[TOM85CJ7]

Ignition module Q\'s......

I am trying to figure out the advance curve for the 258 ignition. In my research I have found that the 258 and the 360 use the same ignition module. Doesn't the ignition module determine the amount of non-mechanical advance? Wouldn't the advance requirements of the 360 and 258 be different? I also found that the Ford 300 I6 uses the same module. Maybe someone can explain the role of the ignition module so I can understand what is going on.

Thanks,
Tom

 

[TOM85CJ7]

Re: Ignition module Q\'s......

After more reading, I have decided that the reason that the same module works on all these engines is because it doesn't have any effect on the timing. It basically acts as a fancy switch reading the reluctor on the distributor and firing the coil. All of the advance on these engines is controlled by the distributor. If I am correct, then the only available advance is the vacuum advance and the mechanical advance. Can anyone verify my theory? Anyone know the advance curve for the 258 distributor?

Thanks,
Tom

 

[jeepgod]

Re: Ignition module Q\'s......

i dont know the advance curve.. but it is handled by the distributor... and not the ignition mod..

 

[LEVE]

Re: Ignition module Q\'s......

1. By Jove, he's got it!
2. Tom, never sell yourself short...
3. You're learning stuff,
4. At a fantastic rate!
5. And sharing it.
6. Good Job, keep it up!

 

[959]

Re: Ignition module Q\'s......

correct for advance.
but, actually it depends. On the stock ignition of a computer controlled 258 the reluctor signal is fed to the MCU then to the ignition module. This gives the MCU the ability to retard the timing.

when you do the howell upgrade. you rewire the ignition so that the signals go directly to the ignition module.

 

[LEVE]

Re: Ignition module Q\'s......

In reply to:

This gives the MCU the ability to retard the timing.

[/ QUOTE ]1. The MCU controlled the timing,
2. Via the Knock Sensor.
3. If a Knock was sensed,
4. The MCU retarded the spark,
5. Thinking the knock was a ping.
6. It makes sense,
7. But I wonder what happened,
8. When a rod or the crank started knocking?
9. But, the MCU only had the ability,
10. To advance or retard about two degrees,
11. Per the knock sensor.

 

[959]

Re: Ignition module Q\'s......

In reply to:

10. To advance or retard about two degrees

[/ QUOTE ]

it cant advance really. to do that it would have to fire the plug before it got the signal from the distributor. ie, it would need to guess.

In reply to:

7. But I wonder what happened,
8. When a rod or the crank started knocking?

[/ QUOTE ]

this is 100% why I went with the Howell system over the MPI. I think today they could filter the knock sensors signal to detect a real ping, maybe with some DSP. but I think they were just using a band pass filter.
I think I read its about 5K center.
but, on dale Becket's web page he said you can test the knock sensor by knocking on the intake manifold. Which means that the filter doesn't filter much. Which would mean on a tiered old engine, the timing could be wiggling due to lifter noise or any noise.

 

[RRich]

Re: Ignition module Q\'s......

Yes, two degrees - at a time. But 2+2+2+2, one after the other retards it as needed.
When the knock sensor "hears" what it thinks is ping, it pulls the timing back instantly 2 degrees. But it doesn't immediatly go back advanced again. Another noise, another -2 -- and so on until the "ping" stops.
So the end result is it can pull the timing back fast, but the timing advances slowly.
As you drive the timing's always seeking the most advanced position - most efficient - but too much jumps it back a little at a time.

Yes, since the knock sensor is just a little microphone, a rattle can sound like a ping to it. I've seen several times an unrelated item - like a loose alternator bracket, even a deep socket left on the intake manifold accidentally, cause enough noise so it won't advance - it keeps pulling it back. It cannot retard at all, it cannot go back any further than intitial base timing, it just prevents advance.
Symptoms - low power, sometimes it's intermittant. Sometimes a hair puller!

 

[JimmyZ]

Re: Ignition module Q\'s......

A long time ago, in the life of this board, TeamRush said that only the 1979 258 cu.in. distributor had a longer slot to allow for more advance.

 

[lindy80cj7]

Re: Ignition module Q\'s......

Also, when you put a gm hei distributor from a 250 the curve is not the correct one for a 258.

 

[TOM85CJ7]

Re: Ignition module Q\'s......

I am liking all of these great responses to my recent posts. I could write a GM-Duraspark ignition primer.....

Thanks,

 

[LEVE]

Re: Ignition module Q\'s......

In reply to:

Also, when you put a gm hei distributor from a 250 the curve is not the correct one for a 258

[/ QUOTE ]1. Correct,
2. The spark curve is tended more for Highway RPM's,
3. Than developing low end torque of the 258...
4. But for most average usage... it's fine.
5. There's always the Crane kit,
6. To adjust the advance
7. And make up for the deficiency.

 

[lindy80cj7]

Re: Ignition module Q\'s......

I agree with you but it's beyond the capabilities of alot of people. I think if you could use the gm hei module like it's been posted before I think it would be best. Low end torque is where it's at in a jeep.

 

[OlllllllOCJ]

Re: Ignition module Q\'s......

There is a great article about how the "Emissions Controls" work for the mid 80's 258, written by Dale Beckett and is here on off-road.com. Contains excellent explanations, pictures and wiring diagrams on the interaction of the MCU, ignition module, vacuum controls, well just every damn thing including the Nutter bypass.

Look Here A Primer on the Jeep 258 with Carter BBD & Pulse Air Emission System

 

[RRich]

Re: Ignition module Q\'s......

Actually the curve required by the 258 is almost exactly like the GM HEI - when the HEI is in the GM product.
But when you install it in a Jeep engine and use the Jeep carb - or anything other than the original GM carb used with that GM HEI distributor - the vacuum curve is wrong.
The carb and distributor work together.

The GM's have a very low ported vacuum, so the stock advance diaphram is sensitive. Most everything else has a stronger vacuum signal, so it causes the advance to come in too quickly on the GM HEI - not the best for an off roader when you want torque, not HP. It works, but not the best. You want it to come in slooooow and late.

The adjustable unit from Crane will let you dial it in correctly, but most folks do it wrong, making it even worse. -- Biggest mistakes with those - not using the limiter cam, not using the original stock springs and weights, and not setting the Allen screw adjustment to the LEAST SENSITIVE position - counter clockwise.