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  #1  
Old 02-14-2007, 09:37 PM
Blue 72 250's Avatar
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Rebuild a Switchgear

72 114 - Has anyone rebuilt a transister for the ignition before? Is there a diagram of the circuit to be found? I cant imagine it would be that hard or expensive for the parts.

just wondering

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  #2  
Old 02-14-2007, 10:21 PM
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I am sorry could you be more specific? I have no idea what you are talking about.
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  #3  
Old 02-14-2007, 11:20 PM
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There isn't much to the switchgear. Mostly some good size capacitors and a TO-3 size transistor or two.

It's pretty hard to kill one but over time the caps will dry out and fail. MB electronics are also know to have their share of failed solder joints. To be fair, it's not MB's fault. Single layer PCBs without through plated holes tend to get broken solder joints.

Michael
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  #4  
Old 02-15-2007, 11:41 AM
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Thanks 250c- I have lost spark on the old girl and I am working through the system and saw the cost of a rebuilt one and wondered why it was so expensive.

I bet with the number of people going to petronix and the like there are some surplus parts out there.

If I find it is the problem I will ask around the forum.
blue
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  #5  
Old 02-15-2007, 12:18 PM
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I just replaced mine. $5 for one at the junkyard. You may get as lucky as I did.
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  #6  
Old 02-15-2007, 01:04 PM
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Quote:
Originally Posted by Tomguy View Post
I just replaced mine. $5 for one at the junkyard. You may get as lucky as I did.
One of the advantages of owning an old Benz.....
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  #7  
Old 02-16-2007, 01:06 AM
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You may want to read 230/8's piece on kettering ignitions. You can set yours up just as he says, bypassing the switchgear and the car will run. The switchgear was intended to extend the life of the points and provide a consistant spark but does not alter timing or dwell as a modern ignition does. It is also NOT a capacitive discharge ignition.

The debate on hall effect VS photo diode will rage on for ever but I went with the Crane XR700 in the 280C because it was $45 (remanufactured) and the XR3000 in the 250C because Pertronix didn't list a kit for it.

Here is a link to all kinds of good stuff if you can get past the fact that it's intended for MG's


Michael
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Last edited by 250 Coupe; 02-16-2007 at 01:33 AM.
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  #8  
Old 02-17-2007, 01:02 PM
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I rigged her up the way that 230/8 layed out on the other thread and got her running again. While I was at it, I reset the dwell and timing.

However, now when I am accelerating she is missing. I used both of the ceramic resisters, (one blue and other silver band) I think .6 and .9 ohm. I have a new bosch blue coil. I am wondering if I have too much resistance with out the switch gear in the mix.

One other "funny" thing about the new setup is my digital dwell meter is not reading a smooth reading. It kind of jumps around. Do you guys think that too much resistance is causing a weak spark, and a "jumpy" meter reading?
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  #9  
Old 02-18-2007, 10:52 AM
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The box can be opened and the transistor inside replaced. It's easy to do, once you know the right kind of transistor. There was a page up at www.mbz.org that had the info. It got lost in one of the system crashes a few years back.

I found a nice page that describes rebuilding a distributor cap. A handy thing for an M189... http://articles.mbz.org/engine/gas/dist/

But that's a subject for a different thread. -CTH
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  #10  
Old 02-18-2007, 12:46 PM
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Quote:
Originally Posted by Blue 72 250 View Post
I rigged her up the way that 230/8 layed out on the other thread and got her running again. While I was at it, I reset the dwell and timing.

However, now when I am accelerating she is missing. I used both of the ceramic resisters, (one blue and other silver band) I think .6 and .9 ohm. I have a new bosch blue coil. I am wondering if I have too much resistance with out the switch gear in the mix.

One other "funny" thing about the new setup is my digital dwell meter is not reading a smooth reading. It kind of jumps around. Do you guys think that too much resistance is causing a weak spark, and a "jumpy" meter reading?
I think you only need one resistor for a straight point ignition.

