Lotus Elan

New Alternator

PostPost by: Robbie693 » Thu Jun 16, 2011 8:41 am

Thanks very much Bill - that's just what I was looking for!

Cheers

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PostPost by: alan71 » Thu Jun 16, 2011 8:44 am

My starter was getting a bit sluggish. I found the voltage drop across the engine earth strap was 0.6V, replaced the strap and it came down to 0.1V.
This was with a 2kW starter so a 0.1V drop = approx 0.0005 ohms. You can?t measure resistance that small on the ohms scale of a meter, the only way is to measure the voltage drop with current flowing.

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PostPost by: bob_rich » Thu Jun 16, 2011 9:15 am

Hi Robbie

suspect as other posts suggest that earth would be the first thing too look at. Electrical wiring in cars, even when in good new conditions does have a measurable volts drop that is a significant proportion of 12V. 4 sq.mm wire (fairly hefty) for a 45A alternator feed would drop 0.2V cold and perhaps 0.28V hot. The original alternator may have used thinner wire because it would be rated for perhaps only 25A say. Volts drops will be significant on all circuits and this is a common problems with 12V systems. Over time the wire does not usually "go off" but the terminal can, voltage drops in switches, and fuses ( Lotus did not use many of them!) can all add up and individual circuit item a bulb or ignition coil for example can quite easily be some 2V less than the battery terminal voltage.

for the earth I would expect battery terminal to engine block to be <1V on a starter motor cranking where current can approach 250A. if this is OK then main earth is OK for the alternator return and voltage drop for individual circuits should not be significant so U may be chasing lead/terminal drops. With a good earth then measuring from any convenient point on the engine block to say each end of the alternator feed wire might give a clue.

For small voltage changes DVMs are the best choice

Hope this helps best of luck

Bob
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PostPost by: billwill » Thu Jun 16, 2011 2:05 pm

alan71 wrote:My starter was getting a bit sluggish. I found the voltage drop across the engine earth strap was 0.6V, replaced the strap and it came down to 0.1V.
This was with a 2kW starter so a 0.1V drop = approx 0.0005 ohms. You can?t measure resistance that small on the ohms scale of a meter, the only way is to measure the voltage drop with current flowing.

Alan.



I am surprised that a drop of 0.6 volts made a significant noticeable sluggish starting.
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PostPost by: alan71 » Thu Jun 16, 2011 4:20 pm

It wasn?t a huge difference, and if it happened gradually I may not have noticed, but I had just changed the engine mount.

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PostPost by: Robbie693 » Thu Jun 23, 2011 10:45 am

Hello,

I managed to take a few readings before going away. This is what I measured:

+ lead to the - solenoid terminal, - lead to the alternator case = 0.1V

+ lead to + side of solenoid, - lead to alternator case = Same as battery with the with the engine off, 13.5V at idle (850rpm), 13.8V fast idle, but down to 12.2V with the head lights on.

+ lead to battery +, - lead to the + battery clamp = 0.1V

Then I did resistance test:

Between the cam cover studs and the alternator case I got 0 ohms (subtracting the lead resistance)

Between the cam cover studs and the chassis I also got 0 ohms.

However, when I did the same resistance tests after the engine was hot the resistance went up! - I got 0.2 ohms on the alternator and over 0.5 ohms on the chassis (it fluctuated a bit).

Does this mean my earth leads on the engine are not up to the job?

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PostPost by: billwill » Thu Jun 23, 2011 6:34 pm

It certainly sounds as if it is worth cleaning up the contacts of the engine earth lead, or fitting a new one.
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PostPost by: bob_rich » Thu Jun 23, 2011 7:30 pm

Hi Robbie

How did you measure the resistance? the ohms function on test meters are notoroiuosly inaccurate on low ohms settings. I dont trust them much below about 2 ohms which is far higher than drops in car wiring should produce. Best way is the pass the known current through and then measure the voltage drop. A current in the region of 2A or so should give a reading on the millivolt range of the DVM and this would be a good guide 2 the resistance

best of luck

Bob
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PostPost by: Robbie693 » Fri Jun 24, 2011 2:20 pm

Thanks both,

Bill - I think I will get new earth leads, I did clean the connections when I fitted new engine mounts, which was a fair while ago but the symptoms remained the same.

Bob - If I understand correctly, do you mean connect the meter between the engine and the alternator case / chassis as I did with the resistance test but with the meter on a millivolt setting? If so, what kind of reading should I expect if all is well?

Regards

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PostPost by: bob_rich » Thu Jun 30, 2011 4:49 pm

Hi

Sorry about the delay in replying. Yes the idea is to measure the voltage drop in mV across any suspect lead. You do need to know what the current is though. A trick i use is to use an old headlamp bulb with say the dip or the main bean filament burnt out. power this from say a battery charger and include in the circuit the lead you wish to check. Then use your DVM to check the voltage drop. Say you have around 4A (check with DCM on current range) from you bulb circuit then measure the voltage drop across the suspect wire. Divide the voltage drop by the current and that is the resistance. with good connections and modest lengths of leads encountered in cars (>1 metre) the the resistance should be virtually that of the conductor. As an example 2 metres of 56/0.3mm ( typical for alternator?) should be 9.4 milliohms so with 4A would give a reading on the DVM of 37.6mV so if you get a reading no more than say 10% more than this things should be OK.

Passing a decent current and measuring the voltage drop is the only way to check low resistances.

Hope this helps

Best of luck


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PostPost by: Robbie693 » Thu Jun 30, 2011 9:54 pm

Thanks again Bob.

I haven't had chance to do any more investigations as the car has been pressed into daily service but I'll do some more testing soon I hope.

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