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83 zxt dizzy not grounding for spark


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I hope the title makes sense.  I have the dizzy out with my multi meter, both digital and good old needle, and when spinning the slot past the opt sensor, it will go from 5v to 4.75.  the digital wont pick it up fast enough but the needle move quick.  I thought it needed to go to 0v so the computer will let it spark.  I have it wired exactly like the printout from moby.  this is the only thing that is keeping me form getting this car alive after 7 years of working on it.

 

please help.  ideas?

 

jimbo

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You're probably just measuring at the wrong pins, or wires.  The drop to 4.75 is probably "on" and 5 is "off".   There's a voltage drop when the photo diode is switched, I would guess.  I wish I had one of these, they look like fun to play with.

 

Nobody ever shows what wires they're measuring so this problem, and any solutions, never get clearly explained,  I think that what Nissan calls a photo diode is actually being used as a photo transistor.  When the light from the LED hits the photo diode, current flows and it's "on".  LED light goes away, it's off.  Or it may actually convert the light to current but that seems like it would be a pretty weak pulse, converting one LED light in to a pulse.  Who knows for sure...

 

Anyway, draw up a diagram, or take a picture and show where you're measuring voltage.  There are guys on this forum who know exactly how these things work.

 

Note that there are actually two different wave forms generated.  Ideally, you would confirm both.

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it is a steady 5v until it hits the slot and then momentarily goes to 4.75v then back up to 5v.  I am measuring the at the signal wire back to the computer.  I thought it went to 0v.  so I have not bothered to put the laptop to the car and hit the starter button to see if a signal come in.  I guess I will do that here and see.  what the hell right.

 

jimbo

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  • 2 weeks later...

About halfway down the first page in this link there's a diagram (little yellow box) showing the distributor wiring.  Connect power and ground to a battery and measure voltage on the "tach signal" from the distributor while you spin it.  If you get the zero and five, then your problem probably isn't the distributor.  You might need the pullup resistor to get a consistent signal.

 

http://forums.hybridz.org/topic/23244-megasquirtnspark-mssmsns-installation-guide/

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I have a solid 12v from battery and the tach signal side I have 5v and when I turn the dizzy very slowly I can see the v drop to 4.75v.  that's it.  I thought that it might be so small of an area that it drops that the volt meter cant read it so I plugged everything back into the car and hooked up the lap top and still got nothing.  I have wired it up several different times with the same results.  per mobys diagram.  maybe edis is in order.

 

jimbo

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I took the signal wire off of the relay board to measure.  4.75v.  then I rewired the whole thing with scrap wire per the detail, same.  then I wired everything back to the relay board to get tuner studio involved.  same.  I bought a 30zx cas thinking that the cas was messed up.  same.  dude, if I had hair I would have pulled it out.  this is the only thing from keeping me at the track.

 

jimbo

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What happens if you just measure voltage at the tach signal to ground, with power and ground to the main distributor wires?

 

I'm just curious as to how the tach is designed to work.  The FSM implies that it outputs a voltage on the two output wires, zero or something (1982 EFEC-61).  I think that the pullup resistor might be for the Megasquirt side, not the distributor side.  Maybe.

 

I thought I understood the pullup resistor for the HEI module because it pulls up one circuit (G and W) so that it can control the other (B and C).  Normally G and W would have negative and positive voltage from the VR pickup.  I don't get the purpose for the distributor tach line, since the "control" is from the other side.  Unless it's completing a circuit to ground through the tach line and somewhere, like the distributor body.  

 

Just thinking in writing.  Is your distributor body well-grounded?  Maybe you're missing a ground circuit somewhere.

 

Don't place too much value in any of this.  Basically, I'm just bumping your thread until someone who knows speaks up.

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What happens to resistance on the tach line with power and ground connected to the distributor, but nothing on the tach line.  If it's a switch like the HEI module it will go from low to high.  The FSM says it's a pulse,which may or may not be a voltage.  Maybe you just need a bigger pullup resistor.  More guesses.  At the bottom of Moby's writeup he says that a 2K resistor is for 12 volts and the 1K is for 5 volts.

 

From the writeup:

____________________________________________________________
*Pullup/Pulldown resistor quick course
You use a pullup resistor to get your line(or pin,connection) to some voltage (5,12, etc). Suppose you want a pin to have 5volts. If you just add a wire to 5 volts there is nothing to limit the current and if the pin happens to be an input to a transistor that is switched to ground then you have a short to ground from 5 volts and you burn stuff up. So instead you add a resistor to the 5 volts and the resistor will limit the current according to ohms law. Current is voltage divided by resistance. If you put a 1k ohm resistor to 5 volts then if the pin is switch to ground by internal electronics it can only draw 5 milliamps max(the resistor current-limits the path). 

A pulldown is similar except that you want a pin/wire connected to ground and you want the path to be current limited. 

To size the pullup or pulldown correctly you should know what the circuit is that you are adding the resistor to. But as a general rule you can use a 1k resistor for 5volts and a 2k resistor for 12volts. That limits the current to a few milliamps which is conservative. Sometimes the circuit may require more current to operate properly.

Edited by NewZed
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Might as well post this for photo-diodes.  It seems like the Nissan system would just put out a voltage, with only power and ground.  Maybe it's between tach wire and distributor body.  Almost the bottom of the page, Optical Triggering.  Don't forget that MS is set up with VR trigger even with an optical distributor.

 

http://www.megamanual.com/ms2/pickups.htm

 

 

Good luck.

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that's what I thought.  5-0.  not sure when it is 5 and 0 but it goes between the 2.  for most of my testing, ms is not involved at all.  computer is not on.  just a power source.  and I have the dizzy resting on my intake or another surface for grounding.  I guess I can try and run a wire from the dizzy body to a grounding source and see what happens.

 

jimbo

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Last post - I think that the 5 that's measured with the factory system comes from the ECU.  You have a 12 volt supply on your pullup line so should be seeing 0 or 12, I believe.  The 5 volts that you're seeing might be a coincidence.  The voltage is not generated from within the distributor, it's just allowed through.  The 5 and 4.75 you're seeing might just be the result of a voltage drop (from 12) from a short somewhere.

 

Another guess.  Sorry to clutter up your thread.

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Don't know if this will help, but I measured voltage on the four pins to my 1995 Pathfinder CAS, with it disconnected from the distributor and key On, and I get one ground wire (zero), one 12 volt, and two 5 volt.  The Pathfinder has the same type of optical CAS with a high resolution and low resolution trigger out.  So the 5 comes from the ECU.  The "trigger" might just be the phototransistor grounding the circuit.  I think that much of what is written about the "trigger" voltage coming from the distributor may be mis-guided or wrong.

Edited by NewZed
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When troubleshooting mine when it had a dizzy, I could repeatably ground out the signal wire (white wire) in the ms harness and it would trigger the ms to fire. That was with a pullup wired in as per Moby's diagram. This leads to believe that the stock cas is going 5v to ground and, with the pullup resistor, its going 12v to ground in the ecu

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