After doing some major PM on my 1988 780 B280F -- new coolant hoses, injector seals, knock sensors, and TB cleaning -- it won't start. Initially it would run for about 5 seconds and die. If I touched the accelerator it would immediately die. While trying to diagnose the problem I was surprised to have the motor turn over on its own when turning on the ignition (position 2). I traced this down to the coil producing spark when the key is turned on (position 2) -- it's a continuous spark! Assuming it's a false signal to the Ignition CU, I started unplugging sensors, including the Injection CU. It made no difference, the only way to stop this behaviour is to unplug the Power Stage or the Ignition CU. Though this sounds like a failing Ignition CU, I'm more inclined to think it's a wiring problem as I moved it about quite a bit while pulling the intake manifold. Any suggestions or thoughts as to what the problem is with the coil producing a spark without the motor running would be welcome.

In the end I decided to try a different ICU. It took some while to find one, but when I plugged it in the continuous spark was gone! I plugged everything back in, hit the starter and it fired right up.

Thanks to everyone for your help to solve this problem.

Seems like some sort of wiring or connector problem. The Hall affect sensor on the distributor supplies pulses to the pwr stage which amplifies it and creates the coil spark. Looks like you may have had the knock sensor and throttle position switch disconnected. Do you have the plugs mixed up?

This is a v6 which has two knock sensors and the RPM sensor. The wires to the sensors are not longer than required so it would be almost impossible to mix up the plugs without stretching the wiring harness, though this was one of the first things I checked. On the v6 the RPM sensor plugs into the ICU which in turn is connected to the Power Stage which activates the coil. The problem exists with all sensors connected. I therefore started disconnecting sensors to try and identify the source of the problem until I ended up with just the ICU and Power stage connected. It is my understanding that spark should not be generated until the motor is cranking and the ICU should determine this from the RPM sensor. From what I can determine, the RPM sensor is working but it makes no difference if plugged in or unplugged. The sensor measures 540 ohms across the ICU terminals 10 & 23.

Sorry, don't know all the models very well yet. Certainly is an odd problem. The coil's getting voltage pulses from somewhere to generate the sparking that you see. It can't do it all by itself. Don't know the circuitry but I would think the only way a rapid pulse could be created is by an ICU problem. Good luck....

Sorry, don't know all the models very well yet. Certainly is an odd problem. The coil's getting voltage pulses from somewhere to generate the sparking that you see. It can't do it all by itself. Don't know the circuitry but I would think the only way a rapid pulse could be created is by an ICU problem.

Seems like some sort of wiring or connector problem. The Hall affect sensor on the distributor supplies pulses to the pwr stage which amplifies it and creates the coil spark. Looks like you may have had the knock sensor and throttle position switch disconnected. Do you have the plugs mixed up?

What ignition system do you have?

I had a '90 745 that had a Regina system that would always produce one spark whenever the ignition was turned on, then it would crank a while before it started. I sometimes heard a pop come from the exhaust when the key was first turned.

I was concerned about how long it cranked, but I was told by a tech that the system had to turn a revolution to see where it was. The car always started, but never quickly like our other 745.

Actually, I think that our other 740s produced one spark when the ignition was turned on as well.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks

This is a Bosch system. It's not just one spark, but a constant series of sparks.

By continuous spark you are saying the spark plug you might pull is repeatedly firing whilst you do nothing but leave the key in run spot, though the distributor is not turning. You are not saying you see one spark each time you turn the key.

A tester based on that principle makes use of the fact detonation is only likely to happen at or near a cylinder's TDC compression stroke, and though it might get a motor to turn you can't actually drive it to town that way. You sure you got the hall sender plugged in?

The key is on (position 2), motor not running, and the coil wire is putting out a constant series of sparks, not just one spark, but a constant series of sparks.

It doesn't have a hall sender. It uses an RPM sensor on the flywheel, which by the way shows about 600 ohms and no shorts. Having the RPM sensor plugged in (or unplugged) makes no difference.

My guess is your ignition switch.

Hello, Check the power module for proper voltage when key is on. The crank sensor should have a square wave sequence if checked by a good volt ohm multimeter like a fluke 98. It almost sounds like something has fried in the computer on the right kickpanel(ecm). I would break out the wiring schematic and check for shorts to ground. You might have a pinched wire. I would do that before you condem the ECM. Pauli

There is 12.05 volts to the power module when the key is on. I don't have a nice fluke 98 so can't tell if the RPM sensor has a square wave sequence, in any case this would only be produced if the motor is rotating. The RPM sensor measures 540 ohms across terminals 10 & 23 on the ICU so it should be ok to provide normal spark behaviour. The Injection Control Unit (behind right kickpanel) is unplugged so it shouldn't be causing the problem. I have been checking the wiring for continuity and shorts but so far everything looks good.

10 and 23? I recognize that from the 240 ICU circa 1989 and up. Maybe it works the same. In that, the RPM sensor is definitely NOT a square wave out, like the Hall sensors. That's what the ICU creates from it and uses the power stage to charge and dump the coil accordingly.

First you have to be sure the power stage and coil are not self-oscillating by some means, like a crappy ground or power might cause. You probably already did that by unplugging the ICU. Then you have to find out why the ICU wants to buzz all on its own. Is it getting all its power and ground? Did it get wet? Is there an AC signal on pins 16 and 17? Should be steady DC with no crank motion. Fluke 98, eh? What is that, a pocket oscilloscope?

If you read the original post, he has already done that isolation. He is looking for a clue to which wires got pinched putting that layered intake hardware back together.