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MG MGB Technical - An Electrical Anomaly

Hi guys. I've given myself an interesting problem that has manifested itself because I am attempting to fit led bulbs to the rear tail/stop lights. 1st problem - I attempted to compare brightness between the led and filament bulbs. With the standard bulb in one side and the led bulb in the other I could not get the led bulb to illuminate. After cleaning all contacts there was still no joy. Eventually I removed the other side (ns) and fitted the led. This then lit up as did the other led bulb when swapped over, so the leds were ok. When I temporarily fitted the lense covers and switched the side lights on I had lost the led bulb. I think I had not replaced the os bulb at this point. Further testing showed that when one assembly has a filament bulb and the other an led then the led will not illuminate BUT as soon as the two led bulbs were fitted - bingo - both lights were working.

2nd problem. Switching on the ignition to check the brake lights, this worked fine BUT by accident my wife who was working the brake pedal and ignition when she depressed the brake without the ignition on. To my shock the led tail lights extinguished, coming back on when the pedal was released. Replacing the filament bulbs produced the correct operation.

Unfortunately I had to stop working on the car for the day so I've not yet got a meter on the problem. Some of my thoughts are homing in on the difference in current consumption between the led and filament bulbs when there's one of each fitted to the rear. Is there some voltage drop which stops the leds illuminating because of the differences? Why does the tail lights go out when the brake pedal is depressed when the ignition is off? The front side lights (which are leds) are functioning correctly in all this.

The battery is in good condition and is maintained on a CTEK. The car is a Dec 1972 build MGB roadster.

Any thoughts would be gratefully received before I have another go, hopefully, some time tomorrow pm.

W A Nixson

I would be tempted to try adding some separate earths to the rear lamp units.
Dave O'Neill2

Snap does Seem earthy. Probably find the tail lights are unearthing via the stops
B Anderson

Thanks for the feedback guys. Last night I got thinking along the same lines. A low current would highlight a poor earth where a higher current from the filament bulbs would not show up. I need to empty the boot to have a better view of things. The odd one is loss of rear lights when applying the brakes with the ignition off.

W A Nixson

I think you need to present the same resistive load across both in order to compare them, not necessarily a fault on your vehicle. I would put a resistor across the LED load in order to illuminate both.
Pat Gregory

If you get interaction between the various lamps then you have a bad earth on one (or more) of the light clusters. On chrome bumper cars all the clusters (i.e. not the headlights) earth through their physical mountings. As well as that the lamp can be making a bad connection to its holder, and the holder can be making a bad connection to the body of the light unit. With incandescent lamps fitted - the higher current will give a bigger volt-drop across the bad connection and so be easier to see on a meter - measure the voltage between a known good earthing point elsewhere on the car and the lamp base (if you can get to it), the holder, and the body of the light unit. If you see any voltage at all then that is a fault.

With a mix of lamps with very different current requirements like this you will get even weirder interactions.

Whilst an equal resistive load will change the effect, it's still not likely to give the same brightness as LEDs and incandescent lamps have different voltage/brightness characteristics, and in any case it is the bad earth that is causing the problem. With good earths all round then you won't get any of these interactions, although if there are additional bad connections in any of the bullets or other components in the circuit supplying voltage to the lamps incandescents in particular will be dimmer than they should be.
PaulH Solihull

Thanks Pat and Paul,

Having done some further testing the problem lies with the brake light bulbs - I think. I have pulled out the dual positive termini from the holder and having checked the led (both segments) using a large crocodile clip on the body of the lamp and earthing on the innards of the bulb holder I was able to get the leds to illuminate using my meter lead clamped at the bulb terminal and touching each positive terminal on the led. So no problem with earth BUT after putting all the bits back into the light fitting and with the led refusing to illuminate I pulled the brake light wire to pull the positive termini fully home. At this point the led illuminated for a second while I was gently pulling on the brake light wire. The penny then dropped. I unplugged the brake wire from the back of the holder. Now we have the led and its opposite filament bulb illuminated. Checking the voltages at the led (brake wire) terminal I got 8 volts. On the loom side I had zero volts. Although I've not fully figured out what's going on, my next port of call will be to remove the wire from the brake switch to see if the fault disappears and go from there. I cannot make up my mind if its the leds that are the problem or as I'd already suspected and you have commented on is the differences in current flow between leds and filament bulbs. But it's not the tail light filaments that are the problem but the brake light filaments. I hope to make some further progress tomorrow.

As an aside I had no problems when fitting leds to the front putting just one in to compare light output between the ba9 filament and its led equivalent.

