Double Relay Article

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Amskeptic
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Double Relay Article

Post by Amskeptic » Mon Dec 22, 2008 9:58 pm

Relays allow you to switch heavy duty or far away devices with light duty switches.
In the case of the rear-engined VW, we get to instruct the far away starter to turn over the engine with a very light wire telling the starter relay to hook up the heavy duty battery cable with the equally heavy duty starter motor cable at the solenoid.

The convention with generic German car relay numbering is that the power supply wire is designated #30, the destination wire is #87, the instruction wire is #86, and the ground wire is #85.

Image


In German car wiring diagrams, #30 is used not only on relays, but any time a wire is "hot". If your battery is hooked up and charged, expect full voltage at any #30 terminal you happen across. #15 is found wherever you get battery voltage when the ignition key is turned to the "on" position, that would include the #15 terminal on your ignition coil. #50 is used for any wire that receives voltage when the key is at "start."

So here is our generic, simplified double relay diagram with conventional numbering just for us. All the double relay does is two things:
1) Supply voltage to the fuel injection system when the ignition is on.
2) Supply voltage to the fuel pump under two specific instructions,
a) when starting,
b) when the engine has informed the relay that it is now running under its own power.

Image


Alas, it cannot be so simple. You will also see only little adherence to the German electrical system numbering conventions at the double relay itself. But if you compare the colors used in the above generic diagram to the colors below in the actual current path diagram of your Volkswagen L-Jetronic fuel injection system, you will see the simple functions of the double relay. The ignition circuit is colored in green and it instructs the *power* relay to provide voltage to the injectors, via the voltage dropping "series resistors", and the Electronic Control Unit. The starting circuit is colored in pink. It runs the *fuel pump* relay which supplies voltage to the fuel pump, and the Auxiliary Air Regulator.

Image


Let's run you through the sequence: First, we turn on the ignition. Power relay gets energized and the ECU and series resistors have voltage. The engine is not turning though, so we certainly don't need a fuel pump just yet.
Ignition On - Engine Off:

Image



But, as soon as we turn the ignition key to "start" we need fuel pump and we need some cold starting assistance. (the ignition side is energized as above, but I have darkened the display to keep you focused on the starting side) With the ignition key at #50, voltage is supplied to the starter which is now turning over the engine. Voltage also continues to the double relay where the fuel pump relay is energized and connects supply voltage to the fuel pump and the Auxiliary Air Regulator (it only needs voltage to energize a timed heater). You will note (!) that there is another circuit that is energized OUTSIDE of the double relay function, the 86a > 86 terminals are "bussed" and voltage goes down to the cold start valve quite independently of anything going on inside the double relay. The cold start valve is limited to operating only when the starter #50 circuit is energized, and it shuts off when the Thermo Time Switch cuts the ground path. Like the Auxiliary Air Regulator, the Thermo Time Switch only needs voltage to energize an even faster timed heater than the AAR's.
Starting:

Image



When the engine starts, we release the ignition key. The starter circuit drops out. The CSV and TTS are cut off as well. And inside the double relay, the fuel pump relay is going to want to drop out as well. . . . but, it doesn't. Thanks to the Air Flow Meter, which has determined that the engine is running all by itself, the power relay voltage that has been supplied to terminal #88a to the AFM all of this time, now gets to return to the fuel pump relay through 86b, illustrated with a shocking blue/red striped wire. There is a diode inside the relay that prevents the AFM voltage from running out to try to engage the starter and cold start valve, so all that happens is that the fuel pump and AAR are allowed to continue to run only so long as the AFM is reading air flow. If you stall the engine, well, consult "Ignition On - Engine Off" diagram.
Engine Running:

Image


Actual wire colors and locations are unfortunately not nearly so beautiful as depicted. This might be a good thing. All wiring in the L-Jet fuel injection system is white. Vehicle wiring is color-coded.
Actual Wire Colors:

Image


Now this is disappointing. After that nice elegant distribution of terminals in a logical way in all of the above diagrams, I have to show you the actual layout. It looks willy-nilly, but it is what VW/Bosch had to do to separate the vehicle harness from the fuel injection harness, and still get all the tracks and bus bars laid out inside the double relay. The view of the relay underside is as if you laid it on its firewall "back" side. The plugs are oriented as if you just pulled them off and stared down at them. The left plug is the vehicle harness, you can see multi-colored wires. The right plug is all white wires leading into their harness. People get confused trying to read the terminal designations on the relay and then transposing the correct terminal number onto the correct plug "slot", so here I have labelled both the double relay terminals and the plug terminals.

Image



Note that with the correct wiring colors on the preceding diagram, you can catch that blue/red wire on the left and know instantly that it is the fuel pump supply wire coming from relay terminal #88d. From here on out, you will know that if your car refuses to start readily when cold, you can double check the red/white wire from the starter solenoid #50 terminal to make sure it is sending voltage to the double relay #86a where it buses right back out from #86 to fire up the CSV Cold Start Valve.
Enjoy. This is not terribly complicated stuff, just annoying when wires and terminals get all bunched up and you can't read designations. More to come. . .
*
*
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . . 75,355 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 67,233 miles

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Post by Ritter » Tue Dec 23, 2008 9:39 am

Good God, man! That is a fine tech article!
1978 Westfalia 2.0 FI

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Post by chitwnvw » Tue Dec 23, 2008 1:06 pm

Nice. Thanks.

