Bosch ign module on VN,VP V8 driving other ign box

[Circlotron]

The new module is in now and the car starts at least. I feel just too depressed about the whole thing to go for a drive right now. Gotta go to work after lunch so I might take the LONG way there and see how it goes. Somehow I am not expecting it to be any better.

[Circlotron]

Nuh, not a scrap better.

[VL400]

Bugger Is there a reason for the custom ignition driver if the stock module works ok? The V8 modules are not that bad - although the failures they get might be from sus circuit you have traced out

[Circlotron]

The next step might be to have a look at what the ECU bypass line to the module is doing. If it is momentarily dropping low unexpectedly (I know it's not supposed to) then I expect it would shift the timing from wherever it is on the advance map back to the static timing point provided by the distributor. I have seen jumps of 19 degrees retarded from the working point in earlier tests.

I would like to change some of the ECU parameters but I don't have a spare memcal and I don't want to trash the one I have, eprom being soldered in and all. I want to have something that works to fall back on because this car is my daily driver.

So, a led and a resistor on the bypass line and see what happens.

[Circlotron]

The reason for the add-on module is much the same reason as for hacking the ECUs – you know YOU can do better and you are free to make whatever choices you like. The original modules are a bit of a toy as far as modules go. They operate entirely open loop in that they don't take the actual current into account when setting the dwell from cycle to cycle. It must be all in a lookup table in the ECU - there's no worthwhile intelligence in the module. At speeds above 2000 rpm 8 cyl you really need to keep an eye on what the current is doing if you are going to make a serious spark at hi revs. Those things just take a guess and it's nowhere near as good as it could be. I have seen a car like mine but running on petrol and it picked up I think 6 hp at the wheels when one of my ignitions was piggy backed on. I never heard they had any problems like mine either so that's what's got me stuffed.

I think most of the failures of those Bosch modules are cracked solder joints around the connector pins as a consequence of thermal cycling and cheap-ass single sided boards. Good-better-Bosch does not apply here.

[Circlotron]

Now that the weather has turned colder I am having much less of a problem. Not zero, just less. I have been thinking the root cause of all of this drama just might be an idle mixture problem in that it goes leaner the hotter the engine gets. I posted the following musings on another somewhat less technical forum as a possible explanation:
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"Been doing a bit of thinking about LPG setups with and without a cold air intake. It seems to me that as the engine temperaure goes up, naturally so does the water temperature and consequently so does the temperature of the gas coming out of the water-heated gas converter. If the engine air intake is sucking hot under-bonnet air then this air comes through the radiator and so it's temp more or less tracks the temperature of the gas coming out of the converter. So far so good.

Now consider the situation if you have a cold air intake. When the engine is cold the gas/air mixture might be just right but as the engine water heats up the gas coming out of the converter heats up and gets less dense but the cold outside air it gets mixed with with does not. Therefore the mixture goes gradually leaner the hotter the engine gets.

That means for example that you could set the idle mixture just right when cold but it just wont idle right when hot, or vice versa, good hot idle but bad cold idle because it is too rich.

Trying to think of some way to get around this. If your car has closed loop mixture control on gas then lucky for you, but even so, it probably runs open loop at idle so would still have the same problem. My best thought so far is to get some kind of very small trans cooler thingy with the s-shaped pipe and cooling fins and put it inside the airbox and run the gas through the pipe before it goes to the throttle body / mixer. That way the gas is made to equal the temperature of the air going through the cooling fins, and it tracks that temperature as it goes up and down so the air/fuel ratio should stay more constant."
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We'll see.

[Circlotron]

Righteo, have narrowed it down considerably now. It is definitely temperature sensitive – when the temp gauge is above 1.5 major divisions it comes on just like clockwork. Also rpm sensitive in that if I bump up the idle to 800 rpm in D via the throttle body then it is (almost?) nonexistent.

My latest trick was to wind the idle way up so that the IAC was fully shut then unplug it while the engine was running so it stays shut. Turn engine off then unplug 4 pin connector from Bosch ignition module so that module is triggered only by Hall effect in distributor, not ECU. Start engine and adjust throttle body to ~500 rpm in D and guess what, it idles as smooth as a baby's bum for as long as you like up to any temperature you like. Also means Hall effect is okay. Pump the brakes and there is no stumble like before, and pull it from N to D and the revs go from 800 to 500 just as good as you like, with about 100 rpm undershoot instead of a lot more and a stumble as well.

So it seems the ECU is doing funny things. Things that DON'T show up when the Bosch ign module is running the show alone, only when the Bosch module is driving mine. I can only think that the Bosch module has enough excess dwell that the ECU can move the moment to moment firing point around quit a bit without cutting into the dwell enough to cause a misfire. I know it has been said that the ECU doesn't switch back to firing on dist ref pulse once it has started but I just might have a look at that line to the module and satisfy myself that this is so.

Is there any intended reason that the ECU might wiggle the firing point around abruptly at idle, above a certain temp and/or below a certain rpm? Even if the engine was idling in a stable manner just before the event, ie ~presumably~ not an anti-stall thing?

[Circlotron]

P.S. Have tried a replacement ECU but with same memcal. No difference.

[VL400]

So it seems the ECU is doing funny things. Things that DON'T show up when the Bosch ign module is running the show alone, only when the Bosch module is driving mine. I can only think that the Bosch module has enough excess dwell that the ECU can move the moment to moment firing point around quit a bit without cutting into the dwell enough to cause a misfire.

Dwell is controlled by the ECU too. At idle you will have over 7ms of dwell, and it will never chop it down to a level causing a misfire. Dwell start is a pretty simple calc, it works out how long to wait till it needs to fire for the desired advance and subtracts off the desired dwell. So it will always deliver the intended dwell time, its just that the firing point might be off if the engine RPM is rapidly changing. There is also a dynamic dwell function to add more in if the engine is determined to be accelerating. A burn time of 600us is always maintained but on a V8 its only after about 4000RPM that dwell goes below about 3ms.

I know it has been said that the ECU doesn't switch back to firing on dist ref pulse once it has started but I just might have a look at that line to the module and satisfy myself that this is so.

You can force it to go to bypass spark whenever the throttle is closed. There is an option flag.

Is there any intended reason that the ECU might wiggle the firing point around abruptly at idle, above a certain temp and/or below a certain rpm? Even if the engine was idling in a stable manner just before the event, ie ~presumably~ not an anti-stall thing?

There is a function to stabilise idle using spark advance so you often see the advance jumping around the place, but its been awhile since I have done much on VN $5D code but think that it was introduced in the VR.

[Circlotron]

You can force it to go to bypass spark whenever the throttle is closed. There is an option flag.

Wow! That's what I want! I'm at work a the moment but when I get home I'll have to look into that. What is the flag called?