Setting timing question

[Sloweracing]

Checking the timing on my syclone. My question is it normal for the timing to advance about 5 degrees when you rev the motor. I have it set at 0 at a idle but when you rev the motor it advances 5 degrees. Brown wire is disconnected. Truck does have a MSD ignition

 

[Eric Sy GN]

Re: Setting timing question

The module seems to have a backup or "limp" mode advance curve, in case it stops receiving input from the EST line from the ECM. I can't seem to find any info on the exact specifications though.
The next chance I get, I'll do the same test as Dave to see if it advances the same amount. At least we'll have some empirical data.
Thanks for testing that Dave.
Eric

 

[Aeroking]

Re: Setting timing question

Is there an inductive delay setting in the factory ECM?

There is an adjustment in standalone ECMs to compensate for different ignition setups.

EDIT: and disconnect the MSD ignition. That box will screw with your readings. Disconnect the box, and test with the tan wire unplugged.

 

[Eric Sy GN]

Re: Setting timing question

Yes, the stock programming has compensation for time lags in the ignition system. They don't call it inductive delay, but it's the same thing (they call it time domain correction). But this advance we're seeing here is different.

 

[Couger1968]

Re: Setting timing question

Sorry for a hijack, but I see a similar thing. Is this why when I set timing with the wire disconnected at 0 then reconnect and check with a hand held scanner and an adjustable timing light at idle there is 4 or 5 degree difference. Example timing on the scanner said 29 and it is actually 33 with the light. Should I set it actual timing or keep setting it with the wire disconnected and 0. I have a msd box and knock off distributor, with a ac Delco D1943A module.

 

[dgoodhue]

Re: Setting timing question

If the zero is set properly I would expect the scanner and timing light to read at least in the ball park. It has been when I have checked the timing with the brown wire connected.

 

[Aeroking]

Re: Setting timing question

Sorry for a hijack, but I see a similar thing. Is this why when I set timing with the wire disconnected at 0 then reconnect and check with a hand held scanner and an adjustable timing light at idle there is 4 or 5 degree difference. Example timing on the scanner said 29 and it is actually 33 with the light. Should I set it actual timing or keep setting it with the wire disconnected and 0. I have a msd box and knock off distributor, with a ac Delco D1943A module.

First temporarily disconnect the MSD box from the system, because it will mess with your readings.

If you disconnect the tan wire, the timing light should read 0, regardless of what the scanner says. If the timing light does not say 0 with the wire disconnected, then adjust the distributor to 0. If its adjusted correctly you should be able to plug in the tan wire, and instantly have the timing light match the scanner. Well, ballpark, as Dave said.

Our distributors are not vacuum or mechanical advance. They are electrical advance, so all of the timing adjustments are made outside of the distributor, in the ECM. The ECM thinks that the distributor is set at 0. So you need to verify that it is.

 

[Don W.]

Re: Setting timing question

I'm definitely not an electronics whizkid so this has always been a question of mine. If the dist is mechanically set to 0 it will always "trigger" at TDC. So how does the ECM advance the timing?

 

[Couger1968]

Re: Setting timing question

I was wondering if that ac D1943A was giving me trouble. I read somewhere that it has advance built in to it, but thought it was at higher rpms.

 

[Aeroking]

Re: Setting timing question

The distributor is mechanically locked for adjusting timing. Nothing under the cap moves the pickup. The brains of the distributor is the ignition module (D1943A)

Your ECM is programmed so that 0deg advance on the ECM/scanner is 0deg advance on the distributor. So the ECM understands that anny advance or retard in the ignition timing is done by the tune. The ignition module does several things. First off, it is a crank sensor. It tells the ECM where the crankshaft is in its rotation, so the ECM knows when to fire the ignition and injectors.

The second job the ignition module does is translate between the ECM and distributor. The distributor cannot advance itself mechanically. The ignition module sends the "fire" signal to the coil either early or late, depending on the advance or retard you have in the tune. The tan wire that you disconnect is the signal communication between the ECM and the distributor. so by unplugging it, you isolate the distributor from the ECM commands.

So in order to get a true proper timing reading with a timing light on the balancer. You need to disconnect any external ignition controller (MSD box). Disconnect the tan communication wire, and get a reading for #1 cylinder. It should be zero, because the ECM cannot communicate with the distributor. When you plug the tan wire back in, then your commanded timing advance will show on the balancer.

