The Question
I’m having difficulties starting my MGB Roadster. Last year I had to replace the starter with a reconditioned one. This appeared to solve the problems then but soon after problems returned. I even resorted to hitting it with my wire wheel hammer when trying to get it to work, all to no avail.
The car has had the twin 6 volt batteries replaced with one 12 volt and when checking it shows 12 volts on the meter. The car lights are dim, the ignition light is on but dim. When the ignition is switched on you can hear the fuel pump working but turn the key and nothing happens. The light intensity doesn’t change when trying to crank the engine.
Answer
There are several different faults that can be present here and the 12v measured at the battery indicates a battery with only about half charge as a fully charged battery should show a no load 12.4v to 12.6v reading. Nevertheless, the dim lights and ticking fuel pump at least confirms that there is some electrical continuity but it clearly isn’t all good just from the dimmed lights.
Firstly, I would keep the voltmeter connected to the battery terminals and switch the ignition and headlights on again, confirm that they are still dim and then read the voltage. If the battery has a problem then there will be a large voltage drop. If so then I would be suspicious of the battery condition and I would take it to a good motor shop or garage for them to put a proper load test on it and see if it fails or passes.
Failure obviously demands a new battery, but I always suggest that a single 12v battery should have a minimum rate of at least 50 Ah (amp hour) if it is not to be stressed by the voltage drop of its distant mounting from the engine, the heavy draw of the typical MGB starter, and the often limited power generation of the typical MGB generators (dynamo or alternator) often aggravated by long standing periods when not connected to an intelligent battery management charger.
Typical 12v 063 series battery just fits into a single MGB 6v battery box, with modified clamping and new earth cable. These do not have the reserve of the original twin 6v batteries but offer a viable alternative today as cars are generally used for pleasure and so tend to be connected to battery management chargers when not in use so the battery is 100% charged ready for use.
If the battery passes the load test, and I am doubtful it will, then give it a long session on a battery charger and before refitting thoroughly clean the battery posts and the inside of the battery terminals, and when refitting and making sure they are good and tight. (Note that the original factory inverted cup type battery connections held with a single self-tapping screw are really very poor and ideally should be replaced by a proper clamp type).
It would also be an excellent idea to clean the earth strap connection to the body and ensure that it is a tight fit and to also do the same for the engine to body earth, be that around the front left engine mounting on chrome bumper cars with the square 4 bolt mounts, or underneath between gearbox and body on later cars.
At this point the state of the battery and all the main connections will be good, so if problems persist the focus next moves further down the main battery cable to the starter solenoid, (separate for inertia type starters, integrated on starter for pre-engaged) which is used as a convenient power distribution point for all electrical distribution, the main power feed to the starter, and the input connection for power from the generator. Whilst the battery is disconnected it’s a good opportunity to remove all the connections on this clean then and refit firmly. I am not confident that this will be the only fault though.
Line drawing of the connections on the typical Lucas pre-engaged starter motor used from 1968 on MGBs. In this instance this shows the added ballast resist ignition system connection introduced with rubber bumper cars.
The described fault condition indicates two separate issues; dim lights are not associated with no cranking. The lack of any reaction to turning the key to crank the engine is not what would be seen in the case of a poor battery state or very bad connections. The fact that there is some electrical function with lights, fuel pump and ignition warning light although clearly weak, the expected reaction when turning the key to the cranking position would still generate a reaction.
The first reaction would be as the starter relay is engaged it would generate an audible click as this is a low draw component like the fuel pump. If this happens then it indicates that the relay is either faulty, and not switching the heavy load power to the starter solenoid, or there is a problem with the wire between relay and solenoid.
The metal body type of starter relay arrowed as applies to the 1970 to 1976 model year cars.
Later cars use more common plastic bodied relays and cover more functions.
The second reaction is when the starter relay works correctly and would immediately trigger operation of the starter solenoid with a heavier sounding clunk which is clearly coming from lower down in the engine bay. This would indicate that the starter solenoid is engaging but it is not connecting the battery direct to the starter motor windings.
The third reaction would be for the starter to turn the engine confirmed by the sudden heavy electrical draw dimming normally bright headlights as a confirmation of the starter motor windings absorbing most of the batteries energy. However here when poor connections (or weak battery) are present then the heavy demand from the starter will simply be too much and the result is electrical dropout and everything goes out.
If the issue was just a weak power supply then you commonly get the ‘machine gun’ effect. This is because the dropout is like switching everything off so the starter relay and solenoid return to their off positions, but with the key still held in the cranking position, immediately, with no heavy starter power draw, there is power available again that repeats the cycle over and over again whilst you hold the key in that cranking position.
So if you just get a single light click then that indicates an issue with the starter relay or the wire connecting it to the solenoid. If there is the heavier clunk this points to a starter issue. Commonly this can occur when the internal solenoid connections of the starter have become dirty and the key turned to the cranking position and is a more common problem for cars that have had longer standing periods.
Now we need to consider the other potential issue. That dim light issue of the headlights and ignition warning lamp could be a simple poor connection, but after sorting the battery, its earths and especially the starter solenoids distribution wiring, the dim light issue should have been solved. However, if the problem remains, put your the voltmeter between the bottom fuse in the fuse box in the engine bay (2 or 4 fuse types) and a good earth.
The voltage reading should ideally be the same as at the battery, or within 0.2v of it and test it with both the lights off then on and note any difference. However headlight dimming can be from a fault within the feed or earthing of the actual lighting wiring, so as the ignition light was also dim, do a second test on the second fuse up from the bottom with the ignition switched on, (top fuse on 2 fuse box types) and check voltages.
A larger voltage drop for both ignition and non-ignition circuits firmly points back to the distribution wiring at the starter solenoid again! Beyond that common junction both power and earths become separate for each circuit so you need to look deeper into the affected circuits, but it is far more common for issues to be located where there are earth connections, first direct to where these are bolted into the body, and for heavier current drawing circuits like headlamps also in the many bullet connections within the black wiring.
Starter solenoid replacement options
Inertia remote mounted:
- BCA4501 - Round type
- 13H5952 - Square type
Pre-engaged type:
- NAF10001
Preferred approach: replace starter motor with small PMGR Bosch-style Lucas reproduction (GXE4441).
GXE4441 - Small Bodied Pre-Engaged Starter Motor MGB
The wiring at the front of an MGB is rather exposed to letting the weather into these bullet connections, and where one bullet is affected then those around will usually benefit from the same cleaning. Sometimes the spring steel holder in the bullet receivers can crack and with no spring pressure to firmly hold the bullets dirt and corrosion quickly builds.
Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
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