Is it safe to Scope coil driver output?

[gofastant]

Hi, I just bought the hobbylab scope and am pretty happy with it. As a Mech. Engineer I've never had the need to use a scope, not even in college. So this thing is pretty cool to me, easy to use and informative.

Now to the question at hand. I have EDIS 4 and I was wondering if it's safe to scope the coil driver wires (pins 10 and 12 on the EDIS module) while everything is hooked up and cranking? Should I be concerned with voltage spikes or no? I ask because it wont start and with a timing light every plug is firing at TDC, and I do get a backfire now and then when attempting to start. It seems like coil 2 should only be firing at BDC as seen on the crank.

thanks

[Matt Cramer]

It depends on what type of scope you have; the one we have around the shop can do this but must be protected with an attenuator in order to do this safely.

[gofastant]

OK, good to know. Thanks!

[pb3]

Probing the coil drivers, ie the negative side of the coil you will see 300+ volts, all be it a very short spike but enough to kill your scope input if it can not handle the voltage. As state you will need to attenuate the signal first!

[gofastant]

After reading Matt's reply I planned on using regular 12 V bulbs to get a view on things (to see if they alternatively turn off). Should I expect these to blow up?

[gofastant]

So I wired in some small light bulbs like below. Using a remote switch I cranked the engine (with plugs in the engine but fuel pump fuse out so no combustion). I watched the bulbs and was able to confirm that they are in fact, turning off alternatively, like they should be.

But I think I see a problem. The bulbs themselves are very dimly lit when the engine is cranking and they turn on and off. Shouldn't they receive a full 12 volts? It appears that they only get 2-4 volts as the elements in the bulb are just glowing red during this time.

If I shut the car down and remove the key and then hard wire 12 volts from battery to the positive on the bulbs they are much brighter. The grounds (pins 10 and 12 on EDIS) are grounded when the car is off, they have 19 ohms to ground iirc. But if I measure the bulb voltage, they only have 6 volts and change across each bulb, meaning they must normally be wired in series inside the coil or something.

Can anyone provide some input here?

[Zelectron]

I don't have EDIS, but have been an electronic tech for 30 years, you need a 10x Scope probe, about $ 35 for a good cheap one, that will reduce the voltage your scope see's by factor of 10. They do make 100x probes, but they are harder to find. Most scopes have 8 vertical graticule divisions, and the max per division setting is typically 5 volts per division, some scopes may have 10 or 20 for max vertical, & that's before you turn the variable attenuator knob back.. Assuming yours has 5 volts for max setting for vertical, that's 40 V Peak to Peak (8 divisions times 5), straight into the scope with a 1 to 1 probe, with a 10x probe, you could have 400V peak to peak( 8 divisions times 50) With a 100x probe 4000V peak to peak.

Those 12 volt lamps you are using won't work good there, you have 12V on one side of the lamp, yes, but on the other side, there is a very short pulse type of signal that's triggering your EDIS coil, & it is happening rather fast, those lamps are slow to react, & they also are very low ohm load until they light, then there impedance goes up, nice try for an electronics newbie, but won't work.

[newtyres1]

One could always make up a 10:1 voltage divider circuit with 2 resistors, to save buying the probe for a one-off measurement.

[gofastant]

Thanks Zelectron. I do have access to 10x probes so I will give that a shot. Fortunately the bulbs were informative as far as making sure they don't fire at the same time.

[FixItAgainTony]

Replacing the light bulb with a resistor in series with an LED would probably give a pretty good light show. As already noted, the light bulb is slow, but the idea is right on. An LED is much faster:
From http://www.megamanual.com/ms2/V3trouble.htm :

6) You can use an LED in series with a 330 ohm (or 270 ohm) resistor and probe the input and output of various circuits...

The LED + resistor is a useful thing to have around for quick checks.

- Charles.

[gofastant]

Instead of trying LED's I used some 10:1 leads and cranked her over, again with no fuel. It looks like the spikes only get to 10 volts and there's no super high kick back spikes. It appears that both coils are firing every 180 degrees. Does EDIS do this until one cylinder catches and then goes into alternating mode? Without a combustion event, the resistance is low in every plug, maybe that's why there's no high kickback?

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