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Mon Jul 04, 2011 9:22 am

Almost all the Vanagons I've seen have used a typical 4 pin relay (like the fuel pump relay) to power the control module and other engine components. Recently I encountered an '89 Westie (with a no start complaint) that required a hybrid 5 pin relay with an unusual pin 87a.

Can someone explain what the purpose of relay 321 919 505A serves when used as a main relay in an '86-91 Digifant system ? The Bentley does not show this relay or state it's purpose.

Thanx, John

Mon Jul 04, 2011 9:48 am

If you are talking about the " Digifant control unit relay " at the left hand of the black plastic box in the engine bay, then it make no different if the terminal 87a is there or not. Terminal 87a makes contact with terminal 30 when the relay is not energized. This relay is on page 97.124 - 36 in the Bentley.

Mon Jul 04, 2011 7:04 pm

At location 97.124.36 a relay pin #15 is shown. Also there seems to be no way that pin 87a or 15 can contact term 30, key on or off. If the relay control circuit did contact term 30, the relay would stay energized with the key off and the parasitic current draw would kill the battery.

John

Mon Jul 04, 2011 7:32 pm

I have this relay on my 88. There is a diode inside the relay. Think of it as a surge protector for your ECU.

Mon Jul 04, 2011 7:39 pm

It is a special relay, not just an SPDT, it has the fifth pin and at least an extra diode internally, it is shown in Bentley, and I have no idea why it's there. The extra pin as I recall communicates to a pin on the ECU.

Mon Jul 04, 2011 7:57 pm

JP,

the relay you are asking about is on Bentley 97.124 which illustrates the diode and its connection to the ECU. Compare this to the relay on Bentley 97.106

Here is the best explanation that I was able to quickly and shamelessly clip from the web......

Protection diodes for relays

Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.
Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

Mon Jul 04, 2011 8:16 pm

I don't have my book nearby, but I looked at this a lot before and it's not a common coil surge protection diode, it's on a separate circuit inside the relay. I think this relay may have the regular protection diode across the coil as well.

Mon Jul 04, 2011 8:23 pm

Great explanation but as Det. Columbo (rest his soul) might say..."just one more thing".... I drive a '91 DoKa and the main & fuel pump relays are identical 4 pin relays coded #53. My DoKa came with a 022D Digifant control module so it's not clear why an '89 US Westy needs spike protection and a CAD DoKa does not. I think there's more to this story.......

John

Mon Jul 04, 2011 8:27 pm

It is strange that VW added a terminal to the relay. It should not be marked as 87a but should be marked as 15. The relay now get its power when you turn the key on through the diode that is connected from terminal 15 to 86. But terminal 86 is connected to the ecu pin 23. As we don't know the inside of the ecu, I can only speculate the ecu can now hold the ecu control relay closed after we turn the key off. If owner of 88-on van can connect a voltmeter to the terminal 86 of the ecu control relay and see if there is still power after the key is turned off and for how long if it indeed has power.

BTW, the protection diode for a relay is connected across the coil to prevent the back emf generated by the relay coil from damaging the driving transistor.

Mon Jul 04, 2011 8:37 pm

The harnesses are from the early years that are wired for the 5 pin relay.
Once the D series ECUs and ICMs were made the harnesses were all 4 pin and had plain relays, both 53s.

Mon Jul 04, 2011 8:43 pm

Just went out to take a look at my 90 parts van. Yes, there are only 4 connections to the socket of the control relay. Don't know why Bentley show the 5-pin relay from 88-on van.

Mon Jul 04, 2011 8:49 pm

The Bentley shows the early Vanagons were wired for the 4 pin and the later had the 5 pin set up.

John