In a message dated 4/14/98 8:15:53 AM Eastern Daylight Time, DUHART@symbol.COM
writes:
> As a side thought,if I measure the resistance of the wire between the IGN
> light and the Battery, and I find a large reisistance could this be the
> source of my problem? Howabout between the IGN light and the Alt?
John,
No, a high resistance would not cause the problem. What you are looking for
is a low resistance to ground on one side of the bulb or the other.
Unfortunately, there is no reasonable way to measure this on the battery side
of the bulb. You could pull the plug from the alternator, and look for low
resistance to ground on the brown/yellow wire. I can't tell you what should
be the minimum resistance though, as it depends on the particular bulb you
have installed, although anything less than 5 ohms would defineatly be too
low. I went out to my shop awhile ago and measured the resistance of 11 spare
bulbs I had on hand, and found the resistance of 9 to be between 5.5 and 8.4
ohms. One measured 28 ohms, and the other measured between 35 and 120 ohms,
depending on where I placed the probes. (This is cold resistance - a 2.2 watt
bulb calculates to have 65 ohms resistance when hot. This is why you get an
inrush of current when you first turn on your lights. Untill they get hot
enough to glow, the resistance is quite low). All 9 lights worked perfectly.
Resistance should be AT LEAST equal to the resistance of the bulb. If it's
less than that, you have found the problem, but it could be more, and still be
the cause of the problem. If you think my answers were long before, you
should see how long it would be to explain that! Basically, though, the wire
from the lamp to the ignition switch doesn't just connect to the ignition
switch, but it also connects to other things as well. The resistance to
ground depends on these other things as well. What these things are depends
on what year your car is, how it is modified if you use a harness from another
year, which I think you said you did, and on the status of these items -on,
off, etc. On earlier cars, the wire can be disconnected from the switch to
eliminate these other items, but the connection is made via a splice in the
harness on later cars, and can't readily be disconnected. Etc.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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