Fuel pump priming

[FABombjoy]

The long-winded explanation of what's happening

The tach signal wire (white/slate) sprawls out around the relay compartments, ecu compartment, and up to the dashboard. If a small voltage spike gets on the tach wire, the RPM relay will run briefly. When wires run parallel to each other, it's easy for a wire carrying high voltage to induce current into the surrounding wires.

Spikes like this are typically caused by a coil of wire. Relays would be the prime suspect if they have defective or missing "flyback" diodes, which suppress the voltage spike. If wire is actually physically in the shape of a coil, that could sometimes be enough.

The ignition ECU turns voltage on and off to the ignition coil. Voltage on = charge primary coil, voltage off = field collapse into the secondary windings, creating high-voltage spark, but also a small reverse-current high-voltage spike back into the coil primary wires (one to 12v, one to the RPM signal white/slate wire). The RPM relay detects this spike created by the ignition coil and briefly turns the fuel pump on as these spark events are detected. The dashboard tachometer detects this spike and registers the engine RPM. The idle system also uses this to control idle speed.

When you turn on the headlights, the corresponding relays are energized. When you turn them off, the magnetic field that was holding the relay open collapses which creates a spike. A functioning flyback diode prevents the spike from leaving the coils, but if it's missing that spike will travel back down the wire and possibly affect anything nearby in the harness.

A EFI car can use a relay that has the armature and flyback diode removed. 5v goes in to the coil wire, and when switched off, the subsequent voltage spike drives the tachometer. This lets you directly drive the tachometer without tying it into the actual ignition system.

There are lots and lots of coils in our cars that can create this kind of noise: Ignition coil, all relays, AC compressor coil, ignition vacuum solenoid. Probably lots more that I'm forgetting. The noise suppression caps really do quite a bit more than just clean up AM radio reception.

[Chris 16409]

The stock RPM relay has no design built into it to prime with power on. It just happens to detect an ignition pulse. My solid state RPM relay will always prime with power on. That process is built into the software on power up.

You need to get Dave's solid state relay, it is far superior. If you don't want to have to send your existing relay in for conversion, I can provide you a compatible core.

https://www.deloreanmarketplace.com/...te-conversion/

[titofiedler]

The long-winded explanation of what's happening

The tach signal wire (white/slate) sprawls out around the relay compartments, ecu compartment, and up to the dashboard. If a small voltage spike gets on the tach wire, the RPM relay will run briefly. When wires run parallel to each other, it's easy for a wire carrying high voltage to induce current into the surrounding wires.

Spikes like this are typically caused by a coil of wire. Relays would be the prime suspect if they have defective or missing "flyback" diodes, which suppress the voltage spike. If wire is actually physically in the shape of a coil, that could sometimes be enough.

The ignition ECU turns voltage on and off to the ignition coil. Voltage on = charge primary coil, voltage off = field collapse into the secondary windings, creating high-voltage spark, but also a small reverse-current high-voltage spike back into the coil primary wires (one to 12v, one to the RPM signal white/slate wire). The RPM relay detects this spike created by the ignition coil and briefly turns the fuel pump on as these spark events are detected. The dashboard tachometer detects this spike and registers the engine RPM. The idle system also uses this to control idle speed.

When you turn on the headlights, the corresponding relays are energized. When you turn them off, the magnetic field that was holding the relay open collapses which creates a spike. A functioning flyback diode prevents the spike from leaving the coils, but if it's missing that spike will travel back down the wire and possibly affect anything nearby in the harness.

A EFI car can use a relay that has the armature and flyback diode removed. 5v goes in to the coil wire, and when switched off, the subsequent voltage spike drives the tachometer. This lets you directly drive the tachometer without tying it into the actual ignition system.

There are lots and lots of coils in our cars that can create this kind of noise: Ignition coil, all relays, AC compressor coil, ignition vacuum solenoid. Probably lots more that I'm forgetting. The noise suppression caps really do quite a bit more than just clean up AM radio reception.

You are the best. Thank you!


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[titofiedler]

You are the best. Thank you!


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Already have one. Actually, I got th core from you a few weeks ago!

T.


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[FABombjoy]

You are the best. Thank you!

Sure! I figure it might be easier to understand how to solve the issue if the root cause is made more clear.

I just re-read my post. The ignition ECU switches the ground on/off to the ignition coil, not 12v. The coil gets 12v via the ignition resistor.

Also I'm missing a few commas for clarity, so here's some extras: ,,,,,,,,,,,,,,,

[cis6409]

Fascinating stuff, great explanation
Thank you!

Shane

[Elvis]

Might be interesting for Europeans only - I can update the original RPM Relay to prime everytime
you turn the key.

Of course I can also repair them like all ECUs.