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MG Midget and Sprite Technical - Calling all auto electricians..!

I have finaly tracked down my hot mis-fire/won't-restart problem, or at least the cause, which appears to be the electric cooling fan.

With Ignition on but engine not running I have 12.6v over the battery, and 11.4v over the coil. When the engine remains not running and the fan is on, the voltage over the coils drops to 10.7v (i.e. the current draw from the fan drops the voltage over the coil by 0.7v).

The mis-fire is exactly co-incident with the fan coming on, and can be initiated by manualy turning fan on and off. I have contained the issue by putting a manual switch in series with the fan thermostat so that I can turned the fan off when starting hot, so I can at least get it restarted.

So potential root cause are;

1) The fan isn't meant for a midget it's actually from a modern vehicle, so potentialy the fan is just not suitable for the car which is running a dynamo (I do not want to change to an alternator), or perhaps needs to be slowed with a (fairly large) resistor to draw less current. You may think that this may harm the cooling, but this fan moves a lot of air when it does switch in it cycles for only a very short time to get the temps back down, so a slower fan I believe would be ok. But I'm slightly surprised that 0.7v drop over the coil would make it's mis fire that readily.

2) My coil needs replacing (because it should tollerate greater voltage drops).

3) Currently teh fan is wiried through the ignition switch. Should I try wiring the fan direct to teh battery or using a relay? So that it does not hsrae source witt the ignition?

4) Other ideas please suggest. Please suggest!
James Eastwood

With the engine running and the fan on, what voltage do you get at the coil?
Dave O'Neill 2

You have very excessive voltage drop in the circuit feeding both the IGN and fan. The drop from 12.6 to 11.4 is already excessive by far, and the fan makes it worse. This would include batt cables, hot and earth, browN wires from solenoid to IGN switch, White from IGN to coil and fusebox, and whatever you are taking the fan from, maybe Green from fusebox. Check and clean all the connections and cables, all bullet connectors in these circuits. Feed the fan via a relay from the solenoid/battery connection. Max voltage drop from the battery should be 0.5V at any point in the circuit(s)when loaded. I generally try for 0.2V.
Depending on where the fault(s) are, they may also be causing poor charging of the battery. The battery should be able to carry any short term load of the fan; then the dynamo will catch up.

FR Millmore


A relay is essential...

The current draw of the fan will burn out the ignition switch in short order. In fact I would check the voltage drop across the switch as the contacts might already be compromised. Are you a MGOC member? Did you not read the articles on wiring a fan and other accessories recently?

If you intend to fit a resistor to slow the fan down you will need to know the resistance of the motor or the current draw. From there you can calculate the series resistance required and, more importantly, the power rating required. I'm guessing the current draw will be in the region of 10 amps but do let us know.

Finally, adding extra accessories means the battery and/or the dynamo are working harder. This set-up was marginal at best with original equipment (in the cold, dark and wet!) and an alternator is to be recommended.


M McAndrew

Agree with above comments re relay.

But would have thought the coil would be happy with 10.5v or more, since the battery volts will drop somewhere around there when cranking on the starter...

Anthony Cutler

Hi James,

Is this a dynamo equipped car or alternator?

You've far less charging system reserve on a dynamo system, maybe the charging circuit can't cope.

Agree with comments re the fan circuit overloading the ig switch though.

Regards Steve.

SR Smith 1

This may not be relevant, but I have a similar set-up to James (modern electric fan) although mine is through a relay and I use an alternator. But with the car on tick-over, when the fan kick in there is a very noticeable hesitation and drop in engine revs. It never actually stalls, but runs lumpily for the few minutes until the fan switches off again.
Now I am wondering if it is the same problem that James has. I guess I need to check voltages under different conditions.


Guy, I would guess there is no fault on your system, just the extra load on your alternator/ dynamo. Which do you have? it sounds like you have an alternator, and it is trying to meet all the demand for the fan - an alternator can sap 6hp as far as i remember, and at tickover the engine does not produce much (I'll bet single figures)
Dan Cusworth

1 hp = 746Watts
12V charging system x max output of 34A = 408W nominal
Put in some losses, you get about 1hp max loss from full charge state. Generator systems cannot produce anything close to this at anything near idle, so they also cannot load the engine this much.
Yes you can notice this in idle speed, but it should not stall the engine or make it miss or stumble.
If the battery and all connections are good, the battery will take the short term heavy load without allowing system voltage to drop below about 12.5V at worst. If there are bad connections to the battery, the gen/alt will attempt to carry the load directly, but since most of the connections are the same ones, it will also fail to do so, which is what is indicated by low voltages anywhere.
Depending on where the bad connections are, it is possible that the alt/reg will show abnormally high voltages (14-15V+), while those further downstream will be way low. like <10V at the soon to be very dead battery. Or, the alt to battery circuit can be good at say 14V Alt/13.8V Batt, but the rest of the system shows unacceptable drops, not uncommonly down to 10V or less.

FR Millmore

for Danny & Steve who haven't read the original post:
"....the car which is running a dynamo (I do not want to change to an alternator),"

James - what was wrong with the mechanical fan as designed? (i.e. why did you fit an electric one?)
David Smith

Gents, a huge thank you for all this detailed input, I will sort through these. It appears even with the ability to cut out the fan the hot issue is still there. Although the fan does make it worse. I have driven to the Crystal Palace Sprint (this is an exceptional historic sprint meeting I was spectating), and the car only cut out right at the end of the return journey when I attempted to idle it on my drive way. I am now concluding that it simply doesn't have enough spark and I should try a new coil?

