SSI Repair

Generated 24 October 05; Updated 15 February 06

This page concerns the repair of the Tecumseh Solid State Ignition SSI electronics modules and some trouble shooting procedures. This section has the general information and succeeding sections the specifics for each configuration then repair cases under the different configurations. The introduction of SSI Replacement, a following web page, identifies the configurations and is not repeated here. Similarly, a previous web page; Autopsies, Schematics and How it Works; is not repeated here.

General Tests:

It often happens that a fