strange starting issues with my 71'280se v8 3.5 please help

[body_infinity]

1971 280se (w108) 3.5 v8 (m116)

Hi guys been working thru the 280se v8 and have struck a brick wall with my ignition ;-( firstly the situation so far and setup:

1. Fuel system has been checked all good fuel pressure, filter injectors all tested all good!
2. Ignition is converted - electronic older optical version "optospark 500n usa made" not much info but have been told is very reliable hmmm?? Not sure on this any one else have this??probs??
3. Coil looks new has not been fully tested--yet??

Ok here is the problem which is driving me nuts !!! When I go to start her first thing if I start her normally she will turn but no catch ;-( I went to test the leads if I get a spark from the leads ---

--> no spark..then tested the coil terminals …I get power to the +’ve termianl but also got positive power on the negative terminal??

What the?? Is this normal?? I tested it again once I get her started and there is no power on the –‘ve terminal?? This is confusing? why would it show power when cold??

OK now the strange bit--- the only way she will start first thing is if I ,leave the ignition on for around 1-2mins then she starts first shot..its like something in the electrics needs to heat up or something

The other thing is it sometimes it will shut down after normal idle..just dies for no reason like it looses power/spark..this also happens when I tested the –‘ve terminal on the coil which started to spark when I put it to the negative part of the battery while the car is running??

Conclusion:

1.Coil faulty – how do you test this with a multimeter??

2. Balast resistors? – I have a silver one and a blue one..they are cold no heat even after 5mins of the ignition being left on?? Normal?? Again how do I test it? Both look good no cracks but the blue one looks very old?? but both have power on both ends???I have reda ohm test is not going to show anything-true??
Ok guys any help would be greatly appreciated and drinks are on me if I get it starting first shot !

Thanks again

[250 Coupe]

Only thing I could find on the Optospark 500n is that it dates from the early '80s and uses a shutter like the Crane XR units. If I had to guess, based on the easier start after leaving the key on for a few minutes, that you may have a leaky electrolytic capacitor.

Another thought is that there may be bad solder joints on the board inside the unit.

As for the differing voltages on the -ve terminal, if you check it when the ignition box has just asked for a spark, (same as points being open) the coil has voltage on it but no current through it and thus the -ve terminal will read the same as the +ve teminal.

When you checked it with the engine running, the ignition box was switching the coil faster than your meter can register. The spark came from the collapsing field in the coil which produces about 300 volts at the -ve terminal.

Michael

[Dbglobal]

I had a similar problem, Have you checked the transistorized ignition box? My car would sometimes just turn over and no spark, then all the suden bang it would start with no problem. Turned out to be the transistorized ignition box.

[body_infinity]

Originally Posted by 250 Coupe

Only thing I could find on the Optospark 500n is that it dates from the early '80s and uses a shutter like the Crane XR units. If I had to guess, based on the easier start after leaving the key on for a few minutes, that you may have a leaky electrolytic capacitor.

Hi Michael, I will try again as I had a look at the ignition box (see pic) it all looked good but did notice the capacitor at one end had what looked like corrosion? I think anyway it is rated as 220uf 35v.....an ohm test showed 361 ohm?? hmm could this be it? worth replacing?? anyway any tips or ideas would be great...thanks again ;-)
B-I

[body_infinity]

guys been thinking and reading some posts re the trigger contacts do these have anything to do with the ignition on the 71 280se d-jet? also is it a must to remove the distributer to get these out or can it be done while in place? I never have had to remove the distributer before any tips methods....?

The reason i am thinking about the trigger points is that at times when I have difficulty starting the car it would start but run very rough..but again I have no problem once the car is driving and drives very well no miss fire..just at idle some times there is rough idle like running on a min cyclinders???

thanks again....

[alabbasi]

That's a common problem on these old D-Jetronic cars. Try cleaning them carefully before going out there and buying new ones. New ones are around $180.