Weird start/non-start problem - help needed

[HPsauce]

Originally Posted by MikkiJayne

Its just an ordinary relay.

Just clarifying this a bit, attached is a photo of the side of the VAG 204 Relay (431 951 253H).
The extra component in the 85-86 circuit isn't the usual symbol for either a resistor or diode, so what it is? I assume it's providing some sort of protection.
I currently have a spare anyway (many thanks, Frank ) but if I was to order a replacement online (rather than trying to extract one from my nearest VAG spares department) what am I looking for in the specification?

[spannerrash]

My belief is that that is the symbol for a resistor. The one with the diagonal line through it is the symbol for a magnetic coil.

When the coil is energised, a magnetic field is created, which, as we all know, closes the switch. When the coil is de-energised the magnetic field collapses. This collapsing magnetic field has the potential to induce a current in any nearby conducting wires if not correctly managed. This is how an ignition coil works.

The usual way of dealing with this is either to block it from damaging anything sensitive with the use of a diode (electrical one way valve) or use a resistor to disipate the current.

I have some other Audi relays and the symbols are the same on a number of them.

Personally, I would only buy genuine ones. They can't be that expensive.

Cheers Frank

[spannerrash]

Sorry, forgot to put the picture in.

[HPsauce]

Originally Posted by spannerrash

My belief is that that is the symbol for a resistor.

Yes, I think so too, I found some Bosch equivalents online with a specification that implied a resistive protection.

Personally, I would only buy genuine ones. They can't be that expensive.

True, it's just that I've found the local VAG parts people not particularly helpful when just supplied with a part number; they insist on a VIN and identifying the required part in their online database, which for a nearly 20-year-old D2 they really struggle with and lose interest.
It took me ages to just get a standard green Coolant Temperature Sensor out of them, especially as the retaining clip and O-ring had to be identified and purchased separately!

[spannerrash]

It's a shame about your local Audi dealer. Mine, in Leeds are supper helpful and have on occasion even come outside to look over the car as they haven't seen one for a while. They always take my part numbers over the phone without any problems.

I have spent thousands with them over the years though so maybe that helps.

Let me know if you are struggling to get one at all, I'll order one from Audi and send it on to you.

Cheers Frank

[MikkiJayne]

Likewise my local TPS guys ^ Happy to get a quote if you post the part number.

[spannerrash]

Another potentially dumb question from Frank.

What is "TPS"? I have never used them and have never really come across them. Any chance of a Web link please?

Thanks Frank

[moltuae]

Probably a varistor rather than a simple resistor. I suspect they've just simplified the symbol.

Varistors are commonly used in electronics as voltage limiters/arresters. In this case it will be to limit the back-EMF generated by the relay coil, to prevent interference and potential damage to the relay drivers.

https://en.wikipedia.org/wiki/Varistor

A diode can be used instead (across the relay coil), but has the disadvantage of slowing the relay movement, which can result in contact arcing/burning.

[HPsauce]

Thanks for the offers of help folks, but I'll find a way I'm sure. I'm only going to get another relay as backup anyway, MJs F125 diagnosis seems the most likely culprit.

I actually use the local VW parts people for common items as they're much nearer; the Audi dealer makes it almost impossible to get to their parts department for any sensible conversation as the desk in the dealership is manned by morons - last time I ended up talking to someone through an open door into a warehouse on a trading estate where they actually do the work!

[MikkiJayne]

Originally Posted by spannerrash

Another potentially dumb question from Frank.

What is "TPS"? I have never used them and have never really come across them. Any chance of a Web link please?

Thanks Frank

VAG's trade counter https://tps.trade/ They were set up to compete with the likes of Euro Car Parts and GSF and to avoid the dealers having to support trade customers who obviously have different needs to the public (for example delivery, bulk ordering of consumables, credit terms etc).

They were supposed to be exclusively trade from the start but most branches took the regulars with them when they split from the dealers. In recent years though an edict came down from on-high that they don't deal with the public and pretty much all of them are toeing the line. A few branches will still deal with individuals who they already know, but everyone else gets sent to the nearest dealership.

Interestingly, the dealership parts counters appear to have more discount options than TPS do. My discount is decided by the computer based on some opaque algorithm. Dealers appear to be able to discount as far as cost price depending on who is asking.