MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Alternator Output Measurements

Now this is interesting…

In the process of looking under the hood for a solution to a recent erratic engine idle problem – I put my trusty volt/ohm meter across the terminals of my battery. Brand new by the way – Interstate brand – replaced just last week...

But back to the story: with a steady idle of 1,000 rpm - I get the following readings across the terminals...note that all electrical loads are incremental and cumulative...

Baseline idle: 12.40 volts
Turn on parking lights: 12.30 volts
Low beam headlights: 12.10 volts
High beam headlights: 12.05
Heater fan on: 11.98

Are these readings a sign of a weak or dying alternator? It appears to be an original Lucas unit and the belt looks good and deflects less than half inch at center.

Any thoughts if a weak alternator would affect the engine idle characteristics? How much of a fall off should the Lucas alternator be able to support as additional electrical accessories are added? Time to switch to a more modern and higher capacity unit? I have seen several MG success stories using either Bosch or GM alternators, etc.

Thanks for any input into this issue.

PS: I did replace the distributor cap and the idle issue seems to have gone away. Points look good and I did NOT replace the capacitor on the theory if it ain’t broke don’t fix it...

Mike P.
Buffalo, NY

Mike Pelone

Mike-
It's the sign of a car running on a good battery that is not being charged. Either the alt's DOA or there's a (bunch) of bad connections. Either is possible.
Why was the battery replaced?
Does the IGN warning lamp light, engine not running, and then running?
What is Batt V not running?
At 2000 rpm, no load?

Send me a email asking for Electrobabble and I will send you all the info you ever need! Or drive down, I'm 100 mi South.

FRM
FR Millmore

Yes the figures quoted suggest that at the time of measurement the alternator did not charge very much. However for the battery to have a value of 12.4 volts at the start would suggest that it had had some charge previously.

2 things need to be answered
Was the ignition light on?
and what is the voltage reading at the battery when the engine is revved to 2000 RPM?

So on top of that your question would a faulty alternator affect the idle? Not sure this is the problem of an eratic idle, in fact if the alternator is faulty then the idle ought to be stronger as the faulty alternator would not be putting a load onto the engine.

Why have you changed the battery?
Robert (Bob) Midget Turbo

Thanks for the input...

New battery replaced a $39.99 "special" from PepBoys - that battery came with the car...The previous owners receipt showed a purchase date of 1999 in California - as the car was getting harder to start after sleeping a week or more in the garage over the summer...plus the car is almost into the warehouse for the Winter, so I thought what the heck - get a new one! On the other hand, I now appreciate how much new batteries cost (don't ask....)

Yes - the red indicator light was on during the idle test last night as I added more electrical load. But after 15 minutes of fooling around with my meter - then turned it off and took a 30 minute beer break - the red light was off on the re-start...

I WILL do the 2,000 rpm (no load) test this afternoon and take a measurement across the terminals...

FR Millmore - good to hear you are not that far to the south...enjoying the snowfall this weekend?

Robert - since the Interstate battery is new, and the counter clerk indicated it was fully charged - then I would expect full voltage across the terminals, I will check this baseline number and get back with an update...

Gotta go and rake some leaves, then the wife will let me play in the garage..

Mike P.
On The Shoreline of Lake Erie
Mike Pelone

My 1500 charges at a steady 13.2 @ idle & 13.8 @ 2000rpm.

I would suspect alt probs...but check wiring, especially grounds...I had to clean up all earthing points when I first got my car...lights, wipers, charging, etc. were all a bit erratic.

Good luck
Dave
DL Rhine

O.K. - here is what I got this afternoon...
2000 rpm - no load - about 14.3 volts

At warm idle - 1,000 rpm - 13.5 volts (no load)

And after a look around the BBS archives - these results don't seem too bad. However, the headlights are still weak at night (long term chronic issue..) so I might put headlight relays in next Spring and clean up the connectors and grounds...I have decided to leave the existing Lucas alternator in place for now...

Anyway, a test ride this afternoon [after the new distributor cap] produced steady acceleration with no misses plus a smooth and steady idle - but I did turn it up to about 1,100 rpm...may test again tomorrow after a cold start and short run and see if the idle can be lowered a bit...

Wednesday is a fresh fill-up, toss some Sta-Bil in the gas tank, get a NYS inspection on Wednesday then take a short trip directly to the winter storage building - should be a good way to end the summer...

Thanks for the suggestions, etc.


Mike P.
Buffalo, NY
Mike Pelone

Mike-
14.3 is perfect. Nothing whatever to be gained by changing the alt.
You really want Electrobabble! No reason to have crummy lights! My lighting customers complain about all the deer I put alongside the roads after I fix their lights.

Putting it away now is a bit premature - the next month can be the best of the year, aside from this weekend's aberration, that is! Next week looks to be gorgeous.