If you put any type of advance into the distributor, then the ECM cannot see it. For example, using a MSD boost-retard box. The MSD box is adding or removing ignition timing at the distributor. The ECM may be commanding 15deg at full boost, but the MSD is interrupting that signal and retarding the timing, say 5 degrees. The ECM does not know that this is happening, and cannot show it on the scanner. The same goes for manually turning the distributor advance or retard. The ECM/scanner believe the distributor is matched to the tune. When in fact it may be 5deg advance, causign the engine to run at 20deg advance when the scanner shows 15deg.

Hope this helps a little.

P.S. - Sorry for the thread hijack

 

[Couger1968]

Re: Setting timing question

Is it the msd box causing the confusion since it carries a spark for more rotation of the crank. I have had it for years, but never thought to check it before. I always set a 0 with disconnected, that I didn't have adjustable light before. I wouldn't necessarily think so since the msd doesn't know if the wire connected or not.

 

[Aeroking]

Re: Setting timing question

Is it the msd box causing the confusion since it carries a spark for more rotation of the crank. I have had it for years, but never thought to check it before. I always set a 0 with disconnected, that I didn't have adjustable light before.

I'm not saying the MSD is the problem, but it certainly could be. That is the first thing i would try disconnecting. If you have the plug-n-play harness, it should be easy to unplug, and plug the stock connector to the coil. The MSD boxes do funny things to the ignition system. If you have a problem, disconnecting it would be the first thing to do.

 

[Sloweracing]

Re: Setting timing question

Good feed back on this thread. Just a question wouldn't you want the base timing set at the highest timing reading. So if you rev the motor and you get more advance at 3000rpm wouldn't you use that as your base other wise when you plug the brown wire in you will be 5 degrees advance at 3000 or 4000 rpm. You think it's 20 degrees but really its 25. One other thing any chance a sloppy timing chain could cause this.

 

[Couger1968]

Re: Setting timing question

I was thinking the same thing, just to keep the timing from advancing to much. I have a fresh rebuild with only 150 miles on the engine, so the chain should be tight.

 

[Aeroking]

Re: Setting timing question

With the tan wire disconnected, If you rev to 3000, 4000, 5000 the timing should stay at zero, at all times.

This is the problem in the first post. He is seeing timing advance at higher rpms when he should be seeing zero. If it doesn't read zero, then there is a problem in the distributor.

 

[Eric Sy GN]

Re: Setting timing question

Mine jumps up about 12 degrees at about 1400rpm, with tan wire disconnected.
This appears to be a backup mode, so you could drive it and not burn up the cat converter or exhaust valves if the timing was at zero.
I bet the amount of advance and rpm threshold varies with whoever made the module.

This is only with the tan wire disconnected. It does not add on to normal timing when everything is plugged in properly.

Eric

 

[Sloweracing]

Re: Setting timing question

Mine jumps up about 12 degrees at about 1400rpm, with tan wire disconnected.
This appears to be a backup mode, so you could drive it and not burn up the cat converter or exhaust valves if the timing was at zero.
I bet the amount of advance and rpm threshold varies with whoever made the module.

This is only with the tan wire disconnected. It does not add on to normal timing when everything is plugged in properly.

Eric

Ok makes sense. By chance did you ever check timing with wire connected and reved up. Just wondering

 

[Sloweracing]

Re: Setting timing question

My truck when idling at 750 rpm is retarded around 5 degrees. When you bring it up to around 1000 it jumps right to 0 degrees and then will go up another 4 or 5 degrees advanced as you rev it to 4000. Truck runs good set like this but if I set timing to 0 at low idle (750 rpm) and Take it for a ride it starts knocking real bad when it comes up on boost.

 

[wildphil]

Re: Setting timing question

The balancer on these trucks also have a problem with the outer ring spinning. I have had that issue on two of my trucks.

 

[Sloweracing]

Re: Setting timing question

The balancer on these trucks also have a problem with the outer ring spinning. I have had that issue on two of my trucks.

I checked TDC with a piston stop and it was dead on. That's why I was questioning timing chain. Gonna try a new timing light today to rule that out.