The Triumph Dolomite Club - Discussion Forum

Racing over the last three days, the car died on one warm up lap. 35A fuse had blown on the ignition circuit but given there is only the one fuse it took far too long for me to diagnose what was wrong. I first replaced the fuse and in the pits, everything appeared to be working, but back out on the track and there was no overdrive. Subsequently with that wire disconnected at the fusebox, I had no further electrical problems.

However it was an incredibly hot weather, over 30C and track temp a record of 47C. I ran in two classes which meant finish one race only to go straight to the dummy grid and wait 15 minutes before going out again. The electric water pump and radiator fan were working very hard all day to cope with this. That was what I had assumed initially as with those running, I had problems starting the engine. I even wondered until I found the fuse blown that it might be fuel vaporisation.

I now plan to split up the wiring using a new aftermarket fusebox so that each individual circuit can be fused separately. (Nine wires from ignition and four from auxiliary). Am I correct in thinking that a 5 amp blade fuse would be enough for each circuit? The only heavy current draw might have been the radiator fan, but that has been solved as it uses a relay. Is there anything else on that ignition circuit that might draw / need more than this current?

With the Davies Craig 115 L/m pump, its controller box that sits on the dash will run the pump and fan after the engine is off and it has power from both the auxiliary and ignition feeds. I assume the ignition feed is really only so it can identify what mode it needs to be in. Despite the heat I had no cooling issues over ten races. The engine was registering a maximum of 95-100C and there was no coolant loss - I'll post more about those modification another day.

Electrics has never been my strong point, so any advice appreciated.

Geoff

The best is to measure the actual amps drawn. Then use a fuse x2. That would be enough and safe enough. The amps are variable because the watts stay the same and the voltage can vary. When you have a voltage drop the amps will rise. For example the two headlamps are 55w so 110w. 110watts/14volts=7,8 amp. When you have turned more on like a heater, electric pump etc. and the voltage drops to 13v then the 110watts/13volts=8,5amp. When starting and the voltage drops to 10 volts your electric waterpump will draw a lot more current than normal when running.
When you have a shortcut any fuse will blow but it's annoying that in an extreme "normal" situation that fuse also blows.

Almost every DIY multimeter can measure 10amps and that would be enough to measure the electric components.

Jeroen

The current draw is all about ohmes law. Talking first order effects the constant is the resistance, not the power.

So if you have less volts you will have less current too.

The voltage is down to your battery and charging system and its load....hence the volts drop quite a bit when you are starting your car as
the starter load (current) is very high. Volts go up a bit or should when the car is running as the alternator has to put out more volts than the battery to charge it.

The nominal voltages for a lead acid battery are 13.2V at 20C and the alternator should have this up to around 14.5 IIRC.

Your fuse was blowing more than likely because of a short in the stick loom. I make it a matter of practice now to always seperate that circuit
and it needs at least a 10A fuse. Wipers are worth separating and are again 10A - maybe 15 if you have the washer pump as well.

No dolomite lights are fused.

I would certainly put all your davis craig stuff and cooling fans on relays and separate fuses for each circuit.

Jonners

Jonners is the correct explanation (sorry Jeroen). If power were constant your lights wouldn't dim when your engine speed is low, they do because current is constant and voltage varies.

Hi Geoff, this is very much a work in progress, but I've put relays in for both the EWP and the fan, the fuel pump and I've changed the big cabin heater for a small electric one which also goes through a relay...



I'm putting halogen headlights in too so they'll be through a relay as well.

Additionally, I have also changed the std fuse box to a four fuse MGB type....I'm quite pleased this thread has popped up as I was about to ask what ratings to use for the fuses...once I've worked out which green wire does what I'll know which fuse is the all important o/d circuit etc. as the top three are all the same shade of green apart from the light green one!

Tin I think you meant resistance is constant - so as voltage drops current drops too and the lights dim because
the power dissipated in the filament drops.

