The Triumph Dolomite Club - Discussion Forum

HI FOLKS,
OUT IN MY DOLLY 1850 LAST NIGHT AND WHEN I PUT MY HEADLIGHTS ON THE TEMP GAUGE GOES UP AND VOLTS GO DOWN
I HAVE SINCE FOUND THAT EVEN THE HRW OR THE HEATER BLOWER CAUSE THE SAME FAULT.

Hi Tony, I've been thinking about this particular fault over the past few days as its quite interesting electrically.
The curious bit is " TEMP GAUGE GOES UP AND VOLTS GO DOWN ":
Now a couple of things. The temperature gauge is earthed via the engine block via the temp; sender and the thick black wire ( on my 1850 ) down to the battery earth.
Now since you have been able to check charging voltages etc; can you put the ignition on and measure the voltage from the engine block at the sender down to the battery earth.
( Reason being is that if there is a high resistance in this circuit it will back bias the temp; gauge ) There will be extra voltage in the 0V to the temp; gauge circuit. )
This won't affect the fuel gauge as that is earthed to the bodywork at the back of the car.
But turn extra items on and there may be extra paths that the return currents will find.
Slight problem : I would have thought this would have given you starter motor problems.
What voltage do you measure please ?

And a 2nd check: I know many Dolomites have had their wiring modified over the years.
Do you still have the original style instrument voltage regulator nailed to the engine side of the bulkhead ?
If so does it have one green wire and one Light Green wire on it?
And do the fuel gauge and the temp; gauge also have a light green wire going to each of them ?

If so this is good and the next stage of checking is more complicated and involves the alternator.
O.K. while you are looking at the alternator does it have 2 thick brown wires or 1 thick and one thin wire.?
( The reason being that the voltage at the voltmeter can drop under high current but the voltage at the alternator can be higher )

Another question : How much does the temp; gauge rise by and how by much do the volts gauge drop with say dipped lights on ?
Let us know on this and I'll run through some more measurements.
HTH,
Tony.

Hi Tony,
Thanks for that.
Taking each bit in turn :
Those are good voltage measurements; exactly what I would be looking for at the battery ( 14.8V ) and the alternator with the load on. It proves the alternator is actually O.K. It's bit on the high side ( 14.4 V max; ) but lets go with it.
So I think we are looking at connection problems.
Now congratulations on owning a very early 1850. Especially with only 26,000 miles on the clock. I wonder if the chassis number is below 10,000 ?
Now slight problem. It is such an early model that the alternator you have is a Lucas 15ACR . I only found this listed in an early 1850 parts book. As such the connections are rather different to later 1850's / Sprints.

(2) The 2 yellow / brown wires should go to the IGN light and one goes to the internal field winding connection. This looks O.K. as the IGN light is going out and the alternator is working O.K.
The thin brown wire looks O.K. as its regulating.
The thick brown wire is O.K. as this is the main high current output.
Where is starts to get a bit confusing is the high current 0V return. ( Not on your connector).
The Sprint manual is no use on this.
But the Haynes manual shows a 2nd connection by the main high current brown wire going to earth on a mounting bolt for testing. Normally this is done via the casing.
The only other reference to the 15ACR alternator I could find was in the MGB Haynes manual where one ( 1969 /70 ) is shown with the connection and one is shown with the casing as earth.

So … one question for early 1850 owners on here please .
Do you have a thick earth lead from the back of the alternator please ? If you have the 15ACR unit fitted that is.
James / Kyle / Bruce for instance please ?
Thank you. I think for now we have to assume its probably O.K. as the alternator is basically working.

Now to wire colours :
(4) The blue wire out from the temp; gauge is unusual. Blue is normally for lighting. Is it possible this is a green wire / blue tracer ? Is the wire at the temp; sender also Green / blue tracer ?
If so the wiring here is O.K.

(3) The Green / light green connections to the voltage stabiliser sound O.K.
The 12.5V on the stabiliser input sounds a bit low since you have 14.8V on the battery but it should still regulate O.K.
The "7.5 V " output measurement may look odd but in fact this can only be measured with a True RMS meter. The fact that you have voltage and its moving up & down proves at least its doing something. The reason is that you probably have the original type of bi-metallic vibrating thermal stabiliser.

Yes, it is possible to connect the temperature gauge separately. You'll need a new temperature stabiliser. If its a modern electronic one the casing will need earthing. The input can be run with a new wire to the green wire side of the fuse box and the output side can we wired to where the temp; gauge light green wire was.

White and black wires on the voltmeter are correct.
As a further check, when the volts gauge hits the left hand end stop what is the voltage across the voltmeter please under full load and how does that compare with the voltage across the battery ?
Sorry about the length of this.
I hope I answered all the questions.
Cheers for now,
Tony.

Hi,
I think myself and other members of the club met you at Capesthorne last Sunday when you posed this question of falling voltmeter voltage and rising temperature gauge to us.
At the time we noted that you had checked and cleaned all earths and other contacts.
Eric Noble,our Guru,has been speaking to me on the phone this evening and has come up with what we think is a likely cause of your problems.

On your car which is an early one the alternator is likely to be a 15ACR.
These alternators have very poor regulation as the load is increased on it say headlights,heated rear window etc. ie the output of these units will not sustain a good charging voltage when the load is high.
Therefore the voltmeter will indicate,quite rightly,a noticeable fall in voltage. Say 11.5 - 12 for example when there is a large load is on it. So it is working correctly. Precise accuracy is not its strong point.

The problem with the temperature gauge is we believe due to the voltage being supplied to it from the regulator.
The regulator is basically a bi metallic strip with wire wrapped round it as a heating element.
The bi metallic strip is arranged so it vibrates,opening and closing contacts, providing a chopped voltage which eventually feeds the fuel gauge and the temperature gauge.
Normally the regulator provides a chopped voltage of around 10 - 10.5 volts.
If the input to the regulator falls sufficiently the bi metallic strip fails to vibrate sufficiently and the output voltage is not chopped sufficiently and consequently rises,possibly to the level of the input voltage.
This rise in voltage causes the temperature gauge reading to increase.
Although you might not have noticed I suspect the fuel gauge is reading a bit higher than normal when the alternator voltage falls.
You could check the operation of the regulator with an oscilloscope or maybe an analogue meter. Digital meters are no use in this instance.
If a solid state regulator is fitted this effect will not happen. I would suggest you get one from the club they work well.
Hope this helps.
Regards
Paul