Let's see what what tomorrow brings.

Bill - aka willyphixitt
W A Nixson

"Checking the voltages at the led (brake wire) terminal I got 8 volts"

That says to me there *is* an earth problem, if I understand what you have done correctly. Did you have the tail lights switched on as well? If so, and if you have disconnected the harness brake wire from the LED, but see 8v on the LED terminal that wire was taken from, then the only place that voltage can come from is from the powered tail light *because* the earth is bad. However if the tail lights weren't on then I can't see where the voltage at the LED terminal was coming from anyway. Diagram needed!
PaulH Solihull

Hi Paul,

The problem lies with the led bay15d lamp. Having disconnected the brake light filament from the other stop/tail filament bulb I get the led tail light to illuminate. Measuring the voltage at the led terminal for the brake light (now disconnected from the brake switch) there is 8 volts appearing when the side lights are on. If I ground this via my meter the led tail light goes out and there is a current flow of 225ma approx. If I now connect this brake led wire to the positive I get both the brake and tail lights illuminated. Current flow for the tail light only is 75ma and the brake light only it's 260ma. With the standard bay15d filament bulb in the other side the brake filament supplies a low resistant circuit to ground which causes the led tail light to extinguish. I've measured the resistances and I get just over 10k ohms for each of the separate led circuits BUT weirdly ther is only 56 ohms between the two positive terminals. This reading is the same when swapping meter leads over.

With both led bulbs fitted the reason I lose the tail lights when applying the brakes (ignition off)is because of the voltage stabilizer, which again supplies a low resistance to ground until the ignition is switched on.

The low resistance between the two positive terminals does not make sense. I would not have expected to get a reading on my meter as the resistance should have been too high. The brake half of the led array should be reverse biased and consequently no current flow. Similarly, with the side lights off, the tail light leds would be reverse biased when the brakes are applied.

I hope the above makes sense. The circuit diagram that refers to my model is on page Nb17 of the workshop manual.

W A Nixson

It makes very good sense to reduce the pull of the side and numberplate lights on the battery. The same goes for the two interior lights. Although I doubt anyone will be tempted to use leds to replace the dual filament side/brake after Willy's experience, I did try them in the Summer with the Winter in mind. The result was perplexing too until I realised that the Chinese-made 380 dual filament 'bulbs' were actually single filament working on the two terminals at the base without involving the case of the bulb in anyway other than as a support for the light structure. If anyone is tempted to go for LED sides allround, just swing off on each of the terminals to check it really is dual filament. The led third brake light works well and is really effective though.
Happy hols everybody
Roger W

"Measuring the voltage at the led terminal for the brake light (now disconnected from the brake switch) there is 8 volts appearing when the side lights are on."

Willy, this is definitely bad earthing then, somewhere along the line. I'd measure the resistance as I describe above, starting with the earth terminal of the LED array, and also connect a flying lead from there to the earth terminal of the battery, and see what happens.

The coil and fuel pump are going to be supplying a lower resistance path to earth than the instrument voltage stabiliser, as they are all 'in circuit' with the ignition off, unless you pull the green circuit fuse.

Without knowing what is in the LED array, besides the LEDs themselves, it's not possible to say just what conditions you might get measuring resistance between the two illumination terminals. If they are for both positive and negative ground cars there could be a bridge rectifier in there somewhere, or some other way of preventing reverse connection. The fact the resistance is the same in both directions tends to indicate that. Even if they are labelled as being for negative earth only there could (probably is) still be some form of protection in there.
PaulH Solihull

Hi Paul,

Thanks for your input. You're right with the fuel pump and coil. I had only traced the green wires and found the voltage stabilizer at the end of one of them. I should've pulled the fuse but I've now been able to suss out a bit more on these leds, although not 100%. Getting a bulb on the workbench and hooking it up to a battery etc. it looks like there is only one circuit containing 127 smd leds. An odd number as it does not divide by 3. There is 7 segments on the periphery each with 15 leds and they look like they are set in a series of 3 leds. Oddly there is 22 leds on the cap. I am guessing that there are two resistors in series to drop the 12 volts to around 6 volts at the leds assuming a 2 volt forward voltage for each individual led. A rough calculation makes me think that the second resistor in the series is around 24 ohms. This together with the 56 ohm gives me a lower intensity to the output of the whole array (tail). Some leds looked extinguished but are in fact on with just a pin hole size of brilliance. When you apply 12 volts to the brake terminal you bypass the 56 ohm resistor giving full brightness to the leds. What is odd is the fact that the 3 levels of leds on the periphery closest to the base are the brightest (tail light terminal). So there most probably some electronic skullduggery inside. If my assumption is correct a schottky diode between the brake positive terminal and the junction of the 56 ohm and 24 ohm resistors would have prevented feedback. Careful selection of the diode would be appropriate as there is a wide range of forward current drops with diodes, so a low voltage drop of no more than 1/2 volt would be preferable. I could make up small adaptors each with a diode and a couple of bullet connectors and fit these between the fly leads from the lamp assembly and the loom bullet termini. Had there been two separate circuits within the led bulb this odd problem would not have arisen. I definitely do not have any earthing problems as such within the car.

