The right scope for ignition diagnostics? Help, please?!

My car is an oddball classic Toyota Corolla GT-S with a throttle-body EFI and a special ignition system that Toyota only used for 4 years (84-87) with this engine series (4AGE). They call it the “VAST” system, and it is peculiar in that it uses one reluctor coil in the distributor during cranking and startup to tell the ECU “really, it is safe to run the fuel pump and make sparks, the engine is physically turning over”. On moden cars this is usually replaced by a cmashaft sensor but with dual overhead cams, using the distributor makes sense.

The complication is that immediately after the engine fires up, the car switches over to a “real” ignition signal from a mysterious ignitor assembly attached to the coil.

Right now, the car won’t start in hot weather. Read that carefully: It runs fine, has recently done thousands on highway miles without a glitch. REstarts on the highway when the engine bay is way hotter than the outside air, so in theory the heat problem has nothing to do with anything under the hood (coil, spark systems, starting systems) and in theory, the ECU in the cabin panel, or the fuel pump (submerged in the tank) is the problem.

But these cars predate ODBC diagnostic plugs, and the shops don’t have scopes anymore. And since it will crank and fire and then quite–I’m suspecting the dual-spark-systems or the handoff to the ECU, so I need to confirm that two spark generators (the reluctor coil signal, presumably 12V) and the main ignitor pack (12v to the coil and ECY?) are both making pulses properly, which means I NEED A SCOPE.

Which probably will have to troubleshoot the ECU (damned if I know how, but at least to see that it is changing output channels/signals or getting the spark signals) and I’ve heard a scope can be used to troubleshoot a fuel pump as well.

I used to know scope basics, I actually own a classic dual-channel Tek that’s a thousand miles away and too heavy to ship.

So it is time for me to own a Velleman! Any advice as to which model is “enough” for automotive diagnostics, and how to find more information on performing those diagnostics, would be greatly appreciated.

I’m the original owner of the classic troublemaker in question, so I’d really like to bring it back to life. The dealer is quite serious about having one tech who’d love to work on it–but could run anywhere up to 16 hours of labor to do it. (KACHING!)

Time to buy a scope, I know enough so that I should be able to learn the rest, once I can see the actual activity on a scope screen.

I’m thinking that a 4 cylinder 4-stroke engine idling at 800rpm should generate about 400 pulses per minute at idle speed, each pulse 12V on the control side, 30-40,000V on the spark side. She redlines at 7500rpm and cuts out at 8000rpm, so I’m still not needing extremely response on the scope, although I’d like to make sure it can handle “modern” vehicles and their systems as well. I plan to get a lot more use out of it.

HPS140 is most likely suitable, however, we do not carry probes to allow measurements on the HT side of the ignition.

Kind of reminds me of the FORD Escort I used to drive. I don’t even remember the model year, but it had a throttle body with what was called Central Fuel Injection. For some reason the car would start initially, but wouldn’t restart on hot summer days. I had to lift the hood and let the engine cool off for a while. Then by chance I discovered if I slammed the hood down it would work without having to let the motor cool off.

I now drive a 1991 TOYOTA Corolla with the 4A-FE engine.

I actually have a book by TEKTRONIX, “Automotive Electronic Troubleshooting Using A Digital Storage Oscilloscope.” It cost $9.95.
On the back of the book: 7/97 FLG4034 40W-7899-1
I wish I could be of more help.