Plus, leaving a new battery to sit a long time is cruel and unfortunately usual punishment. Arrange a trickle charger or start it up once a month if possible.

FRM
FR Millmore

that is fine Mike, a good number of alternators have a high "switch on" when charging. That means until you rev the engine the alternator does NOT switch on and thus you end up with the ignition lamp on when you first start the car. The one on my car at the moment is such an animal and I always need to blip the throttle when I start the engine which then turns the lamp off and starts the alternator charging.

As you state you have increased the speed of the idle and that has smoothed it out. If that is the case then by the sound of it your carburettors could do with a good set up. Not a difficult job although many owners seem to struggle with doing the job so if you can not do it find someone at your local club who can!

midget lamps are poor at the best of times with or without relays. Have you had this kind of headlight before or are you comparing them to a modern vehicle?

I put a set of halogen bulbs in a prefocus lens once but that was also a waste of time. :-)

However any improvement is better than none. :-)
Robert (Bob) Midget Turbo

FRM: My car gets put away later next week due to the limited amount of space in the warehouse...If I wait too long - then I risk hearing "...Sorry..There is no room at the Inn..." I purchased a set of car skates last year which allows me to twist and turn the Midget into a dry corner that is out of the way. Car can't be started (or easily accessed..) during the Winter due to air quality issues (there is still an active business upstairs above the cars)...However, I might want to protect my recent investment in the new Interstate battery - any suggestions on brand or features for a trickle charger?

Robert: I removed the dual SUs when I first got the car and replaced with a single 1.75 inch larger SU on a Titan manifold. Easy to set up and no throttle linkages to keep synchronized, etc. I may take the dome off the unit and see what needle is actually in there - but when I pull out the spark plugs - the tips are a "thing of beauty"...must be close to what I need since no black soot or fouling ios observed - plus the car pulls strongly up to the orange zone on the tach...sometimes the Universe smiles on those who drive MG Midgets...

Mike P.
Nickle City USA
mspelone@gmail.com
Mike Pelone

Mike

Any reason why you can't remove the battery and keep it charged up while the car is in storage?
Dave O'Neill 2

Mike,
Harbor Freight sell a "float charger" I believe it is on sale right now for $6) that has always worked quite well for me. Rather than a true "trickle charger" that constantly sends a small amount of current to the battery, these do nothing until the battery voltage falls a bit, then charge it back up. I prefer this approach.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=42292
David "some HF stuff works..." Lieb
David Lieb

Mike-
Prof Lieb has it right - I subsume those floaters under "trickle chargers" - no room for a new subheading at this point!
Dave O'Neill is also correct, but I sort of dislike taking Spridget batteries out if not absolutely necessary, especially if it's a big one. Hurts too much, especially when you realize you put it in wrong way about and have to do it again.
"Midget lamps are poor at the best of times with or without relays" Hogswallop! Fed correctly, they are no worse or better than any other car with single 7" lamps. In the US, that is better than any other system we've ever had, except some recent high end ones. It does have a good bit to do with the lamp quality, but no one else in the world has the garbage the US has had to put up with until recently. Lamps change quite easily.

FRM
FR Millmore

""Hogswallop!"""

Oh dear I seem to have fallen foul of the all knowing Fletcher again!!! Must learn to drop to my knees and pay hommage!

Never mind I am entitled to my opinion and if FRM says a sealed beam lamp is better or as good as modern day American headlamps then who am I to argue? I lie in the UK and here in the UK modern headlamps are far superior to the old sealed beams. IMO
Robert (Bob) Midget Turbo

You should check what volts you have on the headlight bulb; if you're loosing more than 1/2 volt across the bulb compared with the battery, you should clean up all the connections from the battery to the bulb (battery, solenoid post, large spade, brown-brown bullet connections, ... all the way to the bulb; then do the same for the earth (actually, test and clean up the earth side first - should be easier).

You might find headlights appreciably brighter after re-doing these joints.

If you fitting a solenoid - you should still check the headlight earth connection as this part of circuit stays the same.

A
Anthony Cutler

Dave: I did check out that link to Harbor Freight - for $6 you can go wrong...unless the warehouse burns down! I will check into the closet 110v outlet when I roll my car into storage later this week...

Anthony: Good thought on cleaning up those connections - but I will put that on my Spring Cleaning list for 2010 when I retrieve the car. I can't take a chance on any electrical gremlins or broken connections, etc that I might create..

Wasn't too sure about the Hogswallop comment but it appears that there actually is a "Dexter Hogswallop" registered on FaceBook - now I kinda' feel sorry for the guy...On the other hand there are a lot of Lipsh*tz on the planet...

Thanks for the commments!

Mike P.
Buffalo, NY
Home of the Buffalo Jills
Mike Pelone

This thread was discussed between 17/10/2009 and 18/10/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.