Double Relay Explained. . . In Color!

[sun-bug74]

I ran all of the tests in the electrical checking and testing section of the AFC manual with my son. All tests that ran on pins that I actually have checked out, except one. The pin ten to ground checking for voltage did not work with key on. It is zero volts with key off but also with key on. My understanding is that power to the ECU comes through the firewall on the driver's side. That would be part of the vehicle bundle. Can you help me narrow down just which wire in the vehicle bundle powers the ECU?

Since this whole thing started with me jostling the harnesses that connect to the ECU, I would have jostled the vehicle bundle. If there is an intermittent (and perhaps now completely failed) break in the wire to the ECU and that comes through the vehicle bundle, that could be the issue. As noted above, it is the only diagnostic test in the manual that failed as well.

Does that logic may any sense?
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[sun-bug74]

Edit - whole thing started by jostling the harnesses to the DR
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[Bnanwel]

Mayor Ratwell has provided everything you need for this question in this diagram:

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‘72 Karmann Ghia Conv.
‘73 Karmann Ghia Coupe
‘74 Karmann Ghia Coupe
‘70 Deluxe
‘72 Deluxe

[sun-bug74]

Indeed. Will have a good look at what is feeding 88b after work Monday. Thanks.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[sun-bug74]

Indeed. Will have a good look at what is feeding 88b after work Monday. Thanks.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[75 Westy FI noob]

[sun-bug74]

Thanks. I have never replaced the TS2 and this problem came and went without touching the TS2 before. I will keep that in mind though. I want to see if there is continuity between 88b and pin 10. If there is, then I need to understand why pin 10 does not show 12 volts with key on. If not, I need to find the break in the wire between the two points.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[SGKent]

bad double relay contacts probably

Pin 88Z gets 12V from battery
Pretty sure that when relay is off there is no voltage at 88b
When relay in on there is 12V at 88b
88b provides 12V directly to the fuel injectors, and pin 10 of ECU so if there is no voltage at 88b there will be no voltage at pin 10 or the injectors.
It also provides 12V to pin 39 of AFM thru pin 88a

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"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin

[sun-bug74]

Success!

First, I want to thank everyone who provided information and know-how. As I noted, today, after work, I checked continuity from the wire that clips to the 88b terminal on the DR to pin 10 on the ECU connector. Also as noted, I had already run all of the tests from the service manual and only one had failed - 12V to pin 10 with key on. When I ran the continuity test, I found that had a good path with zero resistance.

Next, I took 12V from the coil with key on and jumped that to the wire that clips to the 88b terminal at the DR end. I then checked voltage at pin 10 and now DID have 12V to pin 10 with key on. So, I established I had a good path and that voltage was flowing properly along it.

My brand-new DR came in today, along with my used but great condition resistor pack. I wanted to rule out that either of those components were causing a problem and I will just put the "old" ones aside as spares if ever needed.

Next, I looked at which wire to the DR brought power to the DR with key on. That is the wire to terminal 86c on the vehicle harness side of the DR. I unclipped that wire and tested it for 12V with key on - nothing.

So, I just used a jumper wire from coil positive to terminal 86c on the DR and it fired up and stayed running very nicely. The failure in the original wire from coil positive to 86c within the vehicle side harness was preventing power to that side of the DR and, hence, no power could flow to terminal 88b on the other side of the DR to power the ECU and the resistor pack. The bus would start on the cold start circuit but could not stay running without power to the ECU and the resistor pack/injectors, of course.

As I mentioned on the outset, I have had intermittent problems that were always solved by jostling the harnesses to the DR. The tenuous connection from 86c back to the coil finally went to where jostling would not restore the situation. I am amazed I went about a year in that state without a stranding.

I would say to someone newer to the FI scene, LIKE ME, that the advice given above by SGKent to follow the troubleshooting in the service manual (after he brought me to the service manual) is key. Test every single pathway as shown on page 39 of the manual. Page 40 lays out the wiring very nicely. The screenshot of the Ratwell diagram (above in this topic) sent by Bnanwel added nicely to the info on the pages in the manual. Wildthings and Airschooled were also very helpful and I learned a ton about the system from this experience (plus ended up with a reliable solution and found that my resistor pack was hanging on only by a thread in the process). In my case, since everything passed except 12V to pin 10 with key on, it really gave me something solid to trace back amongst all of the possibilities. Not bad for a former '74 SB guy.

Keep on bussin'
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[75 Westy FI noob]

[sun-bug74]

Thanks for the info. Glad to be driving it again since this is prime season up here.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[SGKent]

[sun-bug74]

I will definitely order some o-rings, thanks. The wire in question runs in the vehicle bundle from the main lug up front and comes through the firewall at the bottom left to provide 12v to the DR with key on. Fortunately, coil positive is also key on, so the short jumper works. I do plan on tracing and replacing but I am very happy just to be back on the road for now.

Thanks again and I agree that the more of us who stick with FI, the farther in the future we will be able to help others.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[SGKent]

[sun-bug74]

I know that it is not the DR. The “old” DR was working just fine until that power supply wire from the vehicle bundle was disturbed. Then, before I tried the fix, I put a brand new DR in place. The bus fired right up with the jumper from coil positive in place and will not stay started if I remove the jumper.

I am using a terminal adapter on the coil to supply the jumper. Is your concern that the coil will be stressed or is it that my jumper just has to be of an appropriate gauge? I can certainly put whatever gauge wire in place that is suitable. All I am doing is feeding the power input terminal on the DR with 12v from a different place.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[Wildthings]

[sun-bug74]

Thank you. I am powering 86c. So, I think it will be fine.
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1978 Bay Window Bus, AC Vanagon Motor, Daily Driver

[75 Westy FI noob]

[Wildthings]

the power to 86c comes from the ignition switch via the #15 terminal on the ignition coil. The original wire from the coil to the relay had the inline fuse for the backup lights attached to it so was an odd duck.

The double relay is J-40 in the upper right of this diagram, it includes everything between the colored wires coming in from the top and the white wires from the bottom. It is two relays in the same shell, in the diagram the Fuel Injection relay is to the left and the Fuel Pump relay is too the right.


double click slowly to enlarge

[SGKent]