Jaguar Repair: jag not starting, voltage drop test, starter solenoid

Question
bought my first car today. a 1975 xj6 jaguar

it was running smooth on the ride home however when i went to start it in the afternoon a metallic clinking noise started under the hood. i asked a mate of mine who's a mechanic and he thought the battery was low and therefore wouldnt start. then  he figured that the starter engine might be shot. could you provide any help??

Answer
Hi Tom,
It could be many things including battery, starter motor, solenoid, relay or even wiring. The only way to find out is to test. You will need a volt meter to run the tests.

Put the volt meter across the battery posts (not the cable ends) and read battery voltage. It should read 12v +. With the volt meter on the battery try to start the engine holding the key in the start position for at least ten seconds. Note the battery voltage while the key is in the start position. And quickly feel the battery posts for heat. If either (+) or (-) feel hot then you must remove that cable end and clean it and repeat the test.
The voltage when in the start position is an indicator of what is happening. The same 12 to 13v says it didn't connect to anything. 9v or below is an indicator that you have a extreme load (locked up engine or a bad starter motor)
Next test is to do what is called a "voltage drop test of cables". Put the positive test lead on the (+) post (not the cable end) of the battery and put the negative test lead on the (+) post on the starter solenoid and turn the key to start position. You should see less than half a volt. Now put the positive test lead on a clean piece of the engine itself and the negative test lead on the negative post (not the cable end) of the battery and turn the key to "start" and read the volt meter. I must not be more than half of a volt (.5v) These are cable drop tests.

If your battery was low voltage on the first test, charge the battery up (12 to 13v) and then put the volt meter across the battery posts (12 to 13v) and start the engine and put the RPM up to about 1500 RPM and read the voltage at the battery posts (not the cable ends) It should read 13.8v to 14v. This is a test of the charging system.
Howard