The Triumph Dolomite Club - Discussion Forum

My Sprint lights stopped working the other day – I only realized in the evening when I got to an unlit road.

First fault I found was the dip beam wire on the indicator/dip-main/horn switch was coming loose (on a less than two year old ‘Lucas’ part). I managed to fix that with solder, but then I had no lights at all. There was no power to brown wire on the main light switch. Eventually I discovered that this wire, from the connector on the battery to starter to the plastic connector behind the dash, had lost all connectivity. I replaced that with a spare from an old loom, and all lights worked.

However while test driving today the car had a new fault, a bad misfire that only occurred under load and only when using main beam or the headlamp flasher. Didn’t happen with the engine at idle or when driving with the engine not under load / light throttle. Only when accelerating. Dip beam fine under all conditions, same with all lights off. Main beam is wired using a relay, but that circuit has been in place for a number of years.

I’m not sure where to start looking for the cause of this, any ideas welcomed.

Geoff

What's the ignition system? Still on points or electronic?

Either way, sounds like the main beams are somehow pulling down the supply voltage to the ignition system.

So my first check would be the voltage at the top of the ballast resister (if fitted) or top of the coil if there's no resister, with and without mains on, etc.

Graham

Thanks Graham

Magnetronic fitted 2011 and no issues since. Fed direct from fuse box. Car built Oct 78 (VA30063). Engine is usually very reliable. Apart from annual servicing, in 2016 fitted alloy carb mounts, new plugs and reset carb jets / mixture.
Did have to replace alternator last month with used one.

With engine running at 1300rpm (on choke), +ve coil terminal reads 9.9v with dip beam, but drops to 8.4v on main beam. Tickover instantly becomes very rough, falters and stalls after about 5 seconds. I also noticed the rev counter drops to almost zero the same instance as headlamps changed to mains but immediately goes back up. -ve terminal on coil reads 6.6v dropping to 5.5v at the same time.

Any suggestions as to what to check next?

Geoff

If there's no ballast resistor, then they both seem like very low values. But if the battery was that bad, I would be surprised it would start.

So I assume there is a ballast in the supply to the coil. That the ignition module is supplied separately, not from the top of the coil would also suggest a ballasted coil.

But with such a large change, it does add to the suspicion that it's a drop to the supply voltages in the ignition causing the misfire. I can't get values off my car, as today I will be mostly taking the carbs off - again.

BTW, I assume the supply to the ignition is taken off the fused side of the ignition switched supply. Have you looked at the fuse? A bad contact could add to the problem, and I've had fuses got off eBay go a bit high resistance rather than blow, which has made for some fun in fault-finding.

But I forgot to mention the earth side of these measurements. In this case, I think you want to make these measurements once with the earth taken from the battery -ve and once with it taken from the engine, ideally at the distributor body (not the blots that hold it down). It shouldn't matter, but it's not that hard to do, and will eliminate any earthing problems as the cause. It's easy to forget that shared paths on the earth side can cause problems as easily as issues on the supply side.

Next, I think I'd measure the change in voltage on both sides of the ignition switch with the mains on and off.

IIRC, the ignition switch is supplied by a white wire on the battery side of the lighting fuse. It supplies the ignition switched supply fuse through another white wire. If I'm wrong about where the easy places to get at the connections to it are, someone will correct me I'm sure.

It's a bit of a stretch to think that turning the lights on would have that much effect on the voltage drop across the contacts of the ignition switch or the ignition supply fuse, but if the voltage at the coil is already marginal for some reason, I suppose it might cause this fault.

gmsclassics wrote: ↑Sun May 08, 2022 7:49 am
Did have to replace alternator last month with used one.

With engine running at 1300rpm (on choke), +ve coil terminal reads 9.9v with dip beam, but drops to 8.4v on main beam. Tickover instantly becomes very rough, falters and stalls after about 5 seconds. I also noticed the rev counter drops to almost zero the same instance as headlamps changed to mains but immediately goes back up. -ve terminal on coil reads 6.6v dropping to 5.5v at the same time.

Any suggestions as to what to check next?

Geoff

Replaced alternator with used one. Couldn't that be the issue? With the engine running I would expect the voltage at the coil to be higher than that. If the alternator isn't producing the goods your symptoms would be explained.

You can't measure that way.

First measure battery voltage over the battery. Must be around 12 - 12,5v.

Then measure again with the engine running. Should be around 13v at idle and 14 when revs from 2000rpm.

Then you know the condition of the battery and alternator.

Do the same measurements at the brown wire side of the fusebox and using the body as earth.

If that is about the same as the battery voltages you measured before the main wiring is ok.

Measure these both again with lights on, low and main.

When there's a large voltage drop at the brown wires measure the same but for each headlamp. First disconnect all headlamp plugs. See what happens to the voltage when switching the lights on without the lamps connected. Still a drop it's a short in the headlamp wiring. No drop then measure with one headlamp connected at the time to find a faulty headlamp.

When all lamps drop the same voltage and adding to the figures you had, then measure if the voltage also drops with the headlights off but all other electrics on like heaterblower, rearscreen heater, wipers, radio, all you have exept the headlights.

Then report back.

These measurements will have an idea what actually is going on or halfway you already found the cause.

Jeroen

Well, you can work through a list of likely causes of low voltage and or too much voltage change with the lights on, and eliminate each in turn. Or, you can work back from the effect to determine what fault is causing it.

