Posted this on my fuel pump thread, but I learned me not to gravedig from that one.
Long story short, my new fuel pump's breaker-to-relay power supply wire is interfering with my ignition. I've found how to make it stop, but I'm not sure if I have a problem with my ignition or a simple fact of life with my original wiring configuration & distributor pickup type.
I have an MSD 6AL box and an air gap reluctor (internal magnetic pickup) distributor. I'm using all-new wiring for the pump - a Summit relay kit supplied the entirety of the wire I needed for what I'm testing out.
The Dealio:
As-installed, with the power wire running along the firewall with all the other wiring (distributor is on the rear of this engine), if I switch the pump on with the engine off, you can hear the distributor go "didididididi" in time with the motor pulses.
The engine idles okay but not quite as well as with the pump off temporarily. My timing light stops pulsing randomly and the tach is a little sticky-jumpy if RPM changes. If I switch the pump off when the engine's running, these things both clear up.
The Fix(?):
I just tried adding on a test strand so the wire runs straight out away from the breaker, around the front of the engine and back to the relay on the opposite side, completely away from the distributor. Switched the pump on - zero ignition noise.
So, the current from the pump running is causing the distributor pickup to trip, but not strongly enough to keep the actual reluctor wheel from giving the MSD a signal to run the engine.
Given this, should I just get my pump power away from the distributor and not worry otherwise, or if not, what do I need to check with my ignition? The distributor is only a couple of inches away from the firewall & wiring.
It sounds like the pump circuit needs a suppressor. It really shouldn't be pulsing the whole system like that.
Is the MSD unit connected directly to the circuit in the distributor that you hear pulsing? If so I wouldn't be surprised if the MSD unit has a problem. It should have decent internal suppression built into it.
HappyAndy wrote:
It sounds like the pump circuit needs a suppressor. It really shouldn't be pulsing the whole system like that.
Is the MSD unit connected directly to the circuit in the distributor that you hear pulsing? If so I wouldn't be surprised if the MSD unit has a problem. It should have decent internal suppression built into it.
Yes the MSD is directly triggering off of that circuit, but I'm leaning towards the reluctor/distributor. It's not the pump circuit outright - it's dependent on the proximity of the power wire to the distributor, not just if the pump is running.
Here's video of my messy junk (yes that crappy taped-on wire is for testing this only):
https://www.youtube.com/embed/rcSMFz61whs
Maybe the pickup is poorly insulated?
EMI
Generally power wires aren't run next to signal wires. Electromagnetic interference from your fuel pump's power wire is being picked up by your distributor's signal wire. Blame it all on how an electric motor operates, but I digress:
I think your options are:
- re-routing the wire away from the distributor (probably the easiest) just like you said and not worrying about it.
or
- shield your signal wire (the one that is currently acting like an antenna, and picking up the EMI). A wire with grounded metal braiding over an insulated conductor would be the ticket (sounds fancy, but it isn't).
In the odd chance that you re-route your wire and continue to run into noise on your distributor's pickup line then it may be time to think about a shielded wire for that run (or it could be shielded already, in which case we'd look for defects and repair accordingly). Most of the time, though, simple separation is enough.
Good luck man.
I am confused. Where is the power coming from for your fuel pump? If you have a relay, what powers and grounds the coil side of the relay and where does the power for the switched side of the relay come from?
Looks like a straightforward EMI problem to me, keep that wire away from your distributor.