Simple. Clear. Not condescending.

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Post by Sluggo » Sun Jan 31, 2010 11:26 am

Added to the "Tech Articles" section.
:vwgauge420:

1977 Bus with Sunroof - "Lucky '77"
2000cc Type IV w/Dual Weber 36s,
Aircooled.net SVDA w/Compufire,
Redline Weber Fuel Pump,
Holley Regulator,
Half Ass Brush & Roller Rustoleum Paint Job,
Incomplete Custom Interior,
Dual Batteries,
Crunched Slider Door.
------------------------------------------------------

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Post by chitwnvw » Mon Feb 01, 2010 1:15 pm

Does anyone know how the wires are affixed in the plugs that attach to the double relay? I need to replace the brown ground wire, but without cutting the thing open I can't see how to get the old wire out and put a new one in.

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Post by Gypsie » Mon Feb 01, 2010 3:11 pm

I believe that the connectors (male and female) are slid into the connector housing and "click" into place with either a tab sticking out of the connector or a ridge in the housing. I have had success pushing these connectors out of housings with a piece of wire or a tiny screwdriver.
So it all started when I wanted to get better gas mileage....

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Post by Sluggo » Mon Feb 01, 2010 5:50 pm

I bend the terminal in with a tiny screwdriver until the little tab comes loose. Then push it out.
:vwgauge420:

1977 Bus with Sunroof - "Lucky '77"
2000cc Type IV w/Dual Weber 36s,
Aircooled.net SVDA w/Compufire,
Redline Weber Fuel Pump,
Holley Regulator,
Half Ass Brush & Roller Rustoleum Paint Job,
Incomplete Custom Interior,
Dual Batteries,
Crunched Slider Door.
------------------------------------------------------

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Post by sgkent » Sun Dec 12, 2010 7:59 am

a little late on my part but - excellent article and well done.
Thank You -

Merlin The Wrench

Machine Work/Shop Recommendations: http://kentcomputer.com/77VW/machinework.pdf

Pointers on rebuilding an engine: http://kentcomputer.com/77VW/rebuilding_a_vw_engine.pdf

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Post by Amskeptic » Mon Dec 13, 2010 7:46 pm

sgkent wrote:a little late on my part but - excellent article and well done.
Why thank-you. I have been waiting here for ages hoping to bask in some stray compliment.
Colin
(I have a backlog of additional articles to post)
*
*
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . . 75,355 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 67,233 miles

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Re: Double Relay Article

Post by Jivermo » Sat Feb 16, 2013 9:46 am

4 years and some along, and it's still a great piece of work. This has helped me today! The drawings are worthy of the Musee du Louvre, German, Flemish and Dutch collections.

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Re: Double Relay Article

Post by Amskeptic » Sat Feb 16, 2013 7:07 pm

Jivermo wrote:4 years and some along, and it's still a great piece of work. This has helped me today! The drawings are worthy of the Musee du Louvre, German, Flemish and Dutch collections.
. . . and would you look at that? I am still here looking for stray compliments to complement my complétement.
Colin
( I refer to this thing too, as my memory slips)
*
*
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . . 75,355 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 67,233 miles

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Re: Double Relay Article

Post by bajaman72 » Thu Jan 16, 2014 11:05 am

I hate electrical stuff. But this actually made sense. Now i'm thinking I should put a relay on my recently installed fuel pump as well.
Projects:
1972 Deluxe Transporter (2nd Driver) 2.0L - 091, dual Dells, Bug Pack exhaust. Camper Converted
1973 Baja (another 2nd Driver &Toy) Stock 1600 DP, dual Kadrons, Stinger

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Re: Double Relay Article

Post by Amskeptic » Thu Jan 16, 2014 3:13 pm

bajaman72 wrote:I hate electrical stuff. But this actually made sense. Now i'm thinking I should put a relay on my recently installed fuel pump as well.
We have articles for that as well . . .
Colin

Image
*
*
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . . 75,355 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 67,233 miles

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Re: Double Relay Article

Post by Steve » Fri Sep 11, 2015 1:20 am

Thank you for taking the time for this write up. What, if any, significant internal differences are there between the double relay as explained and the California-only double relay for the air-cooled 1981 Vanagon?

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Re: Double Relay Article

Post by Amskeptic » Sat Sep 12, 2015 7:15 am

Steve wrote:Thank you for taking the time for this write up. What, if any, significant internal differences are there between the double relay as explained and the California-only double relay for the air-cooled 1981 Vanagon?
There are no significant differences. There are detail differences. For example, the external series resistors were deleted, so they do not have an electrical supply from the double relay.

Compare your '81 California double relay to the bay bus double relay and list the differences in terminals and track down any differences in destinations/sources.

Then post them here. TIA,
Colin :cyclopsani:
*
*
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . . 75,355 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 67,233 miles

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