XDI ECU stopped working.

[rwrink]

1971 911 2.2 Zenith carbs

I drove the car and everything was perfect.
When I tried to restart, no joy.
It cranks but no start.
XDI light solid green during crank which points to a crank sensor problem.
The crank sensor itself shows 662 ohms which is within spec.
12 volts to the ECU and a good ground.
Now the odd part, I only get 2 volts at the sensor when I should see 5V.
Same result ignition on or during cranking.

I have about 500 miles on this setup.

Would love to hear the expert thoughts.

Thanks
Rob

[al lkosmal]

Rob,
I'm not an XDi expert, but I am thinking that they use a Mag sensor for the crank trigger.
A mag sensor is a 2-wire sensor with shield (sometimes) it is not powered by the ECU...basically makes it own output signal as the tooth goes past the sensor magnet....and the signal gets higher the faster the trigger spins. So no ECU power out to the sensor, just sensor power out to the ECU.
So, if you have a mag sensor....you will likely see low voltage output with ignition on and at low RPMs........and, if working properly...higher output at higher RPMs.
The Mag sensor is somewhat sensitive to the gap between the sensor tip to the crank trigger teeth. I would verify that this is in spec.

A Hall sensor is a 3-wire sensor (with shield) and the ECU powers it, typically with 5V or 12V. So you have ECU power in to the sensor and then a sensor signal out to the ECU
With the Hall sensor, you can verify the ECU voltage supply (5V) and the sensor signal output.


So, which sensor do you have and how are you measuring the voltage?

[mysocal911]

Originally Posted by al lkosmal

Rob,
I'm not an XDi expert, but I am thinking that they use a Mag sensor for the crank trigger. What type of crank sensor do you have. Mag or Hall sensor?
A mag sensor is a 2-wire sensor with shield (sometimes) it is not powered by the ECU...basically makes it own output signal as the tooth goes past the sensor magnet....and the signal gets higher the faster the trigger spins.
So no ECU power out to the sensor, just sensor power out to the ECU.

That's a reluctance sensor.

Originally Posted by al lkosmal

A Mag sensor is a 3-wire sensor (with shield) and the ECU powers it, typically with 5V or 12V. So you have ECU power in to the and then a sensor signal out to the ECU

So, if you have a mag sensor....you will likely see low voltage output with ignition on and at low RPMs........and, if working properly...higher output at higher RPMs.
The Mag sensor is somewhat sensitive to the gap between the sensor tip to the crank trigger teeth. I would verify that this is in spec.

With the Hall sensor, you can verify the ECU voltage supply (5V) and the sensor signal output. signal


So, which sensor do you have and how are you measuring the voltage?

Yes, a Hall sensor (magnetically sensitive), a semiconductor - heat sensitive.

[rwrink]

Thanks for the replies.

Al, the engine was built with a Hal effect sensor.
I installed the engine but could not start it.
It turns out, early serial number XDI cannot use
Hall effect sensors. I replaced with a magnetic sensor
and it ran fine.

I’ll check the tolerance on the placement today and
report back.

Rob

[rwrink]

5” pulley had a max gap of 0.05”.
I am less than 0.035”.

Voltage was measured at 2v at the
connector feeding mag sensor.

Ie in the engine compartment.

Rob

[al lkosmal]

Rob,
Per my earlier email, a Mag Sensor does not have a 5V supply from the ECU, but generates its output to the ECU, so I would expect to read low voltage at low RPMs and higher voltage at higher RPMs.....I would verify the following:

The sensor to ECU wiring is properly installed and intact (make sure that the wires are not missing insulation, shorting to ground or to each other and have good continuity.)
check to see if the sensor voltage changes when you rev up the engine (This will verify that the sensor is reacting to the trigger wheel)
Is your tach responding while you are cranking? (this should also verify that the ECU is receiving the sensor input and providing the corresponding tach output)

The ECU does not provide voltage to the Mag sensor, the mag sensor sends a voltage signal to the ECU.......the signal is generated as the trigger wheels teeth/gaps go past the sensor.....so i don't think that you would measure any voltage until the trigger wheel spins......so, the 2V may be ok at cranking......but should increase as RPMs increase.

Have you contacted electromotive tech help, to see what they advise?

regards,
al

[mysocal911]

Originally Posted by al lkosmal

Rob,
Per my earlier email, a Mag Sensor does not have a 5V supply from the ECU, but generates its output to the ECU, so I would expect to read low voltage at low RPMs and higher voltage at higher RPMs.....I would verify the following:

The sensor to ECU wiring is properly installed and intact (make sure that the wires are not missing insulation, shorting to ground or to each other and have good continuity.)
check to see if the sensor voltage changes when you rev up the engine (This will verify that the sensor is reacting to the trigger wheel)
Is your tach responding while you are cranking? (this should also verify that the ECU is receiving the sensor input and providing the corresponding tach output)

The ECU does not provide voltage to the Mag sensor, the mag sensor sends a voltage signal to the ECU.......the signal is generated as the trigger wheels teeth/gaps go past the sensor.....so i don't think that you would measure any voltage until the trigger wheel spins......so, the 2V may be ok at cranking......but should increase as RPMs increase.

Have you contacted electromotive tech help, to see what they advise?

regards,
al

Correct! The reluctance sensor (mag) provides an AC voltage as a function of RPM and is measured with a VOM set to AC volts and not DC volts.

[stownsen914]

It sounds like the 2 volts is constant even when the engine isn't cranking. A mag sensor wouldn't provide a constant voltage, and I think 2V may be too high anyway. When I had to do some troubleshooting on a mag sensor trigger a while back, I was getting 300 mV from mine. Granted it's a smaller sensor ...

Note that I used an oscilloscope to do the troubleshooting. The cycling was too fast to catch on a voltmeter with any accuracy. But some movement might give some comfort that it's giving a signal.

[mysocal911]

Originally Posted by rwrink

1971 911 2.2 Zenith carbs

I drove the car and everything was perfect.
When I tried to restart, no joy.
It cranks but no start.
XDI light solid green during crank which points to a crank sensor problem.
The crank sensor itself shows 662 ohms which is within spec.
12 volts to the ECU and a good ground.
Now the odd part, I only get 2 volts at the sensor when I should see 5V.
Same result ignition on or during cranking.

I have about 500 miles on this setup.

Would love to hear the expert thoughts.

Thanks
Rob

Read here to determine what you have; https://electromotive.com/product-category/crank-cam-trigger-solutions/crank-cam-sensors/

Then you can better solve your problem.

[rwrink]

Thanks everyone.
I talked to Ricked Clewett and he confirmed that the wire to the sensor can break and still show proper resistance.
I have had the engine out twice and it’s possible in all the juggling I tugged a wire.
I ordered a new one and it’s on the way.
I’ll let you know what happens when I switch it out.

Rob

[rwrink]

The magnetic sensor was bad.
Replaced it and she fired right up.
Thanks everyone!!!
Rob