Can a starter short damage the alternator? (Diode test content)

[anthonym]

was happy car working until headlights flickered like they used to when the alternator was slowly dying, wonder if the short on the starter has damaged it.

Given these links are/can be the rare unfused non-ignition switched variety.

Is it possible, probable?

Anthony

[rj]

Anthony,

If you short the output to ground the current drawn is extreme and this can fry the rectifier diodes. Add that the wires inside the alternatror are designed for a specific current and therefore can overheat. This often happens in inverter-APUs that are overloades as they won't be able to keep the RPM at the right level causing extra current drawn and less cooling.

If light starts flickering then you could have blown one of the diodes. Those often short circuit when they fail and over-current is the result...

:-(

[anthonym]

hi rj,

I am confused about where the short was to. The "short" activated the starter motor. Here's where it was, touch that cable to that terminal and the starter runs (whether ignition "off" or "on") and will start the engine (only) if the ignition is switched on. 

so, it's a short in so far as it connects something that should not be, but there's no sparks just an activated starter motor. I am not sure but suspect that cable is carrying 12 volts direct from the battery and that terminal activates the starter whereas the other one does but only when the circuit is completed somehow (igntion).

The solenoid was activating the starter (ignition OFF) with the car in gear for some minutes (5 maybe) before I figured out what the dull clicking was. Then out of gear spinning the engine until I disconnected the battery. Then I reconnected, ignition on, started of course but starter still constantly engaging and drove home, about 60 seconds to my place. And off, disconnect battery. Later in peace, spotted the cable looked too close to that terminal, pulled it way and problem disappeared. 

Was that "shorting"? I have thought not, but it was a big current draw for far too long. The battery was warm, but apparently fine. 

I am wondering if too much current has been sent the wrong way to the alternator, like the 7 ballast resistor is designed to protect from, and so as you say, damaged something - i.e. diode(s). That said, the alternator was not running... 

Seems to me this is all about that same problem warned about when stopping the engine by pulling the red cut-out key.

Or hopefully not..

the flickering so far: one flicker at engine on, lights on, hoping it was just the alternator spinning up. I now have one of the cig lighter volt displays as mentioned often on here; it's a great comfort seeing 14.x volts and interesting to watch the drop and rise during engine start. 

Anthony

p.s. just realised, so if it generates over current I will see it on the volts display - what sort of numbers might that be? I guess anything 15 plus?

[rj]

How high is the current when the starter motor is running?

(The alternator is not charging during a normal start)

The short circuit can blow the diode due to over-current; open circuit over-voltage where the diodes start to act as zener-diodes.The result is the same but the reason is not.

It will not generate over current. The current is a consequense of something else like over voltage or a short circuit.

You should not read more than 14.2-14.5V. Again it would be easily identifyable with a scope to see if a diode is blown. Not so easy with just a voltmeter.

[anthonym]

it's 14.x where x is 1,2,3 or 4

I suppose oscilloscopes are smaller than they used to be.

https://www.picotech.com/oscilloscope/2000/picoscope-2000-overview . Like a bench-top oscilloscope, only smaller and better

loads on ebay, for £20.. really?

https://www.ebay.co.uk/bhp/handheld-oscilloscope

WELL NOW, it seems my multi-meter a Caltek 3920 has a diode test feature http://www.caltek.com.hk/cm3920.htm

A diode blocks current from flowing in one direction, while allowing it to pass through when the polarity reverses. You can use any multimeter to test whether it is functional, but a digital multimeter with a diode check function will give best results. Most modern diodes are made from silicon, but this reliable design can still break down when exposed to too much power.

https://www.wikihow.com/Test-a-Silicon-Diode-with-a-Multimeter

diode test with multimeter caltek

ok so the light begins to dawn 

 .  time index 1:30 diode test is at 2:35 . (desired plus reading is 500-800 millivolts and open loop O/L the other way)

with the engine off, the alternator not activated, the alternator WITHOUT the one-way diode will draw (drain/leach/) current from the battery (and flatten it). Strikes me that a suitable test without any meters is whether the battery voltage reduces when the car is left in the garage/carpark/etc . If upon return it's voltage has dropped then the diode(S?) is suspect. What I do not know at this time is by how much voltage drop over what time, i'e what rate of drain indicates a broken diode.

Obviously, SOME drain is to be expected from always on components such as the immobiliser or whatever.

[rj]

You are probably not able to test the diode in circuit but you can check the output waveform (even across the battery) from the alternator. There will be ripple. If one of three "bumps" is missing a diode (or the coil) is blown.

