scopeing dwell

screamer69 · Post by **screamer69** » Sun Jul 16, 2006 4:02 pm

can anybody explain how to scope the dwell please

ive got a scope and i have the scope pattern for my engine from tec details on it so if im correct if it matches that then everthing should be ok

am i correct and how do it do it

cheers

Bernard Fife · Post by **Bernard Fife** » Sun Jul 16, 2006 4:24 pm

screamer69,

Take your oscillopscope probe and connect the ground strap to ground (no surprise there!)

Then use the probe tip on the coil's negative terminal.

The coil needs a voltage difference between the termnals for current to flow. The positive terminal always (when running) has ~12 Volts.

A high on the negative terminal means there's 12V on it, so no current is flowing,

A low means the negative terminal is grounded, so the coil is charging. The coil sparks when the signal goes high again (i.e., current stops flowing). The signal may go very high when the current stops, start with the highest settings on your scope and probe (100X, for example).

The duration of the low is the dwell, then. So you want to trigger the trace on the negative slop of the signal, this will keep the dwell period on the left side of the screen, and easier to measure (otherwise it'll bounce around on a runnning engine).

You can measure the dwell as the number of divisions the signal stays low, mulitplied by the time/div setting.

Lance.

woh · Post by **woh** » Mon Jul 17, 2006 3:14 pm

. . . pattern for my engine from tec details on it.

What tec details? What is it you are trying to see? Are you trying to see the on time of the coil (ie deg dwell), or how long the dwell should be? (dwell time for MS setting)

Is the car running to make the scope measuement?

If you do connect the scope input to the '-' terminal of the coil, be sure that you are using a 10:1 probe. Otherwise you may blow the input to the scope.

czb83 · Post by **czb83** » Sun Aug 06, 2006 10:09 pm

Hey, I was reading posts trying to learn about dwell and stuff and had a question about lance's method.

IF you have +12V going to the coil, then you have the negative going to MS. Wouldnt you read nothing(0V) and then MS grounds it, so you see 12V on scope for a couple ms(dwell), then when it grounds, voltage drops very low from collapse of field goign back to zero before next spark where it repeats.

I dunno if i'm just reading what you wrote wrong, but it seems as though you say the opposite.

Mike_Robert · Post by **Mike_Robert** » Mon Aug 07, 2006 12:10 am

Disregard... having trouble attaching an image of scoped primary and secondary waveforms....

jsmcortina · Post by **jsmcortina** » Mon Aug 07, 2006 4:28 am

To scope the dwell connect the scope probes across the current sense resistor R43. As this is 0.01R (from memory) V = IR tell you that when I = 7A you will have 0.07V

James

woh · Post by **woh** » Mon Aug 07, 2006 6:03 am

Here is a picture of what you see at the resistor (top). Note the current rises and then starts to level out. From the beginning of the rise to where it starts to level is the dwell time you want (about 4 ms at 4 amps on this picture). If the dwell were set longer, the current would not appreciably go up and not further charge the coil.

The bottom picture is the trigger at the Tsel jumper. negative going is the trigger.