Re: IML: Frustrating Electrical Problem

[Ian Beeby <ibeeby@xxxxxxx>]

John,

I should be very surprised if the starter was routed through the ammeter - this would be very surprising indeed.  Most car ammeters are designed to show +/- 50-70 Amps - which would be the maximum load or charge possible and some.  They are usually moving magnet meters of low accuracy - designed essentially to show any imbalance between charge and discharge.  Starting currents for larger engines can be a few hundred amps, which would fry a car ammeter pretty quickly and also mean that the starter current had to run to the dashboard and back again which presents a fire hazard in the event of a crash.  Normally starters are un-fused but go through the starter solenoid to act as a switch - and the leads are kept as short as possible due to the very high currents involved.

One thing you might look for is a thermal problem in the fuses - I had a problem a few years ago with a fuse that had at some previous stage been overloaded and become exceedingly hot.  The fuse holder and assembly had, unknown and invisibly, partly melted.  As a result the fuse contact had weakened and become slightly resistive.  This was not detectable until certain electrical appliances were operated, then the fuse (cartridge type) would literally pop out of the holder, disconnecting the electrics for the whole circuit that it supported.  It was very hard to find.  The main reason that it was hard to find is that the wretched thing had gained some resistance, such that it got hot and expanded thermally (and then popped out) only when high-ish currents were being drawn - so normal measurements didn't show a significant resistance.

Things have been better since the fuse assembly has been replaced.  On the particular car (not an Imperial I should add), there were only two fuses in any case (wretched British 1960s Lucas electrics - ask any MG, Rover or Austin Healey owner) and so I had the circuits divided between four fuses of lower ratings instead.  This of course had the advantage that there was a) much less chance of such a thermal problem happening at all in the future and b) that even if a fuse does blow, it is not fatal to the operation of the car!

The main problem with vehicle electrics is that quite high currents are involved and it has become typical to route the ground return through the frame of the vehicle rather than run double the amount of copper through the wiring loom (expensive and heavy).  This means that ground return paths are degraded as the ground return points age (rust/oxidisation) and as the frame itself ages (welds deteriorate etc).  That is one reason why car audio systems and alarms are often directly grounded (reduces interference).  The ground return points that handle the highest currents are often in the engine bay where they can often corrode without being noticed (hidden behind cables, pipes and inconvenient things like engines) - so it is well worth looking for poor ground returns.  To do this, disconnect the supply wire to the device and measure the resistance between the frame of the device (or its negative terminal) and the battery negative terminal (for a negative earth car).  It should be VERY low - comparative readings are probably best on each car.  You'll need an ohmmeter capable of reading less than 1 ohm.

I don't know about over-charging alternators - I think that you should generally have an over-rated regulator to match the alternator - then it might charge your battery double-quick (which may shorten the life but get you charged up so much quicker in the winter) but should not boil the poor thing dry.  My Bristol has a high-output alternator and I am sure that the regulator pack came with it.  The old battery died early in 2004 and was at least 8 years old (judging by the part number on it).

The good news is that a constant problem is a failure that should be easier to track down than an intermittent one - which will always crop up when either you least expect it or when you are least capable of having a look for it.

Best of luck in tracking down your problem.

Regards

Ian

On 26 Sep 2004, at 13:44, jvguilbault@xxxxxxxxxxx wrote:

> Hey, guys.
>
> My '76 NYB has a strange problem.  The battery isn't charging and the blower motor for the heating system isn't working.  The amp gauge doesn't seem to be reading anything, either.  I've replaced the alternator, voltage regulator and the battery.  I suspect the amp gauge, but if that went bad, would the car still start?  Mine does, and everything else appears to be working fine.  I've unwrapped the wires under the hood to check for cracks, but everything appears okay.  The car has been garaged its entire life and the wiring under the dashboard looks great.  
>
> Now, there is one other thing to know.  When I bought the car the alternator was overcharging the battery.  I thought I could get it home, but the battery cooked and I had to replace it.  To keep this from happening again, the mechanic at the battery shop suggested I disconnect the negative battery wire and drive home like that.  Well, you all probably know what happened.  After everything under the hood blew up, I had the car towed to a respectable mechanic, who replaced the alternator, voltage regulator, ignition module, and various light bulbs.  It seemed to work fine, but then this problem with the blower motor and charging started happening.  It was intermittant at first, but now it's constant.  The two problems always happen together.  If the blower motor cuts out, the battery stops charging.  I've taken out both the alternator and the voltage regulator and had them checked.  They check out fine.  Any ideas?
>
> John
>
>
>
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Ian Beeby

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