cobalt91

Hello guys. Yesterday car drove fine until later in the day. Leaving to finish up some running for the day and it starts missing. Feels like a big loss in power. So I park it for the night.
Today I start trouble shooting, and find a weak spark from the passenger coil. I have unplugged the coil and no change to how it runs. So I run to the wrecking yard and find one on a 94 ls400. Get it home and hook it up. Car won't start.
I have searched and cannot find the specs on the test for this coil. I Don't know what they should ohm out at.

Also no CEL.
Thanks
Dan

cobalt91

Ok, so worked on it some more and found the that both coils are weak. I will buy new ones in the morning and replace both. Will report back tomorrow on results.

Yamae

I worry about the drive circuit is the root cause of your problem.

cobalt91

How would I identify and check the drive circuit?

Yamae

I observe the waveforms of in and out of the drive circuit using an oscilloscope. Ask your friend if you don't have any.

The input should swing almost 0V to about 12V at peak.
On the other hand, the output, minimum 1.0V when the transistor is saturated and maximum 200V or more when the transistor is just turned off. The output waveform looks like the one in the link below.
http://www.cathalgreaney.ie/diagnostics/d21.html

In case the drive circuit is defective, the minimum output becomes higher and maximum output becomes lower.

dicer

He wants to know where the drive circuit is located.

oldskewel

iTrader: (1)

What test do you do to find if they are weak? (it's obvious to test for a failed one, but how about "weak" or intermittently failing)

I recently had some intermittent running problems (rare for this car) on my '91, and thought it might be the coils, so I replaced them. No improvement after doing that, but at 190k miles, probably worth doing. I saved the old ones as troubleshooting spares, and the problem seemed to fix itself. I believe it might have been related to the fuel pump system and running the tank low. We'll see, but it's running perfectly again.

Good luck, and let us know what happens.

cobalt91

Replaced the coils and she lives again. What I mean by a weak

cobalt91

Replaced the coils and she lives again. What I mean by weak is the color of the spark. I was unplugging the coil and accidently got zapped. Felt more like a nine volt battery than an ignition coil. They really hurt. Plus it wasn't the hot blue spark you would expect. These are the original coils that came on the car. At 227000 miles it makes sense that they may be a little worn.

oldskewel

iTrader: (1)

Excellent tip, thanks. Glad you got it fixed.

dicer

I would have gone with what Yamae said on this one. Or you could have just removed some corrosion skin on the connector when putting in the new coils. I suppose its possible for dielectric break down in one, but usually they punch through and just don't work. Gosh I got an almost 300k mile LS and the coils are just fine.

cobalt91

Hello Dicer,
I really would have liked to check the wave forms that Yamae was talking about. But unfortunately I don't have the training or the equipment for these tests. I would love to have one of snap-on's high end scan tools.

dicer

You and me both. For the cost of one of them we could buy 4 or 5 old LS's.

cobalt91

Following up with a few things I have noticed since the fix. The fuel mileage has doubled. It is also seems to be more responsive to throttle inputs. I'm thinking about pulling the throttle body for a cleaning. I haven't done that yet. I have sprayed it down while it was running, but I'm curious about its condition.
I also want to thank you guys for the input on the problem.

AandSC

Glad to see you have her back to running well, and thanks for following up with the resolution.