also with this car and both times without problem, 'until' I connected a battery to the caravan..... with the caravan battery attached it casued havoc with the car, with all manor of dash lights illuminated and fuses blowing !

Does anyone know if the 17 volt thing means anything toward what I could have done to my electrics, or is 17 normal ???

Thought I'd ask in this section of the forum as I figure you guys are great with electrics

thanks for your time, any help or advice you can provide would be great !!

Paul.

If your engine wasn't running at the time, your meter device is almost definitely faulty or you misread what the number meant - as it couldn't have been near 17 volts. If the engine was running your voltage regulator probably needs replacing (and this does tend to bring things like the airbag light on). Check the voltage at the battery and it should tell you the same, the voltage should be the same throughout the electrical system unless some thing's *really* wrong.

It should not be anywhere near 17V, that is so high it will potentially cause problems

The only ways I could think of for you to get a reading of 17 volts are :

Fault in or misreading the meter (try another meter).

Fault in the voltage regulator in the alternator (and then only with the engine running).

Fault in the tow bar wiring has led to the caravan battery voltage somehow getting in series (rather than the normal parallel connection) with the voltage from the car's battery (but then it should be 24V, not 17).

Please let us know what you finally find out when you solve the problem.

I'd pretty much agree, with the note that Greg's "caravan voltage" theory would surely require the caravan "small power" plug, not the "MVL" plug, to be connected?

I just asked the girlfriend who was with me at the time, amazingly without prompting here, she remembers it was 17 volts, I'll have to connect the meter up again tommorow to see. The caravan was not connected, nor was the engine running (car was in the garage) and the multimeter ( a digital readout with rotary dial to set) was on the anri-clockwise dc voltage side using a scale to show 10-20 volts.

All will become clear in the morning when I check it before I go to work, I'll post the results

cheers

Paul.

If the engine wasn't running and the caravan electrics weren't connected, I don't see how you could get a reading over 12v!

Ok, I've just checked it again, I have measured on the fuse for the sidelights :

15.8 volts with the engine off, and

18.8 volts with the engine on

is this normal ??

Paul.

Have you tried another meter? Sounds like your meter is over reading by around 3v.

Definitely *not* normal. Does the battery completely disconnected from the car read around 15.8v? Disconnect negative first and reconnect it last and ensure you have any codes needed for the stereo etc before starting.

the meter was an ebay special, but to be honest, I've used it for everything from measuring the voltage of AA batteries to the charge state of up to 5 standard car and motorbike batteries, and thought it was reading correctly. This is wierd !

This 'has' to be all wrong doesn't it ! ....... there's nothing in a car that would have any need or any ability to boost 12volts to any more than 12 volts....... I don't know much about car electrics, but I couldn't see any reason for it !

I've got my auto electician coming back to look at my tow electrics, so I'll ask him and get him to measure this, and I'll get back to you

thanks for the help and the interest

Paul.

the meter was an ebay special, but to be honest, I've used it for everything from measuring the voltage of AA batteries to the charge state of up to 5 standard car and motorbike batteries, and thought it was reading correctly. This is wierd !

This 'has' to be all wrong doesn't it ! ....... there's nothing in a car that would have any need or any ability to boost 12volts to any more than 12 volts....... I don't know much about car electrics, but I couldn't see any reason for it !

I've got my auto electician coming back to look at my tow electrics, so I'll ask him and get him to measure this, and I'll get back to you

thanks for the help and the interest

Paul.

Did you have it set on AC or DC ?

Radiotwo

Deffinately set on DC

Paul

Based on your further information I'll narrow my suggestions down to 1 :

Fault in or misreading the meter (try another meter).

Did you have the meter leads plugged into the correct terminals (some meters have different terminals for different measurements) ?

A freshly charged battery may go up to 13.2 V, but after a while it should settle down to around 12.8 V. If you put it under medium load (eg headlights on but engine not running it would go down to 11.8 - 12.5V, depending on load and duration).

