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MG MGB Technical - poor cold starting

Hi all,

I have a '76 Rubber Bumper BGT, and it's never been really keen to start in cold or wet weather. It's still the same after three years(!), having fitted new plugs, coil, ballast resistor, rotor arm, dist cap and leads. And the battery is in good condition. The starter motor is also new. (By the way, I have a Magnetronic ignition installed).

The thing that puzzles me (and I've never experienced this with *ANY* other car) is that if I have trouble starting the car, it often *just* fires when the starter motor is disengaged - the momentum of the engine is still turning it over, and it's as if the spark is given a boost by the fact that the starter motor is no longer being used. Once the car starts, it runs fine - no coughing or spluttering.

This suggests that the ignition is wired up wrong. Any ideas??

Dave.
Dave

Your car was originally wired such that when the starter is engaged the ballast resistor is by-passed and the coil receives 12 volts directly from the starter solenoid. When the key was released to return to the "run" position current was again directed to the ballast resistor. In the installation of the after-market ignition you have eliminated the direct feed to the coil when the starter is engaged.
Charles Edwards

Thanks for the reply Charles.

That figures, since the magnetronic was fitted to the BGT before I bought it, and the symptom has always been in evidence.

Is there any way around this problem, other than by re-fitting the standard points set-up? Or should I look elsewhere for the root cause of the poor starting problem?

Thanks in anticipation,
Dave.
Dave

Dave. The use of the Luminition system, which is what I believe you have, does not require that the input from the starter to the coil be disconnected. My daughter's 77 used this system for many years with no problems. So, the first question is does this happen all the time or only when cold?

If it only happens when it is cold, I would look to the choke mechanism and see if it is operating correctly. Paul Hunt's website has a good section on how to set up the SU carbs, including the choke portion.

If the problem exists all the time, does your car have two wires running to the positive (input) side of the coil? One line is from the second small spade terminal of the starter solenoid and provides a full 12V input during cranking. The second wire is powered when the ignition switch is in the run position and provides a reduced voltage through a resistor wire. You mention installing a new ballast resistor. This resistor was used to power the old Lucas 45DE4 (Opus) system, not to power the coil. Thus, if you have added a ballast resistor in the coil input circuit it may well be getting to little voltage to run properly. However, that would not affect the starting unless it had been wired into the line coming from the starter solenoid. When wired into the white wire circuit, it would cause problems when the car was at idle or being driven. More information on this area would be appreciated.

Do you have two input wires to the coil?

Where is the new ballast resistor wired to?

On all of my RB cars, I have wired in a single input wire to a 12V coil and am having no problems with starting in our "cold" weather (below 40 deg F, but seldom below freezing). Hence, some more investigation of your problem should lead to a working solution. These cars, when properly set up, should have no cold weather starting problems, unless everyone else's cars are also having problems. My MGA used to start in Cheyenne, Wyoming winters when my American car would not. Les
Les Bengtson

Hey Les,

Are you saying that if your replace your coil with a 12 volt coil, you do NOT need the second wire comimng from the starter solenoid? Were "ALL" the coils not 12 volt? I am a little confused. My earlier B, a 1972, does not have 2 wires coming to the couil from the starter.

Please help shed some light here.
Jack Williams

Jack. We were speaking of the later, rubber bumpered cars here. They used a 6V coil with two input wires, 12V from the starter solenoid and 6-8V from the ignition relay. The early cars, like my 68 and your 72, used a 12V coil with a single input wire. One of the reasons it is important to know what year one is discussing. There were some significant electrical, ignition and carb changes over the 18 year life span of the B. Les
Les Bengtson

Dave,
You say you've replaced the balast resistor?. On my '75 rubber bumper the balast resistor was in the loom and not replacable as such. A Pink and white wire which ran across the front of the car to the RHS horn and back again. I wonder if the PO has wired two balast resistors into the circuit for you, and if so odds are they aren't wired properly, ie to the coil through the second hidden resistor when cranking so you're still getting six volts, not 12.
It certainly sounds as if the resistor bypass isn't happening.
Peter

I had the boost wire disconnected on my V8 for some time and didn't notice the difference, and the current crop of geared starters people are raving about don't have provision for it anyway. An MGB should be a good starter, although lack of the boost connection in adverse circumstances (like cold and damp, poor setup of ignition or carbs) could make the difference between starting and non-starting. The fact that it seems to fire just when you release the key is interesting. As soon as you release the key you remove the load of the starter from the battery, and if you catch it at the right time there may just be enough inertia in the engine to throw it towards the next firing point when the battery is back to full voltage, hence a bigger HT spark than when cranking. Prevent the engine from starting by disconnecting the HT (connect the coil HT lead to a plug resting on the block to avoid excessive HT voltages round the coil), and measure the voltage on the coil +ve while cranking. You may need to use an analogue voltmeter as the variation in battery voltage while cranking may not allow a digital to 'settle'. In an ideal world you should see about 10v. If you only see about half that you are not getting the boost voltage from the solenoid. Between 5v and 10v indicates either a tired battery or bad connections in the cranking circuit or both. Measure the voltage on the battery posts (not connectors) while cranking and compare. Same voltage or within a couple of tenths means a tired battery, *higher* voltage by more than a few tenths means bad connections.
Paul Hunt

Thanks everyone.

