Club 80-90 Forums

can anyone let me have one of these please, a damaged one is fine, most of them have the tabs broken, we are just trying to figure exactly what it does and how

many thanks for looking

I'm told it's an Idle stabiliser for the distributor, it is in line with the wiring from ignition amp to disributor, as you can see it's the same 3 wire colours as the distributor

Here you are Facebook had the answer

Advances ignition under a set idle speed DJ manual says 850RPM ( hence why disconnect it for distributor setting)

Thanks Paul
still want one to take apart to see how it does it and hopefully to understand why the various versions are engine specific

I understand where you are coming from.....as the unit simply introduces a reversible "negative delay" to the ignition timing why would there be a need for more than one type of unit when the same 3 wires are being used ?

Either...... internal component development, same function, different bits
or varying limits of the range of timing adjustments
or varying timing response profile taking account the behaviour of other mechanical components like camshaft profile
or something else

I hope you get one and have success in your investigations

Picture of the internals -
https://www.thesamba.com/vw/forum/viewt ... p?t=534974" onclick="window.open(this.href);return false;

937carrera wrote:I understand where you are coming from.....as the unit simply introduces a reversible "negative delay" to the ignition timing why would there be a need for more than one type of unit when the same 3 wires are being used ?

Either...... internal component development, same function, different bits
or varying limits of the range of timing adjustments
or varying timing response profile taking account the behaviour of other mechanical components like camshaft profile
or something else

I hope you get one and have success in your investigations

the distributor wirings not the same, some is reversed and presumably the delay differs depending on model, country of delivery etc, different timing for emmisions and stuff

937carrera wrote:I understand where you are coming from.....as the unit simply introduces a reversible "negative delay" to the ignition timing why would there be a need for more than one type of unit when the same 3 wires are being used ?

Negative delay? It advances the timing to keep the idle speed stable when it drops below the set threshold, the different types have different idle speeds and maximum advance.

I thought that might trigger a response, which is why I put it in speech marks.

I already understood that what the DIS functionally does is advance the timing to increase the tickover (retarding as well to lower it back down to the target rpm)

The question I was thinking about was how does it do it ?

This was my logic, it may contribute to the understanding or it may not, I am genuinely interested in the how though.

When the DIS unit is bypassed ignition happens based on the triggering of the hall sensor alone. For the timing to be advanced the spark needs to happen slightly earlier than would happen based solely on the position of the distributor shaft, so the DIS modifies the triggering of the ignition amplifier. A delay (positive delay) would delay the spark and and retard the timing. As the timing needs to be advanced rather than retarded, I called it a negative delay, there is probably a more specific technical term, but I didn't want to simply re-use retarded.

Just a guess based on application of logic, it might not be right but I thought I would chuck it in to the discussion while Aidan figures the answer out.

This isn't just the DJ engines though, DF and CU both have DIS units so it is of wider interest

I poo poo'd the term negative delay as a form of lingual algebra used to explain the explainable.
How say they trigger a repetitive event early by measuring the time/distance between the two previous events and subtract an amount determined by the error signal which is demanding the advance so as to create the next event early.

Or, it has an inline/inbuilt delay which it can reduce to `advance' the timing?

tobydog wrote:Or, it has an inline/inbuilt delay which it can reduce to `advance' the timing?

I did consider that - if that was the case, then my thinking was that you wouldn't connect the two plugs together when setting the timing initially. The range of the DIS is 10 degrees, so upon engaging the DIS unit base timing would change by the thick end of that amount.

In circuit when setting timing you end up chasing your tail?

Or it works by magic

FWIW, I cannot see how it is possible to "advance" spark timing events with passive components, only mechanical centrifugal or vacuum advance devices can do this.
A triggered event cannot occur before the trigger time!
The only way to do it electronically would be to use mapped timing, but that didnt come along until much later for production cars.
I suspect that it is a simple digital delay device, which somehow looks at the Hall sensor trigger event intervals and has a delay time only slightly less than this interval, thereby "advancing" the timing a few ms for the next spark...

Sent from my S60 using Tapatalk

silverbullet wrote:FWIW, I cannot see how it is possible to "advance" spark timing events with passive components, only mechanical centrifugal or vacuum advance devices can do this.
A triggered event cannot occur before the trigger time!
The only way to do it electronically would be to use mapped timing, but that didnt come along until much later for production cars.
I suspect that it is a simple digital delay device, which somehow looks at the Hall sensor trigger event intervals and has a delay time only slightly less than this interval, thereby "advancing" the timing a few ms for the next spark...

Sent from my S60 using Tapatalk

Unless you set the timing advanced with the unit unplugged, plug the unit in, which then retards the timing to “the right” timing then it always has the ability to retard the timing by dropping the advance that it adds , been a while but I think you set idle and timing with it bypassed