setting the dwell on 280sl transistorized

[userzweig]

(I'm editing this 6/13/06 because my technique doesn't seem to work right every time.  Turns out that you need to run the engine for a little while first, which I guess gets the transistor going.  Otherwise you won't get any reading at all!  Also, be careful that the distributor cap clips don't touch the points.  bz)

If you have a 280sl with the transistorized ignition, which is a nice thing, then it is hard to get a good connection point for setting the dwell angle.  The two choices are pin 7 under the battery (need to remove the battery the first time to get to it; afterwards maybe you can find it by feel) or else you can connect between the outlet of the .4 ohm resistor and the inlet of the .6 ohm (the inlet of the .6 is hard to get to, since it's screwed on to the side of the coil and the inlet is on the bottom).

I had the idea of taking off the distributor cap and connecting my meter to the clip that connects to the points (other end to ground), since this clip also connects to the 7 pin under the battery.  It means measuring the dwell while cranking the engine with the cap off, but it's a heck of a lot easier.  I set the points at .012 inches, and the first time I measured the dwell it this way it came up 30 degrees, so it looks pretty good to me.  Here's a picture of my connection:


Does anyone have a strong opinion about the merits of this approach?

Bruce

PS - I also think that since very little current actually goes through the points (it goes through the transistor instead), then you can go a longer time between point changes, since they're less likely to have pitting.  Haynes says to check or renew the points every 6,000 miles, but with the transistorized system I think you can go a lot longer -- I'm going to try 12,000 at least on this set.

1971 Copper 280SL Automatic

[ja17]

Hello Bruce,

Here is some information not in the BBB. It was sent out in a factory service bulletin. It is for the factory transisorized ignitions.

b]Download Attachment: dwell1.jpg
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Download Attachment:[/b] dwell2.jpg
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Hooking a dwell meter up to the standard ignitions (non transisitorized)normally involves just hooking the red wire up to the coil and the black wire to ground.

The brand of meter used may alter hook up locations. I suggest placing one lead on ground and jump around the second lead  until your meter begins to work.

Joe Alexander
Blacklick, Ohio

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[userzweig]

Thanks, Joe.  Those diagrams that you posted are very informative.

I have a practical issue with the diagrams, however: the connections are difficult.  They want you to either (A) go under the battery and connect the meter to pin 7 (and ground) or else (B) connect the dwell meter to the ballast resistors; unfortunately, the inlet of the .6 ohm resistor is tough to get to.

If you remove the cap and rotor, it is incredibly easy to just connect the dwell meter to the clip where the lead to the points is attached.  Since I'm a little lazy, this really appeals to me.

A neighbor and fellow 113'er, Dan Walsh, also had an idea: he suggested using a needle to penetrate the insulation of the green wire that comes out of the distributor (it connects the points to pin 7), and take the reading there.  I wanted to see if my reading, taken at the clip with the distributor cap off, would be the same as a reading in a normal idle, with the cap on.

I tried Dan's method (the needle) and I got the same dwell reading during idle as I had gotten using my easy method (connecting to the clip and cranking the engine with the cap off).

(Incidentally, my meter actually gives me a reading at the coil as well, but it is several degrees different from the accurate readings taken from the points.)

Finally, measuring the dwell with the cap and rotor removed makes it incredibly easy to set the points, since you don't need to put the cap and rotor back on each time you adjust the points gap.

I guess some traditionalists might say that you need to have the engine running to get a good dwell reading.  Perhaps it's the transistor, but for some reason I get an accurate reading at the clip just by just cranking the engine with the distributor cap off.

If anyone else wants to compare dwell readings using my method and something more traditional, I'd be delighted to hear the results.

Bruce

1971 Copper 280SL Automatic

[hands_aus]

Hey Bruce,

I see on Joes' first pic that there is a 2 screw terminal block between terminal #7 on the transitor module and the distributor.

Does your car have that terminal block?

If so, maybe that would be the easiest point of connection.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[userzweig]

Rob,

Thanks for the suggestion.  Yes, there is a terminal block, but it's a little hard to get to.  I asked Don Czapski (he has the same distributor as I) and here's what he said:

"...it is simple once you know the hook up places...pull out the battery so you can see the hookup.  It is a white porcelain junction piece.  The "pins"
are numbered and you can see the numbers if you look closely.  The bottom piece of metal on the battery tray is not bolted down and it lifts out after the battery comes out (now's a good time to repair any rust and then repaint it).  Once you make a note of pin 7, you can do it the next time by "feel."
Or, you could hook up a small piece of wire to extend up from underneath and tape it off."

I asked Don if you could maybe use a flashlight and a mirror instead of pulling the battery.  He said:
"A light and a mirror would work.  Reach down and feel around under the battery tray.  You are looking for a white porcelain terminal with 3 or 4 wire connectors.  It is rather small, about 2 inches long.  Once you know what you are looking for, it will be easy the next time."

Anyhow, I have to say that I like my way better.

By the way, here is a picture of where I connect a remote starter to get the points to the top of the cam -- it's to the right of the engine block as you face the firewall (the other end briefly connects to red + to crank the engine) :
 

I've been able to get it to exactly 40 degrees this way, and it starts quickly and drives great!


Bruce

quote:Originally posted by hands_aus

Hey Bruce,

I see on Joes' first pic that there is a 2 screw terminal block between terminal #7 on the transitor module and the distributor.

Does your car have that terminal block?

If so, maybe that would be the easiest point of connection.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

1971 Copper 280SL Automatic