If you want to use the multimeter to test the diodes you need to remove them from the alternator. It's not a simple do it in the alps job

[anthonym]

the guy in the youtube vid did it without removing a diode.. shirley?

[rj]

It's beyond my skills to do it like he did and get a valid result

[revilla]

I agree with rj. Too much going on in that circuit to be able to test the diodes in circuit reliably. An oscilloscope check will differentiate between "all OK" and "something wrong" but even then you would probably have to strip it all down to find out what was wrong.

Don't be fooled into thinking that the £20 scopes on eBay are anything like the proper bench scopes. The main issue is bandwidth. In other words, their ability to accurately follow rapidly changing signals. For diagnosing an alternator diode they would probably be fine, but for general use you need a scope that has an upper frequency limit very many times the actual frequency of the signal you are observing if you want to be able to see the actual shape of the waveform.

The response of linear electronic circuits is usually analysed in terms of sine waves, as these are the fundamental components that pass through the circuits undistorted. Any repetitive waveform may be broken down into a (possibly infinite) number of sine waves of different frequencies, all multiples of the overall frequency of the waveform. Just as the original waveform is these components added together, the output on the scope is the response of the scope the each of these components added together. So if you were looking at a waveform that was fundamentally 1khz it would have components at say 3khz, 5khz, 7khz, 9hkz, 11khz, 13khz... If your scope could only handle 10khz all of the components above this would be missing and the result would be a distorted waveform display, even though the scope can apparently handle 10 times the frequency you are looking at (in reality it's not usually a sharp cutoff, the response just degrades as the frequency increases). Related to this is the fact that the components are also phase-shifted (effectively time delayed) by different amounts which also means that the waveform displayed by adding them up no longer follows the shape of the input.

In general, waveforms with sharp spikes and square edges will have a lot of energy in high frequency components and it is these which will be distorted most by a cheap scope. A simple square wave (something switching on and off repeatedly) is actually really hard for a scope to follow accurately as the edges are infinitely steep, meaning the frequencies extend to infinity and there is a lot of energy in the higher frequencies. If you look at even a 1khz square wave on a cheap scope you will see lots of ripples and ringing and the sides will not be vertical but sloping.

I bought a Hantek DSO2090 USB oscilloscope for use with a PC which was a reasonable compromise between an expensive bench scope and the cheaper units. It runs at 100 million samples per second and has a claimed bandwidth of 40MHz and seems to work well for what I use it for.

[rj]

Thanks Andrew.

To put that down in writing would have taken me hours if not days.

I totally agree. You can only tell if you need to strip down the alternator or not with a scope. Then, when it has been stripped, you can start to measure with your multimeter which of the diodes that has blown.

[anthonym]

So, if I run a diode test using  the B+ (battery/starter end (disconnected) as the alty is inaccessible) and earth and it apparently works (as in 5-800 ma one way zero the other), what should I conclude? If it's unreliable which bit? (whether 500ma will get that far seems more like a problem). 

ok so turn this around, come up with a home test(s) that does not involve removing the alternator from the car?

I can think of one, or maybe two (you tell me)

the battery keeps inexplicably going flat - indicates broke diode 

more than 14.7 volts appears when charging (though that's not a diode issue is it?) I saw 14.7 today.

tests don't have to be 100% reliable (what is that anyway? It works or doesn't.), just indicative of further enquiry or not.

 

(my google adventure above ends up with a USB Oscillioscope, as it happens.)

 

[rj]

Anthony,

There are not many tests you can do "on the kitchen table." If you have a clamp amp-meter you can try to measure if there's drainage. Or you can fit your multimeter in series with the thick red wire that goes to the battery BUT you may blow the fuse (if one is inside) or the multimeter due to the overload that could occour.

With the instruments I have I can diagnose that something is wrong but not what and I would never try to repair an alternator myself. I would either take it to a specialist or simply purchase a new. (At least here the latter is often the cheaper).

Mind you that I have instruments collected over 3 decades and if I were to buy them new there'd be in excess of £5k just for what I'd be using for this little task.

My advice would be: Take the car to a specialist.

 

[anthonym]

yeah, I just did that! ;-)

how long does it take for an alternator to reduce a battery's voltage to a measurable degree? For example would over night be enough?

[rj]

Bought for 5k instruments?

[anthonym]

don't understand. 

perplexed of Europe

[anthonym]

Anyway, you mentioned the kitchen table so that set me thinking. I have here a brand new HC Cargo  alternator for my Seven and the faulty old one removed from my Duratec (Mondeo) aweek plus ago. 

Hopefully these pics are self explanatory