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Starter motor chattering
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FAB Cruiser

Joined: 03 Jul 2008
Posts: 77

PostPosted: Wed Oct 29, 2008 4:47 pm    Post subject: Starter motor chattering Reply with quote

Didn't know whether to post this one under mechanical or electrical. Anyway, here goes.

We have a 72 MGB that had been parked up in the garage for a number of years.

I decided to start it up in the spring, so with new batteries and a bit of lubrication, it turned over and stated fine. However, if I allowed the batteries to drain just a bit, the starter motor (solenoid presumably, which is attached to the starter) would 'chatter' and not turn the engine.

Anyway, hadn't stated it for a few months so went through the procedure again. This time it won't start even with the batteries fully charged. I just get the irritating chattering noise from the solenoid.

The difficult thing is the starter is well hidden away under the distributor and because of the narrow garage I can't easily get to it from underneath.

Other electrics seem fine. Very bright headlights!

Any suggestions appreciated.

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peter scott

Joined: 18 Dec 2007
Posts: 6221
Location: Edinburgh

PostPosted: Wed Oct 29, 2008 6:09 pm    Post subject: Reply with quote

Hi David,

It could be that you have a bad connection. Maybe the lead from the battery to the solenoid.

The chattering occurs because the voltage across the solenoid coil is being reduced when the starter motor is connected. The effect of this is for the solenoid to let go again and the motor becomes disconnected thus the voltage on the solenoid coil increases again etc etc

1939 SS Jaguar 2½ litre saloon
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Greeney in France

Joined: 06 Mar 2008
Posts: 1173
Location: Limousin area of France

PostPosted: Wed Oct 29, 2008 6:09 pm    Post subject: Reply with quote

As you suggest your battery is OK, your problem could be voltage drop caused by bad main battery cables or terminal ends including earth cable, connections at battery, bad solenoid contacts, or actual starter motor.
Here are a few things to do
Test power to starter, place one of your test lead probes on the stud on the solenoid that your positive battery cable attaches to and the other probe to bare metal on the engine block or cylinder head, have your helper turn the key and read off the measurement. Note that when the key is not in the crank position you will read battery voltage on your volt meter as 12.6 volts this is of little help and should be ignored for the time.

Getting roughly 10.5 volts or so, your cables and ends are fine and you may suspect the contacts in the solenoid, to test them leave one test probe on the block and place the probe that you had on the solenoid stud with the battery cable to the other stud just below the first one. there will be a heavy wire attached to this stud that goes into the starter motor body. turn the key again and read off the measurement. if you have a very low voltage reading here there is a good chance at this point that the solenoid contacts have failed. if you have the same reading, about 10 volts, than the starter motor must be repaired or replaced.

If you are getting less than 10 volts you have a voltage drop and fun begins. Test first the positive, it's easier, place one test probe on the + positive battery post and the other at the stud on the solenoid that the battery cable attaches to. yes your test probes are at either end of the same wire. now turn the key to crank. a measurement above 1 volt is not satisfactory, it may not be THE problem, but should be corrected before continuing. to pin point the bad section simply place the test probes at any two points along the + positive path. start with the centre of the battery post, and 2 inches away where the bare wire slips into the battery terminal end, one tenth (0.10) of a volt is acceptable per connection. any higher than that (under load, which is to say while the key is held in the crank position) and the connection should be cleaned or repaired, poke around and make corrections until you have under 1 volt from the battery(+) all the way to the solenoid.

Testing the ground path is basically the same with a few exceptions. there is no negative wire for the starter, it gets its ground from being bolted up, and relies on a good ground from the battery to the engine or transmission. Cars with remote mounted batteries often have no negative cable directly to the engine or transmission. they use one cable from the battery to a point on the frame, and then another cable from the frame to the engine / transmission. both cables and all connections should be scrutinized. this method of grounding is not optimum. a piggy back, large gauge cable can be added with minimal effort.
To test the ground path start with one probe on the negative post on the battery, and the other probe on bare metal at the engine block or cylinder head. If you have already tested and repaired your positive cable the odds are that you'll measure more than 2 volts while the key is in the crank position. use the same method as above to pin point the trouble section.

We do these things not to escape life but to prevent life escaping us
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FAB Cruiser

Joined: 03 Jul 2008
Posts: 77

PostPosted: Thu Oct 30, 2008 10:45 am    Post subject: Reply with quote

Many thanks for the thorough replies guys. I'll try the suggestions over the weekend.

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The_ Yellow_Ardvark

Joined: 05 Sep 2008
Posts: 28
Location: Out Sude my head

PostPosted: Thu Oct 30, 2008 9:37 pm    Post subject: Reply with quote

Try this little lot to help you.

Have you “dropped tested” the battery, not the first time I have seen a new battery fail after repeated charging and heavy discharging on a dead car.
Charge the battery; leave it a few hours before in the car.
Using a good Digital voltmeter set to 20 Volts DC. Take a reading across the battery. This is “True Battery Voltage”

Clean all contacts to do with the battery, starter motor, charging and do not forget the coil and “dizzy” connections. Plus all the earth connections as well

Set the Meter to resistance. Connect the “Neg” lead to the negative battery terminal. Test the earth to the Chassis, engine, gearbox and bodywork. You should have a reading of 0.1 ohms. (0.2 is ok). Any thing more than that you will have issues. It is a case of clean and checks to get this right.

When this is done, we are now onto live voltage reading. The tests are:

Battery to Stator motor, should be “True Battery Voltage”
If not the lead or connections are at fault.

Battery to “Pos” of coil, again it should be “True Battery Voltage”
Again, the wiring or connections are dirty. Check and clean.

Now disconnect the small wire to solenoid and connect meter to this. Turn the key, as if you are trying to start. You should get “True Battery Voltage”
If this is this 0.7 volts below “True Battery Voltage” then the issue is with the ignition switch.

Hard test now, you will need a willing helper. Connect meter to the motor side of the starter solenoid and try to start the engine. If you have aprox, 10 volts you are ok for now. If it is less than that, the contacts in the solenoid are dirty. All you can do is remove the motor strip and clean.

If this does not cure it, the motor is dirty and will need to be removed and cleaned. Sorry.

One last test. Connect the meter to the CB on the Dizzy and try to start, if you have a different meter reading to the last test on the meter the Volts are leaking to earth.
Wibble I am an orange.
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Jim Walker

Joined: 01 Oct 2008
Posts: 124
Location: Chesterfield, Derbys.

PostPosted: Sat Nov 08, 2008 12:35 am    Post subject: Reply with quote

Surely the starting point (not meant as a pun) is with the battery? With a voltmeter across the battery posts (not terminals) and with the wire to the distributor contacts disconnected to prevent the car starting, crank the engine. Anything less than 10.5 volts here, charge the battery. If that does not improve scrap the battery.
Remember that the sum of all the voltages across every switch, connection and load (starter motor in this case) will add up to the battery cranking voltage, and you NEED maximum voltage across the starter motor and MINIMAL loss across switches and connections. MEANINGFUL VOLTAGES CAN ONLY BE CHECKED WITH THE ENGINE CRANKING.
Voltage drop across switches should not exceed 0.2 volts and across the solenoid heavy duty (starter contacts) no more than 0.5 volts. You must regard the ignition switch and solenoid operating circuit as one circuit and the switching side (heavy wiring) side of the solenoid and system as a separate circuit.
The voltage drop across the starter motor ought not to be more than one volt less than the cranking voltage. The switch controlled solenoid activating circuit somewhat higher.


Afterthought. Don't forget to include the earth bonding strip from engine to chassis/battery in your checks and also the solenoid earth (mounting bolts).
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