I read through these postings, in conjunction with the manual, and it's a lot clearer now. There *was* an extra ballast resistor wired into the ignition circuit. Removing this has made the starting performance *much* better.

However, I don't have an additional wire from the starter solenoid to the coil - just a single white/green wire emerges from the loom (although this looks like it has been tampered with!).

Presumably I can just run a wire externally from solenoid to coil to provide the voltage boost when the starter motor is cranking....

Dave.
Dave

Dave. All I have are the US spec wiring diagrams. One of the first questions might be did the UK spec cars go to the 6V coil and twin wires when the US cars did? The UK spec cars kept the SU carbs and, I believe, did not have all of the emissions equipment the North American spec cars had. The 6V coil, on the NA spec cars was used because the engines were much harder to start than the earlier SU equipped, non-emissioned cars were. Hence, the first thing to do is to get the proper wiring diagram for your year/specification and examine it to see what you actually should have. Les
Les Bengtson

All models changed to the 6v coil and loom ballast at the start of rubber bumper production. Even though UK spec cars never got the US emissions stuff or had the same dodgy fuels most manufacturers changed to the same system at around that time as it improves the chances of starting under adverse conditions for all cars. The white/green on the coil shows you have the loom ballast, and there should be two of them on the coil, the other one going to an additional small spade on the starter solenoid which boosts the coil voltage some 67% during cranking. Maybe your starter has been changed for one without this boost contact and someone has snipped the wire at both ends to hide the fact. If so it can be replicated with the use of a relay. I didn't notice any difference in my V8 without the boost wire, but 'talking' to someone else this week with the same problem it takes them ages to start without the boost connection (non-standard starter), but starts almost immediately with one jury-rigged. You should have a 6v coil as well, using the loom ballast in conjunction with a 12v coil will also make starting difficult. You can check this by measuring the primary resistances with a digital ohmeter, the 12v coil is about 3 ohms and the 6v about 1.5. I have also seen a 12v sport coil with about 2.4 ohms, so you need to be aware of that too. A friend had some work done at a well-known V8 specialist and they managed to fit a 12v coil in series with the loom ballast *and* and additional ballast, after about 100 miles on new plugs it would barely run, let alone start.
Paul Hunt

Thanks everyone.

After reading your comments, in conjunction with the manual, it all became clear. I had an extra ballast resistor wired into the ignition circuit. I removed this, and it starts fine - even in the cold and wet (we've had a lot of cold and wet in the past few days!).

However, I only have one (white/green) wire going to the coil - no "boost" wire from the starter solenoid. Presumably I can simply run a wire directly from solenoid to coil...

Dave.
Dave

Sorry - I've posted the same reply twice. (I was looking at a cached version of the page, and assumed that my first posting was lost - duh!)

Paul, I think you're right about the loom having been "doctored". You say that I can replicate the boost wire "with the use of a relay". What does that mean? How is it different to simply running a wire between solenoid and coil?

Best,
Dave.
Dave

Nothing stopping you running an addittional wire from the starter to the coil, It might be easier than trying to trace what has happened to the orrigional. Make sure you get the right terminal on the coil though. Obviously should only be live while cranking. If I recollect there were four terminals on the solenoid. Two small terminals & Two big ones. One of the small ones is connected to the "cranking" part of your ignition key and when powered up connects the two large terminals together, power from the battery and the one for cranking the engine. The other small terminal is live only when the engine/starter is cranking and dead at all other times and is the Ballast resistor bypass ie will supply your coil with roughly 12volts.
Don't try to connect your coil direct to the ignition cranking wire or the starter motor wire as weird and wonderfull things start to happen, as I found one perplexing afternoon.
The easiest way might be to disconnect one of the two small wires and if the engine still cranks then thats your bypass, or if the engine ceases to start when you turn the key, the bypass must be the other one.
If you don't have a second small terminal (ie you've got a pre '75 starter motor) Paul has worked out a way around this, hence the rely comment.

There is also a chance that if you don't have the second terminal, it might have snapped off, in which case you can solder on a new one.
Peter

Get the right terminal on the starter, I ment to say, not coil
Peter

Dave - you only need the relay if your solenoid doesn't have the terminal or if it doesn't work as it should. If it has the terminal then check it shows 12v while cranking before running in the wire, if not we can go into the relay.
Paul Hunt

This thread was discussed between 28/12/2003 and 01/01/2004

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