coma

Hi everyone,

We (Risky Whiskey Racing Team) are currently experiencing a strange voltage spike at the cam sensor during fuel pump priming, so wanted to reach out and see if anybody has experience a similar issue in the past and might have some clues about a possible fix.

Our cam sensor is connected to the MS3X (v.3.57) daughter board (crank sensor to the main board) and we are measuring a -7V spike at the cam sensor during the fuel pump priming cycle. As soon as the fuel pump priming cycle stops and the fuel pump relay clicks off, the cam sensor voltage goes back to +5V. This only happens with the cam sensor - all other sensors show the appropriate voltage. It appears that during the fuel pump priming cycle a reverse polarity 12V signal gets introduced into the cam sensor circuit somehow. We are wondering if maybe the protective circuits in the cam sensor electronics got damaged, we have some settings wrong somewhere or have accidentally mis-wired stuff somewhere. The engine runs without any apparent problems, though. We are planning to try installing blocking diodes into the fuel pump relay ground trigger wire and also the cam sensor wiring but would rather find out what the underlying root cause is.

Any hints or tips are highly welcome. Thanks much in advance.
C.M.

Ted75zcar

iTrader: (2)

Generating a negative voltage ~should~ be all but impossible with this HW. As such, I would start by confirming it is wired properly. Measure the voltages to a common ground, not the ground in the sensor plug. Measure the voltages from all 3 pins to ground both with and without the FP running and report back. Will be 6 numbers.

Ted75zcar

iTrader: (2)

Oh and please clarify that it is a constant DC voltage while FP is on, and not a spike. The term spike has specific meaning in the electronics world (transient event).

gooflophaze

There is a snubber in the vns3nv04d-e on the fuel pump output, but putting a flyback diode across the relay might aid it's response time - assuming it's flyback. Where are you measuring your spike at?

coma

Ted75zcar, gooflophaze, thanks much for your feedback and comments. To clarify, we are measuring a -7V voltage between pin 1 and 2 of the cam sensor plug during fuel pump priming cycle. "voltage spike" was probably the wrong word since this voltage is present during the full 2s fuel pump priming cycle, then goes back to +5V when the fuel pump relay clicks off. We already tried to install a flyback diode in the fuel pump trigger but didn't solve the problem. As a next step we will disconnect the fuel pump ground trigger wire (purple) from the fuel pump relay and power the relay directly from chasis ground, then check if we still have the -7 Volts at the cam sensor plug. This should tell us if the voltage intrusion is coming somewhere from the wiring or ecu.

Ted75zcar: Here is the first set of measured voltages on all cam sensor plug pins to engine ground which the fuel pump off (after priming cycle):

Pin 1: 0.008V
Pin 2: 4.945V
Pin 3: 0.003V

I will have to take the other 3 voltage measurements with the fuel pump funning later tonite when my son is home since you need someone to switch on/off the ignition to measure the voltage in the engine bay (you only have 2sec to do it before the pump switches off).

We are currently powering the fuel pump, coils, injectors, actuators and sensors off the same relay (separate fuses after the relay), as recommended by MS3X manual. Would it be beneficial to separate fuel pump and everything else using two independent relays?

coma

Can someone please tell us the correct pinout of the cam sensor plug? Seems we have wired the +5V supply to the center pin, when it appears that it should be on one of the outer pins (1 or 3).

Thanks.

venetox

What year and model car do you have.
If its a MS3X that's from Braineack correct?
Probably get in touch with him with the year and model and he will be able to tell you how to wire it for whatever circuits he built for it.

coma

Ted75zcar

iTrader: (2)

Yeah, and it is a 12V sensor IIRC (connected to W/R)

Ted75zcar

iTrader: (2)

Plus it sounds like you may have your relay coil wired to the wrong signal. You need to have a dedicated FP relay. The FP should be on its own relay, with the coil high side connected to the equivalent of W/ R on a stock harness. W/R is energized in the ON position from the main relay. The coil negative for the FP relay is a single wire directly to the megasquirt. All the other stuff is wired to the W/R (sensors, coils, injectors, ECU). Most people switch a dedicated line from the battery to the FP due to the high current draw of aftermarket fuel pumps and loading on the W/R line. In stock form, the FP relay is switched from the W/R.

again you need a dedicated relay for the FP.

coma

Quick update. Issue is resolved - the assumed fault turned out to be not a problem. We totally forgot that the cam and crank sensors are powered by 12V and not 5V, so we were measuring the voltage difference between 12V power to the sensor and the 5V sensor return signal as -7V during the fuel pump priming cycle when the fuel pump relay powered all sensors during the initial 2s priming cycle.

Apologies for the confusion and thanks much again for all input, advice and help!!!