I don't care for digital dwell meters and tachometers as they are hard to get a trend from. Dwell can bounce around due to weak point springs and/or a worn cam. It can also be from worn distributor bushings.

Dwell is the ratio of on to off and isn't all that dependent on voltage.

Michael

PS, I still have not made it out to the garage yet but will do it today.
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Current have, 1983 300SD 126.120
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  #11  
Old 02-18-2007, 11:30 PM
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After reading through the information on many posts here and frustrated with the old girl I amgiving up on the switchgear. I bought a crane conversion and it should be here later this week. I will post if any problems

Thanks for the help.
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  #12  
Old 02-20-2007, 04:06 PM
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Found it!

Thx to somebody reminding me about archived websites....

http://web.archive.org/web/20010618183627/www.mbz.org/articles/ignition/bbox.html

...All of which, I will quote verbatim here...

Quote:
MBZ.ORG - How to Fix 350/450SE V8 Spark Boxes
By Rob Dickinson

I wish to share a bad experience turned great for us. My Dad had an ignition failure with no spark. The MB agents wanted a small fortune for the "Spark Box" and my elderly father was obviously upset about it. The Mercedes Dealer located the only new one to be found in the country then. My Dad, clutching at straws, mentioned that his son was in the electronics business and asked if he could return this box for credit if I could fix it. With a smile they agreed. You guessed it, on opening up the aluminium extrusion, alI I saw was an array of Bosch numbered components.
A quick multimeter check on "Diode" mode revealed that the largest transistor TO3 Can was open circuit. The search for an obscure (probably In-House) Bosch number revealed nought. Common logic was that this transistor was the main output one that drives the coil, so it had to be worked very hard and had to be beefy. I looked for the highest current and voltage rated, physically equivalent heat sink mounted TO3 can package (Oval shape secured by 2 screws, for those not in the business) NPN transistor that I could find. The 2N3773 I put in there worked! The MB Dealer couldn't believe it, he honoured his credit offer and refunded my Dad. (The actual spec on a 2N3773 NPN Transistor is 140Volt 16Amp).

I also must mention that I checked the passive components around it, all caps etc OK. Nearly 10 years later and this "Spark Box" is still working! If anyone has an old 350 or 450SE V8 and a broken "Spark Box" in the garage cupboard, it is worth a try to spend less than 5 US$ and resurrect it as a spare. I am not going to accept any responsibility for damage anyone may inflict, however as I see it, you will know instantly whether it works or not. If it doesn't, throw the box out.

Rob Dickinson
-CTH
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  #13  
Old 02-20-2007, 07:06 PM
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Blue:

Yer trouble is with the two resistors. You only need the 0.9 ohm resistor. The extra is reducing the spark energy way too much. A transistor system uses two resistors, a 0.4 ohm and a 0.6 ohm. The basis system for the old-fashioned way of creating a sparl only needs a single resistor...a 0.9 ohm to match the MB coil.

But, since you seem to have given up on points, best of luck with the Crane (but, I'd keep the 0.9 ohm resistor, points and condensor just in case something fails on a dark night.)

230/8
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  #14  
Old 02-20-2007, 08:03 PM
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Actually, you need to match the coil with the restistors. Keeping the blue coil means keeping both resistors (one blue and one silver). Going with a red coil means get a red banded resistor. The gold band resistor is 0.9 ohms and goes with a black body coil.

Of course these color codes apply to stock benz balasts and genuine (rhymes with whine) bosch coils.

The ideal coil setup is the red one if you're going with a crane or petronix. Keep the blue coil if keeping the transistorized box. Go the black coil route if all you want is a conventional ignition without frills and switch gear, etc.

-CTH
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  #15  
Old 02-20-2007, 10:04 PM
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Can I run the blue with Crane?

I would like to run the blue with the crane because I just got it. If I must switch to the red I will, but if performance is average with blue I want to stay with it for now.

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