Both methodologies should work. If you've the experience to make a good guess at the list of likely faults, and if the actual fault is in the list you've guessed at, then eliminating them in order of the most probable is likely to be the quickest way to a fix. But evaluating the effect properly and working backward from that will find the fault, even where you haven't got a clue what it is. Also, I would argue, that method will teach you much more and give far more satisfaction than eliminating the faults that you are told it might be.

So. I'd still measure as much as I could around the coil. And if that says the battery, alternator, and light connections is the right direction, go look at them by all means. But if a measurement shows it's, for e.g., in the earth, or the switch, or a fuse, they send you on entirely different paths.

Graham

Forget the coil.

The lights and coil have nothing in common only the overall battery/alternator voltage.

Lowering the car's overall voltage does result a lower voltage everywhere so also the coil.

The coil feed isn't the problem. The lights are.

Systematic eliminating will find the problem within minutes. Solving the voltage drop by the lights will solve automatically the voltage drop at the coil.

Jeroen

soe8m wrote: ↑Sun May 08, 2022 10:47 pm Forget the coil.

The lights and coil have nothing in common only the overall battery/alternator voltage.

The coil feed isn't the problem. The lights are.

Jeroen

Do they have no possible common earth path then - totally separate earth connections all the way to the battery terminal, even when one of the earth straps has a fault?

And here I was thinking the problem that the OP wanted fixed was the misfire. Silly me.

Graham

Thanks everyone for their thoughts.

Jeroen, I ran through the tests you suggested:

Battery, no electrics on - 12.36v (12-12.5). Battery is type 41, 350CCA, lead-calcium, new 3/2000, isolator on +ve disconnected when car not in use.

Battery engine running – 12.6v @750rpm and 13.5 @2000rpm

Brown wire at fusebox to earth on body
engine running – 12.7v @750rpm and 13.6 @2000rpm

Repeat with lights on, both dip, then main
Dip – battery 11.8v @750rpm, 12.2v @2000rpm,
brown wire 11.7v @750 rpm , 12.2 @2000rpm
Main – battery 11.5v @750rpm, 11.6v @2000rpm,
brown wire 11.1v @750 rpm , 11.4 @2000rpm
As brown wire drop wasn’t that different, didn’t measure each headlight

With lamps off and everything else on (measured with electric radiator fan in addition)
engine running – 11.7v @750rpm and 12.85 @2000rpm
plus fan - 11.6v @750rpm and 12.65 @2000rpm

I still have the dead wire to the main light switch which I have bypassed by a new one to account for.

Your thought on what this means would be appreciated. I am beginning to wonder if the replacement used alternator isn't up to the job and needs replacing?

To answer a comment about the coil, I still have the standard 6v type coil fed by a ballast resistor wire and the feed from the starter when that is working. The magnetronic ignition is fed 12v directly from fusebox when the ignition is on.
Thanks

Geoff

Geoff,

the voltage drop through the main feed wires is not that much so probably nothing wrong with these.

The overall voltage drop when turning lights etc. on is too much. Also when turing all other electrics on the voltage is too low. The alternator does not charge and while driving with the lights on you will drain the battery.

The question is what's causing the misfire. Too low voltage at the coil or too low voltage on the Lumenition. I think the last.
Having a voltage drop through the ignition switch also, what is normal, I think the actual voltage at the Lumenition with high beam on is just to low at idle and makes the car stall. You can measure the white wire side on the fusebox with the engine at idle and then again with low and high beam see what voltage is left there as a voltage supply to the Lumenition. A healthy coil can produce at a (too) low voltage still a spark. We all know the relief when an engine just turns over on an almost empty battery and then luckily jumps on on the last possible rotation... So there's still a spark produced then.

An easy way to check is also have a modern car next to it and connect some jumper wires. The modern will produce at idle the normal 14v so the Dolomite while running connected with the jump leads will have that voltage also. Turn on the lights and see what happens.

When nothing happens and the Dolomite still idles fine with high beam on it confirms the alternator is the cause.

Jeroen

I agree with Jeroen (twice in one day must be a record)

Alternator output unencumbered by lights is marginal @2000rpm and 13.6v and well below normal on idle. Once the lights are on, the alternator can't even keep up with load and is showing a relative discharge that would be big even with a dynamo! So the alternator must be the first port of call, it is most assuredly underperforming.

Admittedly the Dolomega is a bit over endowed in the power dept with a 180amp alternator and a 96a/h battery but I can turn EVERYTHING on that car on and still read 14v off the voltmeter at idle. nothing fazes that charging system, which suits me, I grew up in the days of dynamos and poor battery tech, with an auto, i'm paranoid about batteries going flat!

Steve

Yes, Jeroen, it did exactly that. Voltage on the white side of the fusebox, at idle with no lights was 12.6, with the two dip beams dropped to 11.75, with the four main beams to 10.4.

So I'm looking for a new alternator to replace the ACR17 (45A). Options available locally are ACR18 (55A) or A127 (75A). Any downsides with either?

Thanks again

Geoff

I had a chance to duplicate your problem today but just couldn't manage as the voltage was still a little too high and I needed to reach home also. Lights on and having a Lumenition didn't made it stall.

Fitted a new alternator today and have the normal voltage again.

Jeroen

During the trip.



New alternator.



Normal reading again



Voltage at idle.



Voltage at idle with all 4 headlamps on.