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Wed Apr 15, 2020 6:50 pm

So I have been struggling to solve the case of why can't I get my eff bus started...

The problem started when I swapped out my distributor with the one from my donor 78. I posted a separate thread on think my distributor indexing might have been off but I'm pretty sure I ruled that out given that is started and ran this past weekend. But let me layout the sequence of events...

1 ) swapped out distributor with my spare
2 ) connected dwell meter to coil
3 ) turned over engine (key in start position) but engine failed to start
4 ) realized "dwell meter" was set to measure resistance (old Sear's multi-function dwell meter)
5 ) disconnected meter from coil
6 ) turned over engine again but engine failed to start
7 ) posted this topic https://www.thesamba.com/vw/forum/viewtopic.php?t=...highlight=
8 ) confirmed distributor gear indexed correctly
9 ) turned over engine again but engine still failed to start
10) walked away from it all (this was back in December)

This past Easter weekend, I had a couple more thoughts on what might be the problem. Had a gnawing concern that the "ohm meter" connected to the coil may have damaged my double relay or worse still my ECU,

So I proceeded with the following

1 ) grabbed the double relay and ECU out of my 78 donor
2 ) swapped out the double relay (labelled the donor double relay with '78')
3 ) turned over engine AND TA-EFFIN-DA she started
4 ) let engine run for 5 minutes... firing on all four... no sputters... no misses

Mission accomplished right... so my engineering background tells me to confirm the problem has been solved by re-creating the problem by

1 ) swap in the original double relay - check
2 ) verify van won't start - check
3 ) swapping in the 78 double relay -check
4 ) verify van does start - negative

Should have quit while I was ahead... so now I am back to van won't start. Did a couple more things

1 ) changed fuel filter
2 ) installed fuel pressure gauge
3 ) confirmed fuel pump runs when opening AFM air vane (key in run position)
4 ) fuel rail is at 35 psi
5 ) I haven't swapped out fuel pump largely because it ran when I did the AFM air vane test above and have 35 psi of pressure in the rail

I am planning to bench test both double relays but doubt that they both are on the fritz... so I am reaching out the to wisdom of this crowd to help me out with "now what"

PS... are the 77/78 ECU swappable and are the wire harnesses compatible?

Wed Apr 15, 2020 7:03 pm

Fuel and Air
Spark
Compression

Figure which is missing. Use a fuel pressure gauge to see if you have fuel pressure. Use a noid light to see if the injectors are getting a pulse

Pull a plug wire off, put an old plug in it and ground it. See if you have spark.

Sounds like you have compression since it ran.

Wed Apr 15, 2020 7:32 pm

You can easily check the function of the double relay with your multi-meter. You can also do a pin test on your ECU. I am going to guess that a pin has pushed back out of the plug on either the double relay or the ECU.

Thu Apr 16, 2020 3:40 am

A sniff of ether is a good way to narrow down pure fuel delivery problems from other scenarios. It is a very good, 99% , indicator of fuel vs ignition problems

Thu Apr 16, 2020 10:08 pm

Make sure the various terminals in the double relay plug make it all the way into the relay. Don't rely on any solidarity in those plugs for all of the wires to make a perfect electrical connection. You may have gotten lucky once. Take the relay off the firewall, carefully nudge the terminals on to the relay tabs with a really small screwdriver, then screw the relay back on the firewall. Also the resistor block right next door might have a loose plug fit or failing solder joint.

Thu Apr 16, 2020 11:19 pm

Thanks guys... I will check out the contacts on the wire harness and even the resistor terminal....

Fri Apr 17, 2020 4:32 am

cellobus1 wrote: Make sure the various terminals in the double relay plug make it all the way into the relay. Don't rely on any solidarity in those plugs for all of the wires to make a perfect electrical connection. You may have gotten lucky once. Take the relay off the firewall, carefully nudge the terminals on to the relay tabs with a really small screwdriver, then screw the relay back on the firewall. Also the resistor block right next door might have a loose plug fit or failing solder joint.

This exactly!!

Had the same issue with mine. Chased for hours with a meter. Sometimes voltage pass through relay, and then sometimes not. The white plastic connector on the relay had worn to the point that the lock tabs on the blade connectors were not holding the connectors tight. Removed the white block and just pushed each wire connector onto its respective blade of the relay, and, voila! No trouble since.

Just remember you have uninsulated, exposed blade connectors up there. As long as you dont elbow around near the relay, nothing should be near there to make contact or short out.

Fri Apr 17, 2020 5:21 am

Take it out the circuit and manually activate it this is all you need do if you are stuck away from home without a replacement double relay. And don't forget to insulate it once you are finished.

Fri Apr 17, 2020 5:48 am

Nitramrebrab72 wrote:

Take it out the circuit and manually activate it this is all you need do if you are stuck away from home without a replacement double relay. And don't forget to insulate it once you are finished.

As long as you get +12volts to 88 d&b. She will start. The rest are just cold start aux and safety cut out.

Remember if your stuck without a spare double,

'" d&b from 88 =+12" :lol: