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Discussion Starter #1
The Marelli Plex ignition on my Giulia Sprint has stopped producing a spark at the coil.

I checked to see if power was getting to the ignition module and it was (12 volts) so I replaced the module and the pickup cable. I also replaced the high tension lead from the coil.

Still no spark so I replaced the Marelli Magnetti coil with a Bosch MEC 723 which has slightly lower primary resistance but as the module is no longer under the coil I'm hoping this won't be a problem. I also replaced the wires and connections between the coil and the ignition module.

There is still no spark coming from the high tension lead so I am at a loss as to what to try next.

Any suggetions?
 

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Did you check the ground? The oem diagram shows the ground coming off one of the mounting screws which attach the Plex housing to the firewall.
 

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Discussion Starter #3
Thanks for replying Jim,

I think the module should be grounding ok. There is a wire going from the heatsink to one of the screws that attaches the regulator to the inside of the wheel arch. I've brightened the contacts of this wire and tried a jumper wire from the heatsink to the air intake manifold, I even tried going from the one of the bolts attaching the module to the heatsink and still no spark.

I have a copy of a document produced by Centreline about installing the module and it says to check the resistance between the heat sink and ground and also the battery and ground. I did this and I got a higher ohms reading they they recommended but I don't trust my multimeter. I'm not sure how to rectify that problem anyhow.

Roger
 

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MarelliPlex

Some one has suggested testing on the bench to confirm operation.
Could this be done with just one spark plug attached to the distributor cap, a hight tension lead from the coil to the center of the distriutor cap and 12 volts from a battery charger applying the voltage to the positive of the coil? Also, every thing ground together, i.e. spark plug, module and distributor all gounded to the negitive of the battery charger. Plug the charger in to 120 volts and rotate the distributor and a spark at the plug should appear once ever revolution if all is working as designed.
Does any of this make electrical sense. All comments would be appreciated.
 

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See if this helps:

Coil resistance: Primary ohms--0.75-0.81, Secondary ohms--10k-11k, Balast Resistance: none, Pick-up coil reisitance: 700-800 ohms, Pick-up coil air gap: .012-.016, Rotor resistance: 4k-6k.
 

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For bench testing just connect a plug lead into the coil and skip the distributor cap. connect the negative of a car battery to the heat sink and the positive to the coil +. connect the distributor with the 2 pin plug. Rest the spark plug on the heat sink and spin the distributor.
 

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Discussion Starter #7
I sort of did a bench test by putting a voltmeter between the return wire from the pick-up coil and the ignition module and spun the distributor. I was able to detect a very low current of 2 of 3 volts when the lobes on the distributor shaft went past the pickup coil.

I got the same voltage when I put a volt meter between the ignition module and the wire going to the pickup coil (with the ignition on of course). I'm not sure if the igition module is supposed to be sending and receiving such low volts or something is partially grounding and volts are being leaked to the point where the system can't produce a spark.

I'll do some more volt/ohm tests and try some further bench tests as suggested.
 

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You seem to be confusing voltage and current in your description.

The trigger module is the GM 4 pin HEI module. Google it for more information.
Unplug the 2 pin connector from the distributor at the module on the heat sink. Set your meter to AC volts and connect it to the wires from the pickup. Spin the distributor. You should see about 1 volt AC if the pickup is good.

1 volt AC is enough to make the HEI unit fire the coil. There should be nothing connected to the -ve side of the coil other than the wire from the HEI. The module can blow if it is loaded with something it was not designed for.

Make sure that the grounds are good and that you have a healthy 12 volt supply to the coil. These units don't work well if the 12v supply or the ground are marginal.
 

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Discussion Starter #9
Thanks Ed,

You have answered my question about the voltage required by the module to get the coil to produce a spark. I get 2-3 volts between the module and the pick-up wire when I spin the distributor so it looks like there is no problem there.

I'm making sure I'm getting 12 volts at the positive terminal of the coil. I don't know if it is significant but it is since I got a new battery that I haven't been able to get a spark. Before I was getting one, provided the battery was fully charged. The cca rating of the new battery is 500 amps which I believe should be enough.

Unless anyone comes up with anything else I think I'll just have to keep going over things, testing and making sure everything is well grounded.

Thanks

Roger
 

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Try using a temporary heavy gauge connector wire from the battery + terminal to the + terminal on the coil. You might have a high resistance connection between your batteryn and the coil.
 

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Discussion Starter #11
Thanks Ed,

I put a jumper wire (just 4mm) from the battery + to the coil + terminal and it didn't help.

It was worth a try though.

Roger
 

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It's possible that your new HEI unit is bad. There may be some way to test it. I would try an internet search.
 

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Discussion Starter #13
The test I have done so far suggest it is ok as it should be because it is brand new. Apparently it recieves and sends only a few volts which is what mine is doing when the the shaft lobe passes the pickup
 

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Discussion Starter #14
I discovered that I had the pickup coil too far away from the distributor shaft which meant that the voltage increase when the shaft lobes passed the pickup coil was too small for the module to detect.

When I installed the new pickup coil there seemed to be two possible positions. One had the pickup coil touching the shaft lobes as they came around, and the other had the pickup coil a couple of milimeters from the lobes. I initially chose the latter because I didn't think there was supposed to be physical contact in an electronic ignition. Apparently this was wrong and as soon as I re-positioned the pickup coil I started to get a spark.

I noted that the ends of the shaft lobes were bright which would suggest that they were previously touching the pickup coil when the system was working.

I have the car running now and have advanced the iginiton, although I didn't want to take the revs up tp 5100 as the manual recommends untill I'm sure that my engine work (valve clearences and crankshaft removal) are ok.

I suppose we live and learn.
 

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when you put the new pick up sensor did you put the "c" type" magnet below? did you mean that now lobes touching the sensor? this is wrong.
 
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