MGB & GT Forum

Rainy day in North Carolina and I wanted to check on my 1978 MGB no electricity flow problem. Ill explain. My B is my daily driver and usually starts every time and runs fine. New starter, good battery load tested and good alternator. when temp falls below 50F I turn the key and I don't get an ignition light neither can I hear the fuel pump tick. I leave the key in the run position for a minute or two and the ign. light comes on and the fuel pump starts pumping. I start and everything goes back to darkness and silence. Repeat, and this time finally the starter engages and everything is fine until the next morning at 5:00AM. and the scenario repeats. New batt. terminals. clean batt. posts. sometimes I can hear a relay clik but I don't know the source. Only when it's cold. No change depending on humidity. I am thinking relay but which and where. Please give some advice as to where to look. It is hard to trouble shoot at 5 AM and it is 35 degrees out. Thanks my friends.

Do the headlights work (when the car is in the no power state)?

The headlamps are powered from the brown (unfused, unswitched) power circuit. If they work, then the brown circuit is live. If they don't, then something is wrong with brown.

If the brown circuit is live, then the next link in the chain is the white (unfused, switched) circuit. So, you need to find out if power is reaching the ignition switch (via brown) and if the switch is energizing the white when the key is turned on. Both the alternator warning lamp and the fuel pump are powered by the white circuit.

This is from memory - hope I got it right!

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Check the ground straps, at the battery and engine. Take them off the car body and engine ends and use sandpaper on them and make sure they are very tight. While you have the ground unhooked crawl under the car and disassemble and clean all of the wiring and terminals at the starter.

One of my winter beater GTs used to pull the same trick. It was the body connection for the ground strap at the battery. It looked clean and tight but was corroded under the strap where it bolts to the body.

I have checked the Batt. connections and added an engine ground to body but i will trace again the grounds. Thanks Terry and Anthony. No thoughts on relays?. Can they be affected by temperature?

Tomorrow I will check the headlight theory. sounds solid. Thanks.

If you do find it necessary to clean the grounds and starter terminal (always a good idea on our old cars) coat them with a little dielectric grease to protect them.


While I guess it's possible that cold could effect the flow of current at a poor ground or other connection (45 is not that cold) I would look at a sluggish starter relay and try giving it a tap with a fine adjustment tool.

The thing is that there are no relays between the power and the alternator warning lamp and fuel pump. So, if you have no warning lamp and no tick tick from the pump, then the problem is closer to the power source. Hence the check of the brown and white circuits.

Double check the engine ground - I've seen them appear perfectly sound but when you really investigate you find that the strap between the engine and the chassis is cracked.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Is the starter relay the same as what is known as the solenoid? bolted right on the starter housing.

The starter relay is mounted in the engine compartment near the fuse box, but when I suggested the "sluggish relay" theory I didn't realize you had no Ignition Light or noise coming from the fuel pump. What Terry said is correct....


Take a look at your wiring diagram found HERE Start at the battery and work your self to the ignition switch.

No - the solenoid is part of the starter - and it is essentially a very heavy duty relay. Another relay (much smaller) triggers the solenoid.

BUT - neither the solenoid nor the starter relay have anything to do with the alternator warning lamp or the fuel pump. Well, I suppose the copper bolt on the solenoid is a common connection point for both the brown wire that leads to all of the car's electricals - but current to the brown circuit does not flow through the solenoid itself. So, even if the solenoid fails, you would still have fuel pump and alternator warning lamp. It is possible that you have a dirty or loose connection from the battery to the solenoid (and the brown wire that connects at the same point). And it is possible that the bad connection is temperature sensitive.

What did the headlight test show?

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

So far I have cleaned and re-thighten all of the ground and + connections. It is still acting the same way except the wait time has shortened. I do have good bright head lights all the time. I actually turn them on momentarily before starting the car to verify juice. It starts, it just take a longer time to hear the fuel pump and the ignition light to energize. Work in progress just had to replace the clutch slave cylinder. Happy Holidays We have had 16 degrees temp. in the morning here in NC and it starts.

It is possible it is just the battery. Old battery cannot keeps with when the temperature is getting colder basically your battery diminished rapidly with the cold. How old is the battery ?

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1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

In order to help you trouble shoot we really need to know whether you have power on the brown and/or white circuits when the problem manifests itself.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

I don't know answer, but this Q could help OP and many others re. good battery.

With voltmeter on battery, what should it read when cranking?

Depends on the ambient temperature - but I would hope that the battery voltage would not drop below 9v when cranking.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com