Bosch starters use a electromagnetic solenoid for activating the starter mechanism. Like any electromagnet when they get hot the electromagnetic force is substantially reduced. Often to the point the solenoid can't pull hard enough to activate the starter. ALL Bosch starters suffer from this problem, particularly when old and always when installed on an automatic transmission (right under the exhaust manifold, a brilliant piece of engineering)
Symptom: You turn the key and nothing happens. Nothing. Not even a click. The voltmeter does however show you sucking major current.
You can wait for the starter to cool off (about an hour usually) or you can get more current to the solenoid so it can finally develop enough force to run the starter. Although the wiring is fine it's not heavy enough to pass the current required by the solenoid when hot.
Solution #1 get a long screwdriver and short from the main starter terminal to the solenoid terminal. Works great. Makes lots of sparks. Destroys screwdrivers. Sometimes cars.
Solution #2 Get a new solenoid/starter. Doesn't always solve the problem.
Solution # 3 Wire in a relay that's triggered by the key switch and has current on a heavy wire direct from the battery.
Bosch makes a relay kit just for this. Seems it's been a problem since VW Beetles roamed the earth. It's called the WR1. In many peoples oprion it's not heavy enough for the job. Solenoids can desire up to 30 amps of current to work. Which makes a small relay a part ready to melt, short out and possibly engage the starter and not let it release. Not good. We need a better relay.
Here's what I did. Went to AutoZone and got what is known around the world as a Ford starter relay. Design dates back to the 30's I think when they used to actually pass the current to the starter through one of these. So it's kinda heavy duty. The part number is Duralast F496. Cost about $10.
You'll need to make a short wire from the key switch wire to the relay post. (S) Another longer wire from the battery terminal to the big post (either one) and another to go from the other big relay post to the solenoid terminal. All of the connectors are available at Walmart/Autozone etc. I always install such relays close to the starter so the current path is short. But realistically you could mount it on the firewall or fender and be ok.
Here's the relay
Here's how the wires attach to it.
I installed it to a tab on the tranny housing since the bracket is the ground of the circuit for this relay.
Here's a closeup of how it's nestled in there.
Here's a wiring diagram I found in another forum but it's nicely made. Note the diagram shows a 15A fuse but that may be too small. Start with 15 and work your way up to a 30A. My hookup uses no fuse at all but I don't really recommend that for others.
Write up courtesy of Moljinar, of VwVortex Check out his website: Project 250
Ron, of VwVortex had this to add:
The heat soak relay fix is not unique to automatics. It is far more commonly needed on automatics.
Here's a link to Bentley forums, the three jpgs linked in the 2nd post are the factory bulletin on the topic.
This factory version of the fix uses a Bosch or other Euro relay and several other VW specific terminals and a terminal block.
The "problem" and cure are actually as old as Bosch starters. I have stories from the 6 Volt days.
Anyway, Moljinar's (Oran's) write up uses a Ford relay.
The Ford unit is a far more robust and dependable relay, it's perfectly suited to this application. The Ford part is definitely the best to use. Can't expect VW to recommend a brand "F" part, can we?