I wonder if all the brake/tail light led units are all of the same design? Do some have the diode fitted to prevent feedback through the brake circuit?. I wonder!


W A Nixson

I did a test with LED's about a year ago on my wife's GT and was amazed how poor the amount of light was.
Especialy for the brakes they where dreadfull and imho not safe.

But they did work out of the box
Onno K

Funnily enough I was lying in bed on Christmas morning when two thoughts popped into my head, both related to things I'd mentioned before, and with your latest comments.

Incandescent bulbs have two separate filaments that only share an earth i.e. a single connection. So thay cannot interfere with one anotyher, *unless* there is a bad earth. LED arrays could be the same, but they could also use a single array with electronics to flash them on and off rapidly i.e. faster than the eye can see to make the tail lights appear to be a lesser brilliance, you can see this effect from time to time on OEM applications. And of course they could simply use a resistor from the tail-light terminal to make them dimmer, being bypassed for brake lights. In this latter case you would see some voltage appearing on the brake light terminal when the tail light terminal was powered, being the effect of the potential divider of the resistors and the LEDs. It's this circuitry I asked about, and what wires you had connected and disconnected, what were powered and what were not, and where your meter was connected rather than the factory schematics.

The second thought related to the comment I made about LED and incandescents being used on different sides of the car causing weird effects, and is down to the different way that the LED array is powered. Because there is a potential divider on the brake light terminal of the LED array, if you connect that point to the brake light terminal on an incandescent bulb, the relatively low resistance of the bulb will shunt most of the current away from the LED tail lights.

You will have to either have only LED arrays connected, or only incandescents. Having one of each *will* interfere with one another unless there are two separate arrays and powering arrangements, like an incandescent bulb. If you want to compare one with the other on the car to see what difference they are making then you will have to power one of them from an alternative voltage source, and not have either the tail light connection or brake light connection joining them together. Sharing an earth i.e. just one connection will be fine ... as long as it is a good earth :o)
PaulH Solihull

Hi Paul,

Had I not wanted to compare led to filament arrangements within the tail/stop light assemblies I would not have come across the way these led arrays were arranged. I was expecting to see 2 separate led circuits terminating at earth - as per oem filament bulbs. Silly me! I did not need to use a separate power source to compare tail light output as I only had to disconnect the brake light on the filament bulb. What had confused me was the loss of tail lights (both leds)when the brake pedal was applied with the ignition off. Whether there's some electronic dimmers fitted within the led units, I doubt it with having the 8 volts appearing at the brake terminal and a resistance value of 56 ohms. The 8 volts I measured was when I had disconnected the brake light from the loom and the tail lights switched on.

I'm sorely tempted to unsolder the base to see how its been put together. If everything is as I suspect then I could fit a schottky diode between the 56 ohm resistor and the brake light terminal. That would make it fully compatible under all the circumstances discussed above.

Thanks again for your comments/suggestions. And to everyone else who has added to the discussion.

W A Nixson

Further reflection came up with the same alternative, i.e. disconnect the brake light connections (on one at least) while you are comparing the tail lights, and vice-versa, should solve the problem of interactions.

However I don't see why any modifications should be needed as long as when finally installed you use a pair of the LED arrays, and not one of each type.

Any diode in series with the brake light connection to the LED array should prevent the incandescent interfering with it when the tail lights only are on. Diodes have a relatively constant forward *volt*-drop irrespective of current, and whilst it can be as low as 0.3v for germanium, or 1.2v for bog-standard silicon, I don't think even the higher value of the silicon is going to affect brightness in braking mode, as the biasing of the LEDs in your array should be well into their 'full brightness' range. For a short-term comparison between both types I would say it is far preferable to disconnect the brake circuit while testing the tail lights, than open up one of the units to insert a diode and risk damaging it. If you want to take a video of them going on and off rather than stills then insert the diode externally.
PaulH Solihull

This thread was discussed between 20/12/2012 and 28/12/2012

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