Re: Hot starting problem (update) - Saab Sonett Bulletin Board - Saabnet.com
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Re: Hot starting problem (update)
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Posted by Dick R [Email] (more from Dick R) on Wed, 11 Jun 2014 17:46:16 Share Post by Email
In Reply to: Re: Hot starting problem, eric in vermont, Sun, 19 Jan 2014 15:05:29
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I have yet another chapter to report regarding my years-long search for a hot starting solution. I believe that this will be the final chapter.

My previous improvement – plugging the PCV valve - provided the most benefit of anything that I had yet tried, but it didn’t always start as enthusiastically as I had hoped. The proverbial “start like a Honda” was not achieved. It never didn’t start, but there were a couple of times that I considered borderline.

I then tried an ignition system upgrade that I had used more than 45 years ago with my 1960 Saab stroker – a capacitive discharge ignition system (CDI). I found an old “Delta Mark 10” unit on fleabay – the same kind of device I successfully used back in 1968. I am pleased to report that installing the CDI has done the trick. I have achieved “starts like a Honda” status, hot, cold, and everything in between. No cranking, only immediate ignition. I re-installed the PVC valve and starting has remained instantaneous. I think this is the final answer.

OK, so why did the CDI work? I have some theories. Pardon the length of this posting, but I thought it would be worthwhile to document my findings for others with similar problems to consider.

1. I’ve read postings on this site saying that Sonett coils have a built-in ballast resistor. That includes the OE coil and the Bosch “Blue” coils. I’ve also read others saying that they had a separate and visible ballast resistor on earlier Sonetts. Since my car has no visible ballast resistor, for the sake of this discussion, I will assume that the coils I have been using (1973 original and new Bosch Blue) do have a ballast resistor built in. If that is the case, then at least some of the hot starting problem could have resulted from the coil’s ballast resistor function. Below a passage about ballast resistors quoted from an excellent and very thorough paper on automotive ignition systems that I found on the internet. This is a terrific resource, written in very understandable language. I highly recommend it: (link)http://www.jetav8r.com/Vision/Ignition/CDI.html(endlink)

“In order to increase the coil voltage at startup some ignition designs incorporate a "ballast" resistor. The resistor is switched in and out of the supply voltage to the coil. Once running, the resistor is switched in place and the coil is actually getting less than 12volts. When the engine is started, the resistor is removed and the coil gets the full 12volts. This provides a much better spark at startup to compensate for reduced battery voltage drawn by the starter. When starting a cold engine, the plugs and the air are cold, the cylinder pressure is up, and the fuel / air mixture is poorly controlled. The oil is thick, the battery is cold and its voltage drops as much as 60% because of the high current drained by the starter motor.”

2. I think the last sentence may be the key to the Sonett hot start problem. While separate ballast resistors are “switched out” of the ignition circuit when the key is turned, that doesn’t happen with a “built in” ballast resistor. And the built-in ballast resistor, like the separate version, takes voltage away from the ignition circuit when the coil gets hot, but it doesn’t get switched out during re-start. The Sonett, having such a long main power cable from the trunk–located battery, is also probably more prone to system voltage drop due to starter current draw.

3. The symptom that supports the starting voltage drop causing a problem on my car is that when it wouldn’t start hot, it always immediately started with an easy hand push. That could mean that along with the hot ballast resistor reducing voltage to the coil, the starter’s current draw took too much juice out of the system to allow for a good enough spark. I tried jump starting through the battery terminals when it wouldn’t hot start without any apparent effect, but I never tried to provide a clean (without the starter’s current draw) voltage source to the ignition. That is except when I push started it without using the starter. That was immediately successful 100% of the time. I may have been able to test the ballast resistor theory out by replacing a warm coil with a cool one when the hot starting problem was happening. I never tried that.

4. I’ve had a Petronix unit in place for 7 years, but that device is primarily a point eliminator that provides some increase in spark voltage like the old “transistorized ignition systems” did. They make use of a transistorized switching circuit which greatly reduces the current flowing through the points while somewhat increasing the voltage going to the coil. This is done by having kind of a transistorized relay that takes the main current load instead of the points. Reducing the current flowing through points stops the points from burning, but does not stop the wear on the point rubbing block. So the Petronix unit totally replaces the mechanical points with a magnetic pickup which is used to trigger the circuit that sends current to the coil. No point burning, no mechanical wear, no timing or dwell tuning required. A good thing.

5. However, the Petronix does not provide the kind of basic change to the ignition system that a capacitive discharge does. With a CDI, the circuit changes from being a inductor energy storage device to a capacitor energy storage device. With a CDI, the coil becomes strictly a transformer, and is no longer a spark energy storage component. Generally, the biggest benefit of a CDI is seen at high rpm service because a capacitor only needs about 10% of the time required to fully charge compared to an inductor (coil). As engine speed goes up, the time available to build a charge goes down, and a CDI system is not very affected by the briefer and briefer charging time that you get at higher and higher RPMs. This keeps the spark voltage relatively consistent throughout the RPM range.

6. Good spark voltage at high RPM doesn’t explain why my car starts so well. But higher spark voltage throughout the range does. A CDI has a significantly higher low RPM spark voltage as well. That’s because the large capacitor quickly stores higher voltages than is obtained with an inductor (coil) this voltage is sent to the transformer (coil) to be greatly stepped up at all rpm levels. It starts higher and stays there. That’s good for all ignition situations. And in the case of my Sonett, it is also probably less affected by the starter’s current draw.

Bottom line is that the CDI system did the job and this investigation probably indicates there may be other potential cures as well. These would all relate to maintaining good ignition voltage during hot starts.


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