Answers in order;
Dave - Starting with 12.6v on the battery, 11.4v at the ignition, then 10.6v with the fan on. Then repeated with engine running as per your question, I get 11.8v at the ignition, which drops to 10.9 with the fan on. I should add that it seems to be able to run fine withthe fan on when cold, but turning it on when hot directly creates the misfire.

Mike, Steve - relay, got the mesage thanks! And also ref burning out the Ign switch.

Anthony - I agree the battery volatge drops to 9v odd when cranking and provided it's not too hot it will catch and fire. It's only when hot thant it won't restart.

David - in an attemp to get better cooling and more power, I opted for elec fan only. Yes perhaps not a great solution for a road car, but I live in the country and the roads are clear, and ... and .. and.. ok shoot me now!
James Eastwood

Those stated runing voltages seem very very very low!
(sounds like not charging at all)
At what RPM where these measurements taken?
Do you have the batery on a charger when garaged?

Even when i had a dynamo fitted it would charge wel over 12V
Though the change to a dynamo was well worth the (very low) effort.

Check all conections and fit new connectors and thick gauge wire where needed.
Power the fan directly from the battery through a relay (all big power consumers benifit from relay setups think lights...)
Onno Könemann

James, agree with you about the Crystal Palace sprint. Were you just there yesterday? We have been today and there was a good turnout of classics on display as well as the sprint. They are apparently already planning next years event.

T Mason

David S - My post was specific to Guys posting - in that when the fan cuts in, the alternator supplies the power which puts extra load on the engine which can drop the revs a little maybe 50rpm or so?. When I set the idle I set is with fan and headlamps on. BTW I have a motorcycle battery and 285 cam which isnt at its happiest at tickover.

FRM, I'm not knocking your calcs, "I read somewhere" on full load, dynamos /alternators can sap 4-6 hp. One of those things once read an committed to memory even though not true.

Dan Cusworth

Onno - I've been over the connections now and improve some of them. I do get 13v at the coil once the engine is over 2000rpm, no issues with charging. I also have a new coil on order.

Trev - Crystal Palace sprint yes very good, they do sellect their entry as they're oevrsubscribed, so I think you need an 'interesting' car to get an entry, either some thing rare-ish, or a car with some racing history, or some thing unusual like the mid engined 3L V6 turbo Golf.

Dan - I think 4-6hp would be a better figure for a whole Front End Accessory Drive including PAS pump, alternator and AC compressor.
James Eastwood

Yep, "read somewhere" can get you. Frequently if you track it down, you find that either the original writer was guessing, or it was some crazy setup. Even taking a 130A system at max 14.5V full load, you only get about 2.5hp alternator. Double that for inefficiency and you might see 5hp, but what system runs like that? Certainly not a Spridget! You can get a fair idea just by belt size and life, a single 9.5mm belt isn't handling 4-6 hp for more than very brief periods.
There is a very noticeable effect on acceleration though, as the alt etc acts just like a flywheel, so it might feel like 4-6hp. That could even show up in numbers, if you were calculating hp from acceleration.

James - Have you checked the voltages now with the fan operating? And have you fitted a relay?

FR Millmore


I wonder what the average stock 1275's hp output would be at tickover? - and a h/c overlapped performance for that matter!
Dan Cusworth

Not to be overly pedantic, but the answer for any engine at idle (tickover = idle) is exactly zero. Power is produced in the cylinders as (fuel energy x combustion efficiency - thermal losses), but it is eaten by pumping losses, friction, parasitic loads, noise, etc. If it all comes to a constant idle speed, then the balance is zero, otherwise the speed changes until it does balance.
If you want to know what it could produce as "output" at idle speed, you'd need to load it on a dyno and see what the maximum load came to, but it would not, by definition, be "at idle", since it would be working to drive the dyno.

FR Millmore

FRM, as I'm also a bit of a pedant what about at fast idle? :)
N Atkins

Still zero. Any condition of constant speed (10rpm or 10,000) with no load means no output. If there is more power than the system is absorbing, the speed will increase, if less it will decrease until it is in balance at zero.

FR Millmore

ah, but what about transmission losses, I bet you were calculating those power figures at the wheels ;)
Will Munns

sorry FRM I was feeling cheeky just pulling your leg on that one
N Atkins

Gents, thanks again for the helpful suggestions, this has triggered a minor program of work as follows.

1) Improved connections to coil to maximise voltage available.
2) Fan wired via relay, in progress.
3) Replace SU carb needles that were a set of hand-filed racing needles, these were profiled to run too rich at idle, changed for a set of standard Cooper S 'rich' needles as per Vizard.
4) Replaced the 25D4 distributer with my other 45D4, with advance reduced to 20degrees. This is now timed at 16 BTDC at idle (bear in mind I have a 288deg cam which prefers more idle advance) and hence 36deg full advance. Both the idle and ful advance recoomendations are taken from my ignition book. Engine pulls well without detonation or run-on.
5) Also now have a new coil, although as yet unfitted, as I don't want to change too many things in one go.

3) and 4) have had the greatest effect with a very noticeable improvement in idle refinement. I haven't had the car 'full hot' yet as it will need a hot day run, but I am optimistic.

Thansk again,
James Eastwood

Chaps, a hot missfire in my highly likely to be a failing rotor arm. Take it off and look for any scorching underneath. Do yourself a big favour and get a new red one from distributor doctor!
Robin Cohen

This thread was discussed between 29/05/2011 and 08/06/2011

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