Lotus Elan and Lotus Elan Plus 2 Forums

Indeed - Battery charge and voltage under load are the first two service checks recommended by Lucas.
The latter should stay well over 10 volts if the battery is good.
In the good old days before computer analysis some of us will remember a heavy discharge tester being used to assess battery health.

measure the voltage at the battery (on the actual battery terminals) and at the starter motor as you try to start the car and if there is a substantial volt drop at both points it?s likley to be the battery . I would expect some other symptoms if it was the motor eg smoke
If the voltage at the battery stays significantly higher than at the starter motor then there is a bad connection somewhere.
Ed

john.p.clegg wrote:"The dimming of lights indicates that the starter motor is drawing a large current " , or more accurately that the battery voltage is dropping on load , which could point to a duff battery...

John

I stand corrected, your definition better defines what is happening. From the OP description though I eliminated the battery as he said the it was fully charged and the car was otherwise running normally.

A faulty starter motor cannot be ruled out but diagnosing the fault in a logical manner is much better than randomly cleaning connections and hoping.

To me, the quickest way to isolate the bad connection is with a voltmeter, with the key held in the 'start' position measure the voltage directly at the starter motor, even easier as a first step would be to measure the voltage drop between the two threaded studs on the solenoid. If there is no reading move the probes to touch the cable where it is soldered or crimped to the ring tag.

If the battery and starter motor are OK then the battery voltage should appear at the motor
when the solenoid is energised. If the motor volts read significantly lower then the missing volts will be very easily
located by measuring each section of the circuit in turn.

Ian

JimJ

To me it looks like your starter is shorting to earth internally, a simple test below

If you connect a jump lead from + battery to the starter and flashes when connecting but does not run and dims the lights, this is confirmation

Good luck

With every intention of reluctantly removing the starter motor today, I just gave it another try and it did crank the engine over, albeit a bit slowly but not desperately so.
Following Markda`s advice, I checked the voltage at the battery which read a little over 12 volts. I got a similar reading across the solenoid too, as you`d expect. From the live on the starter motor to the chassis it reads around 9 volts as the engine is cranked over. Doesn`t this sound about right?
I`m concerned that firstly, it wouldn`t work yesterday, and, secondly, that it`s not cranking as healthily as I would expect.
Thankyou all.
Jim

Jim

When cranking where is the missing three volts?

You might need an assistant but really you need to measure the voltage over each connection and cable.

There will be some voltage dropped across every piece of wire or electrical joint but 3 Volts sounds quite high. It will be the total of the wire and the chassis but even the longest run of wire should not drop anything like that.

When measuring you need to put the meter probes directly on the thing you are measuring , ie move the probes for each test.

Ian

9V is too low.
You do need to check voltage at battery when starter engaged if that remains at 12v then 3v loss would indicate high resistance.
If 10v or less the battery is duff.

MarkDa wrote:9V is too low.
You do need to check voltage at battery when starter engaged if that remains at 12v then 3v is a would indicate high resistance.
If 10v or less the battery is duff.

Yes, I should have said that. You can only find the missing volts when current is flowing.

Ian

I currently have a similar issue but I know that there is a joint in my supply cable under the dash which allows voltage and low current but won't drive the starter.
The existing connector is only 30mm long with one bolt each side and worked loose.
I'm awaiting a proper Durite cable connector to make a decent long joint that hopefully wont loosen.

Pardon my naivety but measuring the voltage from the + on the starter motor to the chassis, wouldn`t you expect to see a voltage drop as the engine is cranked over drawing all that power?
Jim

jimj wrote:Pardon my naivety but measuring the voltage from the + on the starter motor to the chassis, wouldn`t you expect to see a voltage drop as the engine is cranked over drawing all that power?
Jim

Certainly yes.

But three volts is more than it should be. Any conductor carrying current will lose voltage (if you want to minimise the loss you could use silver wire as it has less resistance than copper).

Cable as thick as starter motor lead has very low resistance and even thought the current is very high I would think something like 0.5V would be typical.

If when cranking the battery is three volts higher than at the motor terminals that three volts drop is over ALL the circuit, it could be made up of several separate high resistance joints or just at one joint.

Ian

0.5v is exactly what Lucas recommend for the main conductor.

Yes, one of the reasons for a ballast resistor arrangement of the ignition.
Actual voltage measured is dependent on a number factors, rating of the Battery ( cranking current ) CCA. and TOTAL resistance in circuit, at high current say 150 amps for every 0.01 ohm circuit resistance a 1.5 volt drop will occur.

I've always assumed that the voltage across the coil (and starter motor) would be down to about 9volts when the starter motor is churning.

Most of that drop would be in the battery itself.

Hence ballasted coils designed for 9volts and fed through a resistor that drops about 3 volts from the 12 volt supply when the starter motor is not in use.

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So checking the voltages under load, should start at the battery, next the input terminal of the solenoid, then the output terminal of the solenoid, then the input terminal of the starter motor.

For this you need a second person operating the ignition switch to its starter position, or rig a push switch on a long lead to connect the solenoid input terminal to the solenoid activation contact. Ensure a good earth contact for each voltage check and when testing in the engine compartment do two tests on each of the above points, first with the negative lead on chassis metal and second with the negative lead on engine metal.


You should possibly pull the wire off the ignition coil for such a test as you don't want the engine to start even if the starter motor spins.

Useful information may be gained by trying to crank with the plugs removed. Post results.