I have a little box on the inner wing which I believe is something to do with the alternator? It is marked as dac 4729. I would prefer to change it while I have easy access to hopefully eliminate a future problem. Question is they are no longer available so does anyone have any experience of this part? Is there a compatible item? Do they tend to fail? It’s an 89 v12. Many thanks for any input.

 

Load dump module.

Some good previous discussions in the archives.

I recall the most popular course of action is to dump the dump module; bypass it entirely.

Worth spending a few minutes searching the archives.

I'd try EverydayXJ as a source for a replacement if you wanna go that way

Cheers
DD

 

Ooops. EverydayXJ might not be the easiest source since you're in the UK


Cheers
DD

 

Thanks Doug. I will have a search but don’t have much luck on here for some reason.

 

I'll see what I can find. The conversation may have been over at Jag-lovers now that I think of it

Hopefully Grant will chime in. I think he has some experience with these dump modules

Cheers
DD

 

hi doug,

ive got a load dump module issue as well. i tried searching the site and other than this post, found no others. can you recommend another way to search the archive?

ive also seen a lot of people talk about bypassing the module, but no one talks about the consequences of doing this. all mechanics ive spoken to say that by passing it will fry the battery.

any help would be greatly appreciated

 

​​​​​​Hi,
by dumping the module you will not fry ththe battery. But you might fry your electronics if you disconnect battery while alternator is running.
This module is there for overvoltage protection. It was not used in early cars, but I think it was introduced together with higher amperage bosch alternaalternator.
I would guess that basically this module should have a zener diode and a resisror of some sort. Does anyone have or can take pictures of what is inside this module? I could then comment more on this.

 

The load dump module is there as said previously to prevent the alternator voltage rising to in excess of 200V if the battery is disconnected with the engine running. To put it simply - If you do not disconnect the battery with the engine running the load dump module will never be used.

The load dump was a requirement for vehicles to meet various countries design rules.

 

This is not entirely true. This module was introduced with more powerful Bosch 110A alternator. Voltage spikes can occure not only when battery is disconnected, buy any time when high amperage consumer is disconnected from the circuit. It can be battery or heater fans, or high beams etc. Internal alternator voltage regulator will not be able to act fast enough to drop the voltage, so load dump module is there to soften those spikes. You can get away without this module, but durability of ecu relays, wiring etc will suffer.

 

This is not entirely true. This module was introduced with more powerful Bosch 110A alternator. Voltage spikes can occure not only when battery is disconnected, buy any time when high amperage consumer is disconnected from the circuit. It can be battery or heater fans, or high beams etc. Internal alternator voltage regulator will not be able to act fast enough to drop the voltage, so load dump module is there to soften those spikes. You can get away without this module, but durability of ecu relays, wiring etc will suffer.

 

This is not correct, the battery acts as a large capacitor and as long as the battery is connected it will provide smoothing of spikes caused by the switching of lights etc, these draw far less current than a charging battery. But this is not what the load dump module is there for.

The alternator windings have a large inductance value and will act like an ignition coil if the battery is disconnected while the engine is running and the battery is charging, the back EMF (voltage) can be as high as 200V.

 

I don't know how you came to this conclusion, but it is wrong. Althow battery and capacitor might look similar as they both store energy, but battery will never act as capacitor. Energy in battery is stored in chemical form, but energy in a capacitor is stored in an electric field. This is why batteries are really bad at accepting and delivering energy spikes. Capacitors on the contrary have high energy density and can absorb and discharge energy very quickly.
That's why it takes up to 12 hours to charge your flat car battery as it can't absorb energy quickly.
Reg. disconnecting battery while the car is running it all depends if battery is charged or not. If battery is fully charged it doesn't draw any amps from alternator and disconnecting it will probably not cause any trouble. On the other hand if your battery is discharged or you disconnect it just after starting it will be drawing quite a few amps from your alternator and disconnecting it will cause voltage spike.

 

I came to this conclusion because I have repairing electronics for 40 years, which included SLA battery equipped Military electronics. A capacitor and battery have the same basic makeup an electrolyte plates and insulator. The charge and discharge of either a battery or capacitor use the same Physics of electron flow between the plates.

This is a friendly forum for sharing knowledge lets not get into a slanging match

The information I have posted is correct, and I have only responded because people use this forum to gain information and it is important that information is correct.

 

question related to alternators , put a new(not reman),alternator and its charging at 950idle rpm 15.2 volts ,what say gurus?

maybe battery low , but measures 12.4 V not running?
ron

 

Ron, disconnect the battery and measure voltage 12.4V battery is very low, fully charged should be around 13V.

Charge the battery put it in start the car and alternator voltage should drop to about 13.8v

If a battery is very low the alternator will pump max CURRENT the battery will accept so the voltage will rise.

 

thanks, this is not the Jag, its my Chevy suburban (always neglected), i just put the on a 10amp charge, gonna let go all night, Neg cable disconected!

thanks ,,ron

 

When it's fully charged connect the battery and start her up, I would expect the charging voltage to drop to around 13.8V at the alternator, could be a little lower 13.6V or so at the battery.

If you leave the car sitting for a couple of days, disconnect the battery and check the voltage, it should be no less than 12.6V, if it has dropped the battery might need replacing.

Also as the ambient drops the charging voltage will go up, the gassing off voltage at 0°C is around 15.2V but as the temperature rises the voltage rises so at 30°C the gassing off voltage is about 14.2V. The voltages will vary a bit depending on the battery type but these are typical for flooded lead acid batteries.

 

thanks again warrjon, charged all night 13.8V ,10amp tapered down to 3 amp. 14+ when running.
all is good , sometimes i cant see the forest because the trees are in the way!
ron

 

Sorry brinny we got a bit off track there............

Back to your original question. I would not replace the load dump module just for sake if it. In my experience electronics either fail in warranty (component failure) or last for ever (usually something else kills the electronics).

The load dump module does not suppress transients form switching such things as lights AC blowers etc, these transients are handled at the source of the spike with diodes in the relays and the heater blowers have darlington transistors between the blower and power supply, the battery also absorbs some of these spikes like a HUGE capacitor.

Spike suppression MUST be close to the source otherwise it is of limited effectiveness. The load dump module is connected directly to the alternator to suppress very large spikes caused if the battery were to be disconnected while the alternator is pumping large current.