How to make your own FI double relay

[cowalc]

The double relay that controls the fuel injection system on buses is a fairly reliable component, however when it does go the Bosch replacement is fairly expensive given what it’s made of. This was the case for me when I accidentally shorted it out while working in the engine compartment. I decided to try building one myself with a very basic electronics background, the results of which were quite successful and rewarding. There have been several good articles explaining the function and operation of the double relay, most notably Ratwell's article “Double Relay Demystified” , http://www.ratwell.com/technical/DoubleRelay.html. I highly recommend reading and printing out the Ratwell article for a handy reference; it was my guide for figuring out the various connections.

The standard Bosch component is shown below without the cover, as well as the pin connections with the various components on the FI harness they connect to courtesy Ratwell.

The wiring diagram also provided by Ratwell is a very useful tool in visualizing how everything works together;

The basic components that make up this part are two generic auto relays, two diodes, and a 3ohm resistor. On top of these you'll need plenty of male and female spade connectors, wire and tape to connect your components to the receptacle where the Bosch fit. I found it best to assemble the pieces and connections, create a map of how it all fits together and then connect the pieces one by one. The hardest part is connecting everything correctly, which makes it critical to know exactly what's what throughout the process. Below is a similar wiring diagram to the Ratwell figure. To be clear, the numbering below is derived from general relay numbering, and though similar to the Bosch component is not the same.

I connected the resistor and diodes in line with the wires from the relay to the wiring harness for simplicity. This allows you prepare the setup before getting into actually plugging everything together, a list of connections to assemble is:

Note for some connections the terminals are ¼ inch wide, while others are about half that. I simply trimmed my connections to fit, though you could prep the connections with the exact size beforehand as well. Now the important part, connecting everything properly, here's the chart I used to go from the Bosch terminals to the relays and back.

This is based on the pin diagram supplied by Ratwell with corresponding connection labels. The “Number of Connections out” column serves as a tally of what’s been connected, note that the sum of the connections out is 11, which confirms all the pins on the receptacle side are satisfied.

Once everything is setup, it can be mounted on the firewall again, (a cover is a useful addition to prevent any accidental shorts too).

I hope that was clear, I encourage anyone in need of a new double relay to consider making their own, it's not that hard and it gives you a very good understanding of how your bus manages it's starting and FI circuit. Also, if you ever need to make modifications or repairs in the future, the components are easily swapped out as needed.

[greenbus pilot]

EXCELLENT writeup! I always knew these Double Relays were nothing more than 2 relays united together- its great to see your buildup and success in making one yourself. I have many ISO relays lying around, and they are indeed a universal electrical component, useful in so many applications.
Thanks for sharing this with us!
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Sent from a white van down the street.

[grimace007]

THIS NEEDS TO BE STICKIED
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Brian
68 sedan

Dallas Air Coolers

[Macwesty]

Does anyone know the wattage of the 3 ohm resistor?

I'd like to make sure I don't undersize the resistor and fry the circuit.

Scott
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'82 Vanagon Westfalia
Subaru SVX power!

[Tcash]

Click on pic and zoom in on the resitor.

Select the color of each band on the resistor. Looks like.
http://www.digikey.com/en/resources/conversion-cal...ode-4-band

Then click on search


Good luck
Tcash

[Tcash]

[telford dorr]

From the pic, looks to be 1/2 watt. You could always up-size it to 1 watt.

Personally, I see no need for the diode in series with the ignition coil lead to pin 86b - seems to serve no purpose (the other diode does: it keeps the relay holding current out of the cold start valve and starter solenoid).

Also, I'd add fuses to the battery connections to the fuel pump and power relays. VW didn't include these (but then again, they didn't fuse a lot of stuff...) A 15 amp blade fuse on the pump and a 10 amp fuse on the power relay should work. Should something accidentally short, you won't have to rebuild the wiring harness...
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[scrivyscriv]

[BayCreamPuff]

Here's an inline blade fuse mounted above the relay. Currently just have the power for the pump fused.

_________________
Cream Puff
'79 CA Stock FI Deluxe Campmobile

Earl Grey
'87 GoWesty 2.2 Westfalia

Marsha Mellow
'00 Eurovan VR6 Full Camper

[telford dorr]

[telford dorr]

[Macwesty]

[BayCreamPuff]

[telford dorr]

[BayCreamPuff]

[BayCreamPuff]

Oh duh. I see it also powers 86b as well as the relay coil.
_________________
Cream Puff
'79 CA Stock FI Deluxe Campmobile

Earl Grey
'87 GoWesty 2.2 Westfalia

Marsha Mellow
'00 Eurovan VR6 Full Camper

[telford dorr]

Actually, it powers 86, which is the cold start injector and thermo-time switch heater. 86b actually powers the fuel pump relay coil (through a resistor) from the air flow sensor, to keep the fuel pump going after the starter is disengaged.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Tcash]

If you want to follow along.

Thanks to Amskeptic
Double Relay Explained. . . In Color!

Tcash

[telford dorr]

Late news: opened a double relay. The resistor is 2.7 ohms, 1/2 watt. The diodes are 1N4005.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Tcash]