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MG MGB Technical - Starter Issue

I have a 1973 MGB Gt. That has developed an issue like a Dead Battery. a couple of years ago I had an intermittent issue which turned out be a loose ground strap to the engine. With the motor mount in the way I just added a second strap and things have been great. This was discovered after a new Battery and Starter They both had some age on them. So the other day I was doing some Carb. adjustments started fine several times then just a slow attempt at turning over and then some clicks. I have had the battery tested, removed the Starter had it tested, replaced the relay no difference. I have already replaced the the wire going from the relay to the Starter. Is there any other ground that would affect the start cycle. The Starter Switch is also fairly new. The coil is new as I was have a hot white wire issue going to the switch last year.
Ben Stone

Ben
To test the battery, main cables, solenoid and starter in situ.
If you make up a jumper wire and hold one end onto the main wiring post of the solenoid and then touch the other end on the smaller terminal, if it spins over like that then your problem is in the primary side of things---ignition switch or wiring to or from it-
If it doesn't spin over with the jumper wire, taking in that you've had the starter/battery tested and a new main cable, then your issue is a connection somewhere, As you've had the rest undone for testing ,maybe the connection between the battery earth cable and the body.
Does sound like a flat/dead battery though--is it fully charged
willy
William Revit

The Battery was tested and is fully charged, starter was out and bench tested, tried a different switch, connected a battery cable to the negative post and to the frame no difference, ran the cable from the positive post to the main terminal on Starter no difference. The starter is engaging and trying to turn over before dropping down to just clicking. Been some mention of the Seat Belt interlock having an effect. This is a 1973 . Just weird, Been super responsive and then this in just a matter of minutes it's not like I left the car for weeks and then it Started.
Ben Stone

yep understand that---but you have to test if it's the big fat power supply or the little skinny power supply.
The jump wire accross the starter terminals is the first elimination step----have you tried that--result ?
William Revit

will try that today. Thanks
Ben Stone

So I went to the main terminal on the Starter to the small terminal with my Push button switch I use for adjusting valves . Got the same " I'm constipated grunt" from the starter. So what does that mean ?It's got be some simple connection. would it be something with that seat belt relay that someone mentioned . The seatbelts, buzzers and light haven't been connected for the 20 years I have had the car.
Ben Stone

OK ,by using your press button switch you have bypassed all the primary wiring,switches etc, including the seatbelt switch- -because you have the same result by doing that, you have proved the issue is 'not' in the primary side of the electricals so we're back to the fat wires- The problem can only be either, the starter motor, the cable from the starter to the battery, the battery, the earth lead from the battery to the body ,or the earth from the body to the engine.

First job ,check all the connections, cable to starter, cable to battery ---earth lead to battery and the other endof the earth lead to the body--and your engine earth.

Still sounds like battery, have you got another you can try or some jumper leads just to eliminate the battery.
When you had the battery tested did they load test it.

Turn your headlights onto high beam and try and start it--see if they stay bright (starter issue) or go dull (battery or connections)






William Revit

Any weak connection in the 12v and earth parts of the cranking circuit can cause this, as well as battery and starter problems.

When it's cranking slowly measure the voltage right on the battery posts - not the connectors -, then do the same on the starter battery cable stud and body -
They should be close i.e. within half a volt.

If the battery post voltages are 9v or less (probably quite a bit less) then the battery is the problem.

If the starter voltages are 10v or more when it's cranking slowly than the starter is the problem.

Good at the battery and poor at the starter then it's connections. Measure each path end to end to see which one is showing a high voltage, then at the various connection points along that path. Of course there could be multiple weak connections in both, the most recent one being the final straw.

I use an analogue voltmeter as they average out the fluctuations even in a correctly working system to give a steady reading, digitals may not. It does need to be cranking, it will be much harder to judge if it's only chattering.
paulh4

The first thing I did was replace the battery with new and nothing changed. I had the ground to engine issue a couple of years ago. I added a second ground as I couldn't tighten the original one and that fixed the problem. starter is fairly new. I had it out and bench checked. Has to be something on the ground side. All the lights work fine. I even pulled the plugs to make sure I didn't develop a mechanical issue. Engine turned over fine. I did notice the Choke Cable getting warm. I think that is an indication of bad grounding.

Ben Stone

Choke, heater cable etc. getting warm while cranking _does_ indicate a bad engine earth. To confirm put a meter between a good earth on the engine and a good earth on the body and crank. Any voltage displayed shows a bad engine earth.

Any earth strap or cable is only as good as what it is bolted to on the two components it connects. It's not unusual to add a second earth strap to RB cars as the gearbox strap is so awkward to get at, but a CB is relatively easy. I'd get that off and clean up the surfaces. Make sure the tab on the strap is against the back of the chassis bracket and not the top of the mount plate, and against the engine front plate.

Two 'errors' on that image - one was how the car came to me, the other because the photo was taken while I was correcting that.



paulh4

I had a situation following a clutch replacement with the strap shown in Paul's photo but where the connection was made from the wrong (upper) side of the engine mount to the engine block. This meant that all grounding was through the heater, choke and accelerator cables. Oddly enough, starting was OK until it wasn't, with these cables turning bright orange under a starting load with attendant smoke and eminent fire risk. It took over a year for this error to present itself in difficult cranking.
Glenn Mallory

This thread was discussed between 07/06/2025 and 04/08/2025

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