Electrical Tech Tips for MGB Members
Electrical Dashboard Gauges
The electrically operated gauges (fuel, rev counter, water temperature and on 1968 – 72 MGBs the oil pressure) have been known to fail on occasion.
Here are a few checks on how you can pinpoint the causes.
All Gauges, Indicators and Brake Lights fail.
Probable cause is failure or poor contact of the ‘Green/White’ fuse. Locate the fuse block and check the fuse which joins a white and green wire. Clean the clips with fine emery cloth and replace the fuse. If the fuse was ‘blown’, there is probably a fault in one of the circuits the fuse protects. Try to narrow the field by trying the horn, indicators etc until you find which blows the fuse.
Fuel Gauge does not work.
Probable cause is a faulty sender unit. Check as follows: Remove the Green/Black wire from the sender unit in the fuel tank. Connect one end of your test light (you do have one, don’t you?) to the terminal on this wire the other end to earth. Switch on the ignition. The fuel gauge should begin a slow climb to full and the test light should glow. If the gauge still does not work, it is either disconnected (check continuity of its wiring) or the gauge is faulty.
Temperature Gauge does not work.
Probable cause is a defective sender unit. Check as follows: Remove the Green/Blue wire from the sender unit in the cylinder head. Connect one end of your test light to the terminal on this wire, the other end to ground. Switch on the ignition. The gauge should slowly climb to hot and the test light should glow. If the gauge does register, the sender unit is bad. If the gauge does not read, check for loose wires. If none are found the gauge needs replacing.
How to make it start!
Two quite different symptoms may be noticed in a car that refuses to start. If the engine will not fire when the starter motor turns it over, the fault must be in the ignition or the fuel supply. But if the starter motor itself does not function, the fault must be in the motor or its associated electrics.
It is likely that the fault is not with the starter motor itself but with the power supply system. To check this possibility, in cases where there is no response when the starter is operated, begin again and turn the key one position to put it into the ignition on position. The ignition warning light should come on and the fuel gauge register. If they do not no power is leaving the battery. This will be because the battery is flat or the connections at the terminals are at fault.
If the headlights or any other major power consumer have been left on for any length of time, the battery will become completely flat and must be recharged before the engine can be started in the normal way.
Go to start your car and it goes ’click click click’. When was the last time you checked your battery? Keep batteries topped up with distilled water. Corrosion on battery posts and terminals can be killed using boiling water. Remove terminals, check for corrosion replacing if necessary. Refit with a smear of Petroleum Jelly to the battery posts and the terminals.
Faulty Battery Post Connections
If the battery itself is in order, the loss of power may be caused by faulty connections between the battery and the starter. The ‘post’ connections on top of the battery are always suspect. In extreme conditions – icy weather for example – a starter motor can take 400 amps to turn the engine over. Any faulty connection will prevent such currents from flowing, but may nevertheless pass the much smaller currents demanded by the ignition lights.
A quick clue as to the condition of the connections can be had by touching the positive terminal of the battery, marked +, immediately after you try to start the car. If it feels hot, the connection is poor.
Corrosion may have begun on the inside of the clamp, on the post or both. A quick whisk with some sandpaper will restore the surface condition of both.
Battery clamps often become slightly oval inside due to corrosion. So when you replace the clamp try to place it in a slightly different position so that it presents a fresh face to the post.
With the ‘helmet’ type of connection, a screw through the top locks the connection in place. If it has worked loose tighten it, but if this reduces no result, remove the screw, lift off the ‘helmet’ and clean it and the post with emery paper.
If the battery connection will not come away from the post after it has loosened, trickle hot water over it. Never try to pull or twist it free as this risks cracking the battery case.
Faulty Battery Cables
If you can find nothing wrong with the battery connections, next check the battery cables. The braided metal lead to the earth connection is especially suspect because corrosion can creep along it attacking individual strands of wire until those remaining cannot carry the starting load. It is easy to be misled by an apparently sound lead as the corrosion often affects only the underside. Lift the lead so you can check it all round and if faulty replace it.
The earth connection on the body of the car should also be examined since rust can set up on the nut and bolt connection. Undo the connection, clean its faces with emery cloth and bolt it back into place. The area of metal around the bolt may also need cleaning.
The insulated live lead from the battery tends not to suffer from corrosion as much as the earth lead does, but an overfilled cell or a fine crack in the battery case can allow acid to escape, settle on the lead and work its way under the insulation. To test for problems in this area bend the lead to and fro several times. If it is corroded you will feel the grating of the fractured wires – a sign that it must be replaced.
Turn the ignition key to the first position again and if the ignition light fails to come on try the horn and headlamps. If none work, the battery is completely flat. If the battery is providing power try to start the engine again.
If there is still no sign of life in the starter motor and the lights do not dim when you try to start the car, it is clear no power is reaching the starter. When the starter motor turns it drains off power from the lights and causes them to dim. So if the motor fails to turn and the lamps remain bright there are two possible faults. Either power is not reaching the starter motor or the motor itself is defective.
Replacing the 6 volt batteries fitted to many MGB’s with a modern 12 volt battery
A frequently asked question is “can I fit a modern 12V battery in place of the two 6V batteries fitted to many MGB’s”. The answer is YES.
All Chrome bumper cars up to 1974 were fitted with two 6V batteries. These batteries are of old style construction and lead to poor starting due to corrosion of the terminals. With the introduction of the rubber bumpered cars, the battery compartment was modified to accommodate a single modern 12V battery. The type 063 battery, as fitted to RB cars will fit into one of the two 6V positions leaving the other position available for fitting a useful tool/spares box, the only mod required is to replace the existing battery strap with a couple of large cable ties or similar. Incidentally the type 063 battery actually has a better cranking performance than the original two 6V batteries.
Whilst a type 063 can be made to fit into a chrome bumper battery position, it is a very tight squeeze. A better fitting battery is either a type B36 or type 202. These are the same size as an original single 6V battery and can use the existing battery clamp. As with the 063 you will need to unbolt and move the earth lead over. All these batteries (B63, B36, 202) has a better CCA (Cold Cranking Amps) performance than the original two 6V batteries, so will give a marked improvement in starting and require less maintenance.
Read the Full article:Fitting a 12V battery to a Chrome Bumper MGB
Excellent wiring harness info can be found at www.advanceautowire.com