If your alternator's voltage regulator was really putting out 18.8 V when your engine is running it would cook your battery in very short order (hot, bubbling, maybe even exploding in the worst case scenario)

Maybe use your meter to check power directly across battery. Then maybe try another car.....to verify meter.

6 or 7 dry cells connected in series, and see if that gives the expected 11 (6 oldish cells) to 13.5v (7 new ones).

Digital Meters are easily damaged if the mains voltage is applied whilst set to read say

20 volts DC. The more expensive ones (Fluke etc) have better protection than the cheaper ones. Inside the meter resistors are put in series to measure voltage, and can change there value if the meter is abused,by being overloaded.

I only trust a fluke meter myself.

Ok, I did indeed find out what the problem was. My mechanic actually has an almost identical meter, and confirms that it's actually the battery in the meter thats going flat and therefore gives a wrong reading...... he also admits that the particular meter he was using was a cheap ebay special itself !

I didn't replace the battery in my own meter, so I can't confirm this, but I can confirm that with his meter, the car is reading normal.

thanks to all of you guys for the advice

..... and I'll buy some new batteries !!

Paul.

Paul - seen this sort of thing many times in the electronics trade -little device to narrow down iffy meter readings (if you can't trust meter then you have got problems )

Nip down to maplins and get a 0.6 watt 68 ohm resistor and a 9.1 v zener diode (type BZV85C9V1).One end of the diode will have a black band ,join this(twist the wires together ) to any end of the resistor .Put the resistor(end not joined to diode) to the + of car battery .Put the end of the zener not joined to the resistor to the - of car battery .Any other reading than about 9v means meterv suspect .Whole lot should cost less than 50p (a lot less ) ,and vindicates or otherwise meter in seconds . NB polarity is essential ,otherwise diode drops .7v and resistor gets very hot .

Hope that's some use for future,to you or anyone else .

or put a new normal AAA,AA,C or D battery which you may have lying around across the meter terminals ,which should read 1.5 to 1.6 volts,on a good meter. Most meters include a scale for 2 volts DC max,and 20 volts DC max . You can then test two scales on the meter. This will save you buying specific parts, which you will not use again.

or put a new normal AAA,AA,C or D battery which you may have lying around across the meter terminals ,which should read 1.5 to 1.6 volts,on a good meter. Most meters include a scale for 2 volts DC max,and 20 volts DC max . You can then test two scales on the meter. This will save you buying specific parts, which you will not use again.

Till you pick up "nearly new/new battery" which has been thrown in drawer to slowly short out against some screw etc .Leastways once a calibration source has been made ,it's there for peace of mind at a cost of less than 50 p

Happened on a course - instructor put battery in pocket ,put phone on top and then swore blind battery was ok - fortunately we had a few meters around .

That's why I stated a new battery ,meaning one which is still packaged!. A person may have brought one ,or some batteries say 2 months ago, and still have it or them packaged in a drawer. Taking it even further somebody may have brought two batteries for a torch ,a while ago,and never used the torch,or used it very little. These batteries will still read 1.5 volts ,briefly turning the torch on will confirm by its brightness ,the condition of its batteries. Dont forget we are trying to prove an unknown meter which reads 42% high, not calibrate an Atomic Clock!

A cheap meter when manufactured will only be plus,or minus 10%, ,meaning 12 volts may read 10.8 volts or 13.2 volts ,and stilll be acceptable to use . The new zener diode ,could have slipped through the net when manufactured ,and be anything from a 2.1 volt to a 200 volt one! If put the incorrect way round on a battery,without a resistor it will blow short circuit!. Finally any car battery when measured (engine not running) cannot chemically make 17 volts.

(Physics-each Lead Acid cell reads 2 volts X 6 cells in series = 12 volts, or 14.5 volts if fully charged,(not on charge when measuring)) .

Edited 7 February, 200917 yr by AndyPandy