1971 280s ignition

[Mark N]

The ignition on my 71 280s (m130, twin Zeniths) is making me crazy. I just rebuilt the motor and on the occasions where it runs right, it runs GREAT. But I'm getting an occasional, intermittant cut-out, which is definitely ignition. I hooked it up to a scope to try and get an idea what's going on, and the secondary kv seems low - 7-9kv or so. It's blue coil, which will throw out strong kv when pulling a plug wire back from a plug a bit. Dwell is at 37, timing's at 10 degrees advanced. This is the old transistor/switchbox setup, with two resistors. I'm getting 7.3 volts at the hot side of the coil with new resistors. I had a hard time initially getting it to run, and finally succeeded only after re-installing the original points instead of a fresh Bosch set. The original points are an unknown brand, with one side of the contact having a hole through the middle. I read somewhere (probably here) that those worked better for this system. It certainly works better for me. But not perfect.

I noticed when hooking everything back up after re-installing the motor that there was a wire on the negative side of the coil connected to ground. Since the motor only runs with that wire disconnected, I wonder why it was there?

Is a condensor neccessary for this system? Anyone have a wiring diagram? This info seems difficult to come by - it certainly isn't in my shop manual.

I know one answer to my problems is to go with a Pertronix or Crane setup. And I may end up doing just that, but it really drives me nuts when I can't figure out a (seemingly) simple system such as this. It worked fine prior to the rebuild, so why doesn't it work now? What am I missing?

Oh, btw, I tried swapping in a (supposedly) known good switchbox, to no avail.

Any help MUCH appreciated.

Thanks!

-mark

[ctaylor738]

7.3 volts to the coil is high - I recall that it is supposed to be 3.4 or something like that. I think the dwell is supposed to be 34.

The capacitor is not needed per the MB manuals.

I have a hard copy of the wiring. Send me an email to remind me, and I'll scan it and send it to you.

Like you, I have never understood how the coil could work with the positive side grounded.

[Mark N]

Thanks Chuck. I thought 7.3v was high too. I just put in new resistors so that's kind of puzzling. Yeah I don't understand that ground wire - it was on the (-) side of mine, but won't run with it connected. I'd love a scan of the diagram. I'll send you an email.

Thanks,

-mark

[Arthur Dalton]

ON cars with trans ignition, if there is a second wire on the + term of the coil , this wire is not to grd., rather, it comes from a feed terminal on the starter solinoid to by-pass the ballast for start only. The earlier ign did not have this .
There may also be RFI capacitors on the coil, but these do not directly ground the coil terminals.
I think you are correct about the neg coil ground on switchgear type systems, as the points do not complete the coil circuit . The points are low voltage switching that trigger the transistor in the S/G , and that sends power to coil , which should complete coil through a neg ground.
Anyone have that schematic??

[MunichTaxi]

I have a attached a transistorized wiring diagram, previously attached in this thread.
Transistorized wiring diagram

[Arthur Dalton]

Do you have the front part for the schematic?

As you can see , the ground on the coil is to complete the coil circuit as it gets its positive switched side of the coil from the switchgear. Whereas , with standard points , the switching is done by the points on the neg side of the coil. [ to ground]. In other words , the with S/G , the sig to the coil is swiched before the coil, whereas with reg point ignition, the sig is switched after the coil [ on the neg to ground side.]

[spartonboat1]

That configuration was a transistor triggered by points setup- relay logic if you will. It is very sensitive to having the correct points installed, due to the very low voltage needed to trigger the transistors. It is seldom known that it required a special set of chrome plated points from Bosch, not just standard points. Standard points will develop a light oily film. The voltage across the points in this system is so low that the points are not kept "clean" by the constant make/break voltage, hence the chrome.

So verify that the points are the exact "correct" specified chrome plated units, and then work backward to other issues. Also, the early transistor units were intermittent in high underhood heat, such as on a hot day. But they were upgraded after 73-74 and were less of a problem later.

One other unrelated point- make sure you are applying engine oil to the little cup underneath the distributor...it is not oiled by the engine and if not manually oiled, the distributor bearings will dry up, seize, and tear up lotsa lovely stuff.

Finally, don't mess with the factory settings of the carbs- but do clean them thorough and DO NOT use fuel with any ethanol- it will plug up the fine passages in the carb idle circuits big time.

[Mark N]

You guys are amazing. I have a good understanding of how my ignition works now. Or at least, how my ignition *should* work. It works now, and the car runs great. For about twenty minutes. Then I have to play with the points. Then I get another twenty minutes... Someone (prior to my owning the car, of course ) played with the wiring it seems, bypassing the switchgear box. So now it's set up in more or less conventional fashion, but with too much current going through the points. Even with fresh ballast resistors, I'm getting 7 - 9 volts at the (+) side of the coil, which is apparently too much. I'm baffled. And I'm tired of being baffled. I want to enjoy my nice new motor, dammit. So I swallowed my pride, threw in the towel, and ordered a Pertronix Ignitor kit. I'll let ya'll know how it turns out.

Thanks!

-m

[Arthur Dalton]

Originally Posted by Mark N

You guys are amazing. I have a good understanding of how my ignition works now. Or at least, how my ignition *should* work. It works now, and the car runs great. For about twenty minutes. Then I have to play with the points. Then I get another twenty minutes... Someone (prior to my owning the car, of course ) played with the wiring it seems, bypassing the switchgear box. So now it's set up in more or less conventional fashion, but with too much current going through the points. Even with fresh ballast resistors, I'm getting 7 - 9 volts at the (+) side of the coil, which is apparently too much. I'm baffled. And I'm tired of being baffled. I want to enjoy my nice new motor, dammit. So I swallowed my pride, threw in the towel, and ordered a Pertronix Ignitor kit. I'll let ya'll know how it turns out.

Thanks!

-m

When one converts back to standard ignition [ from transistor S/G ]. they take the two resistors used in the trans system and put them in series between the ign. switch and the coil positive terminal. That give correct ballast and coil voltage. The old systems had a .9 ohm ballast, the series give you 1.0 , close enough.
For best results , guys get rid of the points Crane/Petronix] and go with the Benz red coil with a 1.8 ballast.

[Mark N]

The Pertronix kit is installed. It was so easy I thought for sure I must be missing something. The magnet ring took a bit of tapping in order to fully seat on the distributor shaft, as it seems like it's seated when it still has a ways to go. Use a deep socket and be gentle. Otherwise there's not enough room for the rotor. It fired right up, and seemed noticeably smoother. I also put in a Pertronix Flame Thrower coil, mainly because the old coil was rusty and ugly, and opened the plug gaps up to .034. When I get a chance I'll hook it up to a scope and see what kind of voltages I'm getting now.

With the ignition issues FINALLY straightened out, it was time to put the wheelcovers back on, polish the oxidation out of the paint, and top it off by re-installing the hood star. She runs good. She looks good. Seven months of time, love, and money are finally being reciprocated. Of course, we're not done yet.... carpets, woodwork, sub-frame bushings.... at least the heater blower works

I'll have to put up a picture!

-mark

[Mark N]

I'm feeling kinda like a proud poppa, so here's a couple shots of my 280s, as promised. Had to wait for a semi-sunny day here in Seattle. Thanks to all for helping nurse me through my ignition travails.

-m

[Atchisvw]

Which Pertronix ignition kit did you install?