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A Study of Bus FI Double Relays (based on painful experierience)
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telford dorr
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PostPosted: Thu Apr 22, 2021 10:31 pm    Post subject: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Well, it turned out to be "Everything you 'know' is wrong" day.

Working with Aeromech on an engine swap on a '79 bus. The original engine was getting 'iffy', so the owner sourced a complete replacement '77 engine pulled from a bus that was getting a Porsche 6-cylinger transplant (no comment...) Supposedly it was running good at the time it was pulled. It arrives strapped to a pallet. We (wisely, it turns out) decided to mount it up on a run stand, fire it up, and check it out. It's way easier to debug an engine on the run stand than it is in the bus! And easier to repair, if needed.

Apparently, the 'complete' engine was missing a few bits, like the double relay, chassis FI harness, and fuel pump. It did include the FI controller. No problem, I think, as the '79 was FI and had the harness, double relay and a fuel pump (stuff the suppler likely wouldn't be using for his project). So we hook it all up, added oil (it was shipped dry), then jumped the fuel pump to 12 volts to prime the system. Fuel pressure looks good. Press the start button. It fires, then immediately dies. Do an inspection for missing hoses and big vacuum leaks, etc. All looks good. Press start again. Same results. WTF? OK, time to sit down and figure this thing out.

I noted that the double relay did not click when the run stand power switch was activated, nor did the fuel pump run when the AFM flap was manually opened. Had to be a relay problem.

The 13 pin '79 double relay has all the same pin numbers as the earlier 11 pin double relay, except that it has two extra pins. These pins (88e and 88f) are internally connected to existing pins 88a and 88b. These extra pins supply power to some of the injectors, which keeps the relay connections simpler, as the '79 doesn't have the injector resistor block to group the connections. No big deal, I figure, because the '77 doesn't use these pins. Not having time to study it further, a replacement early double relay was sourced and installed. Engine started and ran fine.

This bugged me, as the old engine started and ran using the 'non-working' relay. Now that the new engine ran, this warranted further investigation. Inspected both relays:

Image may have been reduced in size. Click image to view fullscreen.

Image may have been reduced in size. Click image to view fullscreen.


You can see that the relays are almost identical, with the '79 relay having the two extra pins (88e and 88f, in otherwise unused spots). All other pins are identical - almost.

Note pin 85 (the relay ground connection). On the early relay, it's on the chassis harness connector. On the '79 relay, it's on the engine harness connector. Seriously??? Why in hell did they move that? Inspection of the harness showed no connection to that 85 pin on the '77 engine harness! That would explain why the relay didn't work. The question is, would adding a ground connection to that connector on the '77 harness allow the later relay to be used? Had to try it. Crimped up a wire to a 0.110 sized terminal, stuck it the connector, grounded the other end, and installed the '79 relay. The engine fired right up!

So: the later '79 relay works fine in earlier FI systems if you add the missing ground wire (kinda like having to add the missing wire to the new replacement headlight dimmer relays). Thus, the '79 relay is a universal relay, and can be made to work in all FI engine years up to '79

Follow-up: as to why it originally fired up and died - even with the double relay not working, the cold start jet did work (using fuel pressure from the original pump priming). It's little shot of fuel was enough to get the engine to fire, but obviously not enough to run.
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'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)


Last edited by telford dorr on Fri Apr 23, 2021 9:01 am; edited 1 time in total
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PostPosted: Fri Apr 23, 2021 3:47 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Nice documentation as usual , Telford

Are these relays diode snubbed like modern ones?
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telford dorr
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PostPosted: Fri Apr 23, 2021 8:59 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Not as far as I know (the FI controller doesn't directly drive either relay). They do have isolation diode(s) to keep various inputs from backfeeding each other.
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'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)
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Wildthings
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PostPosted: Fri Apr 23, 2021 9:53 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

telford dorr wrote:
Not as far as I know (the FI controller doesn't directly drive either relay). They do have isolation diode(s) to keep various inputs from backfeeding each other.


Do the aircooled Vanagons use the same double relay?

I have thought it might be nice to get rid of the double relay and go to separate FI and FP relays off of a later VW, something that could be bought at most any auto parts if needed out on the road.
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telford dorr
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PostPosted: Fri Apr 23, 2021 10:22 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

No idea on the vanagon relay - haven't had hand-to-hand combat with one.

Should be possible to use standard cube relays in place of a double relay, as there's nothing trick about the actual relays inside the double relay case. You would have to add your own diodes, etc. The biggest issue would be connecting the setup to the stock harness connectors. I would NOT be in favor of chopping up a stock harness. Maybe you could gut a dead double relay and use its case for the connectors, or something. Or you could lay out a printed circuit board - I know you can buy PC-mountable male blade terminals. If you actually do this, post pics of the results.
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'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)
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PostPosted: Sat Apr 24, 2021 3:25 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Your 24 Ohm snake always brings me back to happy memories of Horowitzand Hill. Iíve built most of the circuits with the dark light bulbs

Laughing
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telford dorr
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PostPosted: Sat Apr 24, 2021 8:39 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Always loved that cartoon..

Haven't read Horowitzand Hill (except a little on-line just now), but I intend to get a copy - looks excellent! Thanks for the reference!

I, too, have built 'dark bulb' circuits, but fortunately few have ever left the bench...
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'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)
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Randy in Maine
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PostPosted: Mon Apr 26, 2021 6:41 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

I was under the assumption that the 13 pin DR was a 1979CA set up. Is that correct?
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telford dorr
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PostPosted: Mon Apr 26, 2021 11:11 am    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Correct.
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'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)
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PostPosted: Mon Apr 26, 2021 12:17 pm    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

telford dorr wrote:
Always loved that cartoon..

Haven't read Horowitzand Hill (except a little on-line just now), but I intend to get a copy - looks excellent! Thanks for the reference!

I, too, have built 'dark bulb' circuits, but fortunately few have ever left the bench...


I think I have a spare copy. Pm me your snail mail and Iíll send when I find it, Iím downsizing.
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telford dorr
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PostPosted: Mon Apr 26, 2021 12:25 pm    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Done! Thanks!
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'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)
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PostPosted: Mon Apr 26, 2021 1:30 pm    Post subject: Re: A Study of Bus FI Double Relays (based on painful experierience) Reply with quote

Yes, early AC vans use that 13-pin relay.

Fun fact: a 13-pin car runs on two cylinders on an 11-pin relay!
Robbie
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