Still trying to figure out barely running 75 1.8 L jet blues

Looked at relay board and found missing rear window defogger fuse which is no big deal but also saw missing power supply relay fuse ???? Unknown what this is for. Then unknown yellow wire.



Click to view attachment

Yellow wire goes to the starter

Thanks, tach bounces when I get can get it to rev upwards of 3,000 rpm.

most of them do, the dielectric in the 40 year old capacitors are not what they once were.

What is the power relay fuse used for ??

not used in your application. Ljet systems have their own relay board called the dual relay. The dual relay turns on the AAV, fuel pump, and the computer.

Thanks again

Timothy,
I know you are the FI guru, but my 18.L L-Jet has the Power Relay.
It is one of the items to check on Bowlby's fuel pump diagnostic checklist.
Mostly, to check for Relay Board operation.
If it is not needed for L-jet, why is it there?
Jason

The 914 was designed with the D-Jet system in mind. When they started using the L-Jet, they adapted the setup from the one on VWs. It had the double relay, so they continued to use it. The L-Jet system evolved into Motronic, and the double relay became the DME relay.

The yellow wire in the picture is to tell the double relay that the starter is engaged. It appears to be hooked up correctly. On the L-jet 914, you do not need any of the relays or fuses on the relay board to get the engine to run.

It's good to question, let me show you the path of electrons for fuel pump operations on the Ljet, (this only applies to Ljet).

Lets start at the dual relay in this picture below:
Unfused power straight from the battery enters the relay at pin 88z. After the points close (won't bore you with the details on how this happens), continuity is made and 12vdc exits the dual relay via pin 88d (red highlighted box). Ultimately, pin 88d which is a big black wire with a red stripe goes to the 12pin connector of the relay board on pin 12.
Click to view attachment
Next: A circuit track inside the relay board connects pin 12 of the 12 pin connector to
pin 13 of the relay board's 14 pin connector. I circled the action in red. Note that there is a junction point at J17 on pin 87. This too is a fuel pump relay, but used for Djets. Even with this relay pulled, pin 13 of the 14 pin connector will still receive power.
Click to view attachment

Here's a close up of the relay board and what's happening:
Click to view attachment
I have the current path highlighted in bold red on how it enters and exits the relay board via the 12 pin connector and out the 14 pin connector. The junction (which I highlighted in yellow) can also inject 12vdc to this current path but it doesn't since Ljet doesn't control the operation of this relay.

With that relay pulled from the relay board, the car should remain running. (because the current path highlighted in red has not been interrupted)

Different year Ljet 914's may have some slight variations on how the fuel pump receives power. Some fuel pumps were mounted near the engine while others were mounted near the fuel tank. The 914's with the engine mounted fuel pump, I believe a short wire from the dual relay traveled straight to the fuel pump.

Huh, very interesting! Sure makes diagnosis more involved when you are testing relays and circuits that don't really matter!
Thanks for the response....you the man!

On my relay board the yellow wire is plugged into slot "II" (2) of the the socket at lower left. Looks like the OP has his plugged into slot "IV". I assume both do the same?
Thanks again,
Jason

Electrically, the two rear ports are the same. Look at the picture and see how they are both tied to each other.
Click to view attachment

I may have answered my own question. Looks like on your flow chart T4bII and T4bIV join up. Junction just above the oval you highlighted.

Yep.....Timothy you respond too fast for me to even figure it out on my own!
Thanks again.

Coondog: I would concentrate on the ignition side. Plug wires, coil, condenser, and points/dwell. Consider installing a electronic ignition module. They hold the dwell perfect, and makes the car idle/run better IMO. Also check the advance is moving the breaker plate smoothly and the advance canister holds vacuum.

Jason, I'm curious now. What's your 914 doing or not doing?

Timothy,
Car sat for a few years. After draining and replacing hose, lines, etc., car would start but then just die. Put in temp switch to manually switch on fuel pump. Installed inline fuel pressure guage, and getting good constant pressure from pump, with car off or running. Car would start and idle, but die out after going above about 2500 rpm. Then i broke a lousy ignition wire, so progressed stopped to order parts.

Decided to replace ALL vacuum hoses, clean all contacts and grounds. Done. While going through the Bowlby fuel pump checklist.....and found a "connection" near pos battery that was not reliable (previous owner--apparently liked electrical tape). Easy fix.
Ordered new cap, rotor, points, condenser, wires. They should come in tomorrow.

I will PM you with results so as not to take this thread hostage.
Jason

Tim made the same suggestion to me for my L-jet 1911cc . HOT SPARK module link

He also suggested that I buy a spare and keep it in the car. If you somehow leave the key on with the unit in a closed position for a long period, the unit can fail. As mentioned, it eliminates all the points/condenser tuning, etc...

$50 well spent.

We need to give TIM an award of some kind

Try unplugging the black/purple wire that goes from the coil to the tach. It probably won't help, but if it does then that is a big hint about what is causing problems. Remember that grounding out that black/purple wire will cut off the spark completely, so an intermittent ground or even just wonky resistance could cause strange problems.

The bouncing tach may be just from the tach itself, or it may be a lousy signal from the points. If the latter, that can cause fuel injection problems because L-jet uses the ignition signal to tell the injectors when to open.

--DD

Not sure if you fixed your electrical issues yet, but even if you did, try this. Remove your relay board and look at the bottom. The boards were sealed with a tar like compound that seems to melt and decay with heat and age, especially below the voltage regulator. If it is degraded, the connectors beneath it will push down when the VR is inserted and result in poor or no contact. Good luck!
Click to view attachment