Centre brake light not working - how to use multimeter?

pippip

Hi folks,

So have a 08 Mondeo Hatchback. Centre brake light is made up of 5 bulbs (5w12v 501). Over the last year or so the bulbs have one by one and now all gone. Changed them yesterday and can't get them to work.

Other two brake lights work fine. Can only see one fuse listed in manual as "brake lamp switch" so assuming this powers all three I'm guessing fuse isn't the issue. I believe this model has two brake switches, one for left and right brake light and second for centre brake light and cruise control, may be wrong on this. Cruise control worked on Friday.

So have never been able to get my head around electrics so using this to opportunity to learn something.

Is any of what I'm reporting below correct in how i'm using the multimeter and results I'm getting -

For testing all below I'm holding red and black pins into each side of the connection to the brake light.

Car open but no key in ignition I'm getting the following -
Set to DC V 20V reading 11v
Set to Ohm 20K reading -15
Set to Continuity reading 712
Set to Amp 20M reading -1.11

Car running and brake pressed in -
Set to DC V 20V reading 13.2v
Set to Ohm 20K reading -15.6
Set to Continuity reading 321
Set to Amp 20M reading -1.37

I've read a few pieces on checking the ground, not sure on this so open to advice.

I have NCT tomorrow so may be dropping this into a garage anyway if any surprises arise and may just get them to sort it but would be interested in at least attempting it myself, at least to troubleshoot it better.

greasepalm

Usual issue broken wires in the loom at hinge as all cars break at some time with opening and closing of boot door.

pippip

greasepalm wrote: »

Usual issue broken wires in the loom at hinge as all cars break at some time with opening and closing of boot door.

Yeh thats down as a possible alright. I'm just wondering can I "correctly" use a multimeter to prove the connection isn't getting the right results.
I sort of expected to see a bigger variation in the numbers between pressing and not pressing the brake which leads me to think I'm not using it correctly.
The fact that I'm picking up the voltage and amps I would have thought my problem may have been them on permanently.

greasepalm

Must have had about 100 cars in for broken boot wires and different issues,number plate lights,fog/reverse light,brake light,rear wiper etc
Go check the loom at hinge as broken wires will pull through and others hanging on by a strand or 2.

I dont use a meter for broken wires as forgot i brought one in months ago to check for broken circuits.Block connector should see 12v and work on those 2 connections.Brake pedal needs to be pressed/jammed down to show other lights working then the middle should work.

pippip

Managed to prize open the plastic covering on the wires between the hatch and the car. Can't say I could see any sign of fraying, all seemed in good shape. Bit hard to "pull" at them as they were secured with electrical tape but I uncovered as much as I could and no sign of damage or wear.

Won't rule it out yet anyway

Dirty Nails

pippip wrote: »

Is any of what I'm reporting below correct in how i'm using the multimeter and results I'm getting -

For testing all below I'm holding red and black pins into each side of the connection to the brake light.

Car open but no key in ignition I'm getting the following -
Set to DC V 20V reading 11v
Set to Ohm 20K reading -15
Set to Continuity reading 712
Set to Amp 20M reading -1.11

Car running and brake pressed in -
Set to DC V 20V reading 13.2v
Set to Ohm 20K reading -15.6
Set to Continuity reading 321
Set to Amp 20M reading -1.37

I've read a few pieces on checking the ground, not sure on this so open to advice..

Put that meter away before you electrocute yourself
Ohms & continuity is the same - not to be used on a live circuit.
Voltmeter connects parallel ONLY. All you need for this test
Ammeter connect series ONLY. Used for current flow.Use highest setting & work down depending on current flow.

To check for power clamp black lead to good body earth & probe lives with red lead.To check for earths,do the reverse.Thats the beginners basics anyway.
There are more tests for voltage drops & bad connections,above is enough to get you started without blowing the meter.

zg3409

pippip wrote: »

Hi folks,

I have NCT tomorrow so may be dropping this into a garage anyway if any surprises arise and may just get them to sort it but would be interested in at least attempting it myself, at least to troubleshoot it better.

Did you pass nct?!
Its a visual fail so retest is free and you can drop back any time they are open. A garage should be able to sort for you but beware typically they are busy and need to book you in.

As said its typically the wiring at or near the hinge. Very common on Micra's.
Also typically one side of the bulb is connected via a wire to bare metal and sometimes this rusts or breaks off.

If you do want to DIY, best to try learn the basics of electrics first. A bulb with wires on it is a good tool. One side to car chassis, other to wire under test. You need to force on brake lights and confirm the others are lit before starting any tests. You can often trace wires by unique colour and stripe.

I didn't tell you but often if middle brake light was 'never fitted' you can pass with nothing there, but if you fail you need to fix it

martinr5232

pippip wrote:

So have a 08 Mondeo Hatchback. Centre brake light is made up of 5 bulbs (5w12v 501). Over the last year or so the bulbs have one by one and now all gone. Changed them yesterday and can't get them to work.

They are not ordinary 5w capless bulbs and they wont work. They will close down the feed from the body control module. Cant remember what bulbs they are we just changed the lamp they are not that expensive from ford.

greasepalm

That surprises me if not 5w bulbs as most lamps are stamped with rated bulb fitting.

Changing lamp suggests like its leds,handbook if he still has that should list specs for bulb.

pippip

So failed nct but only on this so visual retest only.......happy with that to be honest. I had it removed completely from the car and duct tape over the hole.
Listed as Missing / Not clearly visible.

So manual just says 5x w5w same as stamped on bulb holder. Photo of old bulb below -
https://www.boards.ie/vbulletin/attachment.php?attachmentid=539901&stc=1&d=1610988981

greasepalm

So back to hinge to recheck for broken wire.

pippip

Well all sorted. Wasn't a broken wire in the end. Turned out that once all the bulbs went it logs a fault code which needs to be cleared before the car will allow the light to work again. Common on a few Ford models of similar year apparently, crazy design.

Just throwing my recommendation out there to Auto Electrician Mark Priestley (Dublin) who sorted it for me very promptly. 086 267 9990