new points, ignition won't start

[userzweig]

Hi,

I have a '71 280sl automatic, and today I changed the points.  I've done this 5 or 6 times before without any problems.  Today I put in new points, gapped them at 0.012, put it back together, but it wouldn't start.  I put the old points back in, and it still wouldn't start.  It had been running fine an hour or so before I started this, though the initial idle was a little rough.  I screwed around with the gap for a while, but to no avail.

I checked a couple of things that another poster (blariwag) suggested ln 31 oct 2003 (mainly bad insulator on lead out of distributor), but that looks OK.  

I'm trying to figure out how to diagnose if the problem is in the LT or HT part, and then if it's in the LT then where it is.  I hooked up my dwell tachometer between ground and coil minus.  When cranking, the tachometer continually reads zero.  This makes me think that the points aren't breaking, or aren't closing, or else that there's no power going to the coil.  However, I checked the voltage between coil minus and ground, and it was 11.5 volts all of the time.  (I thought maybe it should have gone down when the points closed, but it's probably more complicated than that.)

Does anyone know of a good procedure for diagnosing these things?

Bruce

1971 Copper 280SL Automatic

[ja17]

Hello Bruce,

With the distributor cap off and the ignition on, take a good insulated screwdriver or such, and manually push the points open. You should see a spark. I aasume your car still has the factory electronic ignition?

Joe Alexander
Blacklick, Ohio

[JimVillers]

Joe's test is the first step.  The problem is probably a short in the bolt/connector that goes through the distributor housing.

Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor

[ja17]

Hello,
Jim, you'r correct on the earlier cars with the standard ignitions. But if your car is the later with the factory electronic ignition a spade connector replaces the bolt connection and a short here is a lot less likely.

Joe Alexander
Blacklick, Ohio

[userzweig]

Joe,

Thanks for the suggestion.  I didn't get any spark when I separated the points with the ignition on, and so that told me where to look.  I decided that I needed a test lamp to find out where the problem was exactly.  They didn't have one at Radio Shack, but they did have a 12 v LED for $1.29, and so that plus some tape did the trick.  

Basically, I had some trouble getting the wire off when I took off the old points, and I bent the spade the spade lug enought to just short it out.  I straightened it out, regapped the plugs, and it started after a little more futzing.

Thanks again for the suggestion.

Bruce

quote:Originally posted by ja17

Hello Bruce,

With the distributor cap off and the ignition on, take a good insulated screwdriver or such, and manually push the points open. You should see a spark. I aasume your car still has the factory electronic ignition?

Joe Alexander
Blacklick, Ohio

1971 Copper 280SL Automatic

[userzweig]

I had the shorting problem again today, and I discovered that the spring on the points can short against the body of the distributor if you don't watch out when you adjust the points!  I think it just happens with the late model '71 distributor and points -- the late model distributor is kind of small, the mounting slot on the points has a little too much room for lateral movement, and the points are pretty big, and so it's important to push the spring away from the distributor body when you screw the points in.

Bruce

quote:Originally posted by ja17

Hello Bruce,

With the distributor cap off and the ignition on, take a good insulated screwdriver or such, and manually push the points open. You should see a spark. I aasume your car still has the factory electronic ignition?

Joe Alexander
Blacklick, Ohio

1971 Copper 280SL Automatic