Strange electrical short due to alternator overvoltage, HELP!

[kwren]

Originally Posted by Trevor

Keith,

Excessive current is required to blow a fuse, not an excessive voltage. The fuse link or any fuse can pass megavolts without opening. The only limitation in respect of performance being the surrounding insulation.

The fuse link has battery voltage positive potential at each end of the element, with the battery between the alternator and ground. Please explain how the alternator can induce current across the fuse element such that it will blow.

Hope this helps.

Trevor.

I will have to get back to you on that! Today is a busy day.
Take care,
Keith

[Trevor]

Originally Posted by Svxswede

Say that you have a single lightbulb connected to the battery. Over the battery poles is the alternator. A 100 W bulb will draw 100/12 =8 amps, and the resistance of the bulb is 12/8 =1,5 ohms.

By applying Ohms law, increasing the voltage also increases the amperage.

There is of course a limit to the powersource ability to put out amperage, a car alternator can put out 100 amps up to quite high voltage levels.

Lets say the alt produced 20 volts instead, the battery will act as a buffer for a while but at a level overvoltage reaches the system and if the resistance of a circuit is 10 ohms and normally consuming 1,2 amps it will now draw almost the double.

Any relay will have to handle twice amps and also the fusible link which handels many powerconsumers will be overloaded.

Still havent found the fault at fuse 15.....

Seems a lot of circuits are involved there.

Alex

Alex,

What you advise is exactly correct in respect of the circuits which are connected after the fuse link.

The fusible link is between the alternator and the battery so that in order to blow the fusible link the alternator would be required to deliver what amounts to a very high charging current, in excess of the anticipated maximum for the link. My immediate thought was not to consider this as the most likely scenario.

I have now checked the link for thickness and it would appear to be rated at much less than I had anticipated. However with three 45 amp. circuits fed from the link, one could expect it to be rated at say 150 amps. But then again not necessarily so, as it constitutes a slow rupturing fuse.

Whatever actually happened we may never know, but trying to figure out the reason is an interesting exercise.

I sincerely hope that you have found the final short circuit involved.

[Svxswede]

I disconnected the ac unit and all related such as fans, evap, compressor etc. No short there. So I decided to go to the fuse box. It seems that the fault is somewhere starting from plug E (12 pin connector at the fuse box).

That bundle is connecting a lot plus intruments at pin A15 (us version, we will see what it goes to in my car..)

I will try to trace it, first I will remove the intruments in order to acess the plugs. Hopefully there will be a melted contact or something easy...

Alex

[Trevor]

Alex, you certainly have got a difficult fix on your hands and I admire your tenacity, especially as everything is so hard to get at.

After further considering the causes, I realise that we have been thinking on the basis of the rated alternator output. If the voltage regulator failure took the field winding suddenly directly to ground, this could have induced a transient spike in both in voltage and current, far in excess of any rated figure. One can only assume that the fuses involved were not fast enough in action, to prevent resulting short circuits from causing damage before they blew and cleared the circuits.

[Svxswede]

I removed and dismateled the instrument cluster as I previosly told. I found that one conductor at the back, at the plastic pcb, had melted a bit. Tracing it, I discovered a small pcb inside the housing. According to schematics its a dimmer circuit. I probed it and discovered a faulty powerdiode. Replaced it and woohaa, it now works.

Today I will spend reassembeling the lot of connectors and units. Hopefully it will be hazzle-free. I still suspect the ABS unit being fried. We will see later today.

Alex

[Trevor]

Great fault finding Alex.

I hope you will find that something at the beginning of the ABS circuitry failed and protected the main section, and that it is fixable. Best of luck, as you deserve it.

I wonder if you have replaced the alternator as a unit and may have the faulty unit available to analyze. It could be worthwhile to find out exactly what happened, and whether there is anything which could be done in order to prevent such a destructive fault from occurring.

Cheers, Trevor.

[Svxswede]

Damn! All went together smoothly. Hadnt the time to test the ABS, but the transmission is not functioning properly. I pulled the codes and it said duty solenoid a, b, and c.

I suspect the TCU.....

Trevor, give me a hint how to test it out.

I think it would be easiest to check solenoid resistance (acceptable range?) from the harness, and if the solenoids check out fine, tear out the tcu and probe it. Hopefully there is a protection ciruit.

I still have the alternator in the car, unhooked. My new one will arrive any day. After i swapped it I will check what the hell happened.

I guess its not an everyday fault, but hey this faultfindig was tough. So if anything in this thread can help someone with a similar problem Id be happy.

Alex

[Trevor]

What a bugger! Alex, you are having a hard time.

I too would be very surprised if the solenoids have been damaged. If anything surely it would be a short to ground.

Solenoid A. Resistance specified as between 1.5 and 4.5 ohms. I have measured a sample at 3 ohms.

Solenoids B and C. Resistance specified as between 9 and 15 ohms.

It is apparent that we are exactly on the same wave length and that you appreciate how valuable your experiences could be in respect of others. Special thanks for the offer to do some extra research. Certainly not urgent and you will need rest after you finally get your car back on the road.

I remain at your service, Trevor.