Fuel pump priming

[cis6409]

Hi guys
Wondering if anyone has had this before ..

my fuel pump will not prime with the ignition turned to position 2 .
As a consequence she won't always start first time but she 'll start every time on second go no problem

I have replaced my rpm relay twice but no change .
however I've discovered that if I press on my headlight switch or flash the beam with the stalk it will prime the pump !
none of the other switches on console or dash will do the same thing just the hi low beam stalk or headlight switch ..
What is causing this , I assume it's gotta be an electrical fault of some kind ?? Do they share a common ground with the pump ?
Where would I start to look ?

Cheers
Shane
vin #6409

[Bitsyncmaster]

Hi guys
Wondering if anyone has had this before ..

my fuel pump will not prime with the ignition turned to position 2 .
As a consequence she won't always start first time but she 'll start every time on second go no problem

I have replaced my rpm relay twice but no change .
however I've discovered that if I press on my headlight switch or flash the beam with the stalk it will prime the pump !
none of the other switches on console or dash will do the same thing just the hi low beam stalk or headlight switch ..
What is causing this , I assume it's gotta be an electrical fault of some kind ?? Do they share a common ground with the pump ?
Where would I start to look ?

Cheers
Shane
vin #6409

That is pretty much normal except most stock RPM relays prime with the key. What is going on is an electrical noise is triggering the ignition ECU or causing noise on the 12 volts to the RPM relay. 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.

[5875]

Hi guys
Wondering if anyone has had this before ..

my fuel pump will not prime with the ignition turned to position 2 .
As a consequence she won't always start first time but she 'll start every time on second go no problem

I have replaced my rpm relay twice but no change .
however I've discovered that if I press on my headlight switch or flash the beam with the stalk it will prime the pump !
none of the other switches on console or dash will do the same thing just the hi low beam stalk or headlight switch ..
What is causing this , I assume it's gotta be an electrical fault of some kind ?? Do they share a common ground with the pump ?
Where would I start to look ?

Cheers
Shane
vin #6409

I have the exact same issue with mine. It will always start on the second try after the pump is primed. I find when it's still warm, it will work on the first try, but after sitting more than a day I have to do two tries. I'm going to try your trick and report back. Mine also has a new rpm relay.

[Bitsyncmaster]

If you don't have the capacitor (near the air intake), you should also get a prime just stepping on the gas pedal. My car did that until I added that capacitor. Some cars have that cap and others don't. That cap helps reduce the inductive spike of the advance solenoid which is powered on or off by the idle switch.

[titofiedler]

Hello Shane. I am having the same situation here. When flash beam light it will prime the pump. Wondering if you have found a solution. I have already changed the old rpm relay to the new version from Dave...

Cheers
Tito
Vin #2944


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[David T]

With the original RPM relay it can take a bit of cranking to get it started. Usually the first start of the day and usually if you haven't used the car for a couple of days. One thing you can try is turning the key on and off a couple of times and not cranking the motor. It seems to vary, not all RPM relays act the same and some cars hold pressure a lot better than others. Make sure your cold start system is operating correctly too. If it doesn't work right it makes a cold car a lot harder to start. Acts kind of like a choke on carbureated cars, it adds a lot of fuel to get the motor started and it operates at much lower pressures than the injectors so it gets fuel into the motor a lot sooner than the injectors can. The cold start system consists mainly of the TTS (Thermo Time Switch) and the CSV (Cold Start Valve) and the wiring. Problems are mostly the TTS and the wiring. Also make sure there is nothing plugged into the Hot Start plug near the fuse block. The capacitors were added to the cars to catch voltage spikes that could cause interference with the radio.

[titofiedler]

With the original RPM relay it can take a bit of cranking to get it started. Usually the first start of the day and usually if you haven't used the car for a couple of days. One thing you can try is turning the key on and off a couple of times and not cranking the motor. It seems to vary, not all RPM relays act the same and some cars hold pressure a lot better than others. Make sure your cold start system is operating correctly too. If it doesn't work right it makes a cold car a lot harder to start. Acts kind of like a choke on carbureated cars, it adds a lot of fuel to get the motor started and it operates at much lower pressures than the injectors so it gets fuel into the motor a lot sooner than the injectors can. The cold start system consists mainly of the TTS (Thermo Time Switch) and the CSV (Cold Start Valve) and the wiring. Problems are mostly the TTS and the wiring. Also make sure there is nothing plugged into the Hot Start plug near the fuse block. The capacitors were added to the cars to catch voltage spikes that could cause interference with the radio.


Thanks David.

Actually the problem is just the fact that fuel pump is priming when I flash beam headlight. When I release the switch lever of the beam light, it will prime the pump

Thanks
Tito




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[Bitsyncmaster]

Hello Shane. I am having the same situation here. When flash beam light it will prime the pump. Wondering if you have found a solution. I have already changed the old rpm relay to the new version from Dave...

Cheers
Tito
Vin #2944


Enviado do meu iPhone usando Tapatalk

Your flashing the lights must be firing the ignition ECU. I would make sure your distributor pick up coil connections (the connector) is clean and has good connections. Otherwise I would suspect a bad ground. Make sure your battery is good also.

[cis6409]

Hi tito
No, have not found a solution to it, did buy one of dave s self priming solid state relays so I don't worry about it anymore so much lol! A few other things have taken over unfortunately, head gasket issues and the like, so the primimg of the pump thru the stalk or headlight switch has been put on the long finger..

Thinking about it I've always felt it was an electrical issue so I think Dave is right about a spike being picked up by ignition module or something to that effect.

My car for what it's worth is missing both capicators at the coil and the throttle plates so that might be somewhere to start..

Dave saying it might be a bad ground is another option to I think .
All the headlight switches on dash and binnacle stalks etc are earthed underneath the binnacle in that mysterious earth junction that is near impossible to see. Also the fuel pump is earthed through the inertia switch which shares into that earth junction underneath the binnacle so maybe that's what's causing the spike??

I remember having similar issues with my old Renaults and fiat's down the years, you would press the brake and the indicator would light up instead of the brake light, anytime it was fixed I was told it was the earth that needed fixing..

Correct me if I'm wrong with that info btw!

Cheers
Shane

[cis6409]

When I get the car back together I will check to see what the voltage drop on the negative side is like or for excessive resistance for each earth point on the car using the battery negative as reference.. Maybe that will show up something :)

[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/listings/delorean-rpm-relay-core-for-solid-state-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.