w124 Alternator Charge light bypass?

[Joloke]

Is there any way to bypass the alternator charge light just for diagnosis purposes?

I found this on Benzworld but it seems a little too simplistic and I wouldn't want to blow anything

Alternator Test Cord for W123s - Mercedes-Benz Forum

My battery isn't charging and all the usual suspects have been replaced Battery/Alternator/OVP and I have blamed the OVP but I am starting to wonder now?

I recently contacted Kent at Mercedessource I really wasn't expecting a reply but kindly he advised.............

Don't take it for granted its the OVP just because it worked when you knocked it

He suggested I get a spare instrument cluster and see it it made any difference?

Sadly cant find one that doesn't look more knackered than mine!

So I am thinking If there's some way I can rig up an external battery light and that works I know its a problem in that area

If it doesn't it may still be the OVP

I want to do this properly use the right bulb and diode just as if its reading the one inside the dash cluster

I am appreciate any pointers?

The one on Benzworld I don't know? I mean will that even switch on and off going direct to the battery?

Thanks

 

[Ted]

Not sure what you're trying to achieve here.
The charge bulb is used for initial excitation of the rotor as it allows a small amount of current to flow. So it is necessary that it is in the circuit. (It can be overcome by revving the engine up high - once done it will charge normally until the next start)
If the light is staying on, then there is a high probability of the alternator not charging.
If it is not coming in at ignition turn on then it can easily be tested by grounding the small (thin) wire at the back of the alternator - use a test light to do this so there is no risk of a short. The test light should glow dimly (depending on the wattage) and so should the warning light - Ignition must be on.
If you want to 'substitute' the charge warning light (i.e. the one in the dash isn't coming on) connect the test light between the battery positive and the small terminal on the alternator - it should then perform as the dash warning light.
By far the most common failure on these is the regulator failing - this is held on the back of the alternator and is a cheap and easy fix. You also get a new pair of brushes into the deal. Bargain!

 

[Joloke]

Not sure what you're trying to achieve here.
The charge bulb is used for initial excitation of the rotor as it allows a small amount of current to flow. So it is necessary that it is in the circuit. (It can be overcome by revving the engine up high - once done it will charge normally until the next start)
If the light is staying on, then there is a high probability of the alternator not charging.
If it is not coming in at ignition turn on then it can easily be tested by grounding the small (thin) wire at the back of the alternator - use a test light to do this so there is no risk of a short. The test light should glow dimly (depending on the wattage) and so should the warning light - Ignition must be on.
If you want to 'substitute' the charge warning light (i.e. the one in the dash isn't coming on) connect the test light between the battery positive and the small terminal on the alternator - it should then perform as the dash warning light.
By far the most common failure on these is the regulator failing - this is held on the back of the alternator and is a cheap and easy fix. You also get a new pair of brushes into the deal. Bargain!

I am a little confused to how that idea works? If the bulb is between battery positive and exciter wire on alternator would the battery now power the light whether the ignition was on or not?

And upon starting the engine would this set up/bypass extinguish the light connected to the positive?

Non of my dash warning lights are coming on

New alternator and Regulator and battery it was thought it was the OVP but that made no difference really

I think that gave me a false lead as if i took it out and replaced it they started working but since checked all that it checks out fine but still no warning lights

I like you say am trying to eliminate the circuit,Ie if i bypass it with my own charge bulb and it works the fault is in the wiring somewhere in that circuit

 

[Ted]

Easily done then

To test the warning light bulb, connect a testlight (max 2 watts) between the small terminal (may be a blue wire) and battery. The light should glow the same as the normal charge warning light.
Start the car and the light should go out.

You should be able to do this without removing the wire, but it is best to do it.
While you are getting your hands dirty, just check that you have a good feed to the alternator main terminal with the testlight.

 

[Joloke]

Thank You Ted

Only problem is my alternator has a molded 3 pin plug

When connected I assume the light would com on immediately as its not switched?

But after starting should go out pretty much like the dashboard light?

I am guessing it would then charge the battery as it should?

Only thing that worries me is the circuit on the rear of the cluster has a diode do I not need one for this test lead?

Theory is as I have been chasing the OVP if this works its clearly in the charge/exciter wires circuit

I am loathe to pay an auto sparks unless I really have to?

 

[Ted]

Only problem is my alternator has a molded 3 pin plug

Not so easy - you may have to break into the wire

When connected I assume the light would com on immediately as its not switched?
yes
But after starting should go out pretty much like the dashboard light?
yes
I am guessing it would then charge the battery as it should?
Yes - assuming that there is nothing between the main charge cable and the battery stopping it.
Only thing that worries me is the circuit on the rear of the cluster has a diode do I not need one for this test lead?
If this is on the charge light circuit only, then no. My guess is that it may be there to stop the charge light glowing if the battery is very flat and the alternator is putting out full charge (I could be wrong on this assumption, and probably am)

The exciter circuit is simply the warning light. This provides a small amount of current to get the charge started - it then works a little like a 'bootstrap' and generates more field (rotor) current and this becomes self generating, if that makes any sense.

It should be possible to overcome this by using the very tiny amount of residual magnetism in the rotor by giving the engine some good revs after starting - once this has kicked in, it should charge normally.

I've taken a look at the test cable on the other site. That is a worker (if you can get a plug) and will bypass all other circuitry and prove the alternator is ok (or not).
Make sure you use thick cable for the charge wire as it will get hot if it is too thin.

 

[brucemillar]

From memory (I am old and can smell of wee) the regulator/brush pack is not too expensive and may well be worth replacing anyway. It may save you time and money. I had a similar fault on my 124 and it was the regulator. 5 minute job to replace from above. Two screws and out it popped. Bit of a fiddle to get back in as the new brushes are nice and long.

 

[Joloke]

From memory (I am old and can smell of wee) the regulator/brush pack is not too expensive and may well be worth replacing anyway. It may save you time and money. I had a similar fault on my 124 and it was the regulator. 5 minute job to replace from above. Two screws and out it popped. Bit of a fiddle to get back in as the new brushes are nice and long.

Thanks Bruce but the regulator is brand new

Been out today to try and get an old school screwdriver style bulb tester but gave up as they are so flimsy compared to the old ones

They look as if they may catch fire if they went anywhere near a couple of volts

Ill work something out

 

[brucemillar]

Thanks Bruce but the regulator is brand new

Been out today to try and get an old school screwdriver style bulb tester but gave up as they are so flimsy compared to the old ones

They look as if they may catch fire if they went anywhere near a couple of volts

Ill work something out

I bought a cheapy of the bay, strangely enough after our very own Ted, helped me out on an electrical fault on my Pajero. It's one that has a very sharp spike on the end so handy for pricking into wire shielding.

Good luck with this. I hate electrical problems on cars.

 

[grober]

Have you checked continuity on the earth side of things.Especially the engine earth strap from engine block to chassis [its normally attached around the bell housing] frayed cable can be disguised by breaks inside the insulation sheath or corroded connections at eithet end.
ps did you replace the entire alternator ?-----the regulator pack has the control electronics and brushes but not the diodes.

 

[Ted]

This, from Halfords is a decent quality tester.

http://www.halfords.com/workshop-tools/tools/hand-tools/halfords-professional-circuit-tester

Throw the 24v bulb away and put in a 12v one (unless you fancy working on trucks) and you will be able to sort out 80% of electrical problems on a car.
Add in a decent multimeter and understanding of Ohms law and you can diagnose 90% of problems.

 

[brucemillar]

And as Ted taught me. To check dodgy engine earth, or if unsure. Use a jump lead from the block to earth. If the problem goes away? you have your culprit.