A 60W bulb doesnt always do 60W. Its a nominal rating.....It might do 60W if it has 12V and draws 5 A.

If the volts drop to 10 - You can check the sums...

12 volts = 5 A x R, so in this case R = 2.4 ohmes.

So when the volts drop to 10 say the current drops too...

10 = Q A x 2.4, so Q = 4.1666 amps

So the power drops to 41.6 watts and your bulb is dimmer...

Jonners

The lights example wasn't such a good one because that's the only item that has no load on a car. Your electric waterpump will draw a higher current at a lower voltage.

Jeroen

Not sure what you mean by "load" Jeroen....

If lights aren't an electrical load, what exactly are they?

Must be some different terms in dutch electrical education....

Jon

Jon Tilson wrote:Tin I think you meant resistance is constant - so as voltage drops current drops too and the lights dim because
the power dissipated in the filament drops.

It was the faulty keyboard, honest. That's not me sitting in the corner under the Dunce hat, oh no.

Jon Tilson wrote:Not sure what you mean by "load" Jeroen....

If lights aren't an electrical load, what exactly are they?

Must be some different terms in dutch electrical education....

Jon

Mechanical load. These are electric pumps or fans, wipers etc. They have to deliver some torque and are designed for a certain voltage, draw a certain amps so are certain watts. When the voltage drops they can not deliver the required torque to overcome their mechanical load and the revs drop. At that point the current goes up. That's why for example you should not try to start an engine with a low battery. The starter cannot deliver because of the low battery voltage and then the current rises in the starter and it wil get hot and burn the internals.

There are some nice calculations how much the current will rise by dropping the voltage. It is not linear because of the influence of build up heat but the rule of thumb is half the voltage is twice the current.

So when determining the fuse needed for example a lucas wiper motor of about 30Watt is 30w/14v= 2amp. In max conditions 30w/10v= 3amp or driving home at night with a faulty alternator 30w/8v=3,75w a 5 amp fuse will be ok to have it working most of the time and not blowing a fuse.

Hope this is a better explanation.

Jeroen

The original glass fuses, whilst clearly displaying the number 35 are actually only good for 17 amps continuous load and can still cope with all the cars ignition controlled circuits at once. so a couple or 3 amps per circuit is probably close. Maybe 5 for the power hungry HRW (35 amps is the "blow" rating)

Steve

Put 110V onto a 230V motor/pump etc and see what happens........

Put 380V onto a 230V motor/pump and see what happens......

Measure the ohmic resistance of your household vacuum cleaner or your benchtop grinding wheel, apply VIR and....it doesn't add up.

The simple VIR rules don't work for them because......

I was talking about dc. I'm not sure about ac. The only thing I know about ac is when I smell my drill mixing a bucket of plaster I have to cool it down

Jeroen

Same, here's a link:
http://www.precisionmicrodrives.com/app ... m-back-emf

This is why VIR is perfect for some simple applications, particularly dc with no inductance or capacitance, but as soon as we start using motor theory (wipers, starter, etc) or large inductances (coils, overdrive solenoids) things can change drastically. Then add in the characteristics of lead acid batteries under load and the simple VIR needs a lot more theory.

Very true....
We weren't talking too much about motors though. Fuses was the deal....
But now I see where Jeroen is coming from I of course agree....

The motors that have a fuse in a normal dolomite are the wiper motor, washer pump and heater. The ignition fed fuse does them all and is as has been said rated at 35 amp blow and about 17A continuous. It is a VERY good idea to separate the circuits - but I dont think Ive ever had
a dolly fuse because of any of those motors. Its almost always the gearbox loom that causes shorts.

I think we are all on the same page on that. Agree about not using a flat battery to start a car - hadn't really thought about the current flow
but its complex - because as you "load" a flat battery so the voltage drops and you cant really think of it any more as either a constant current source or a constant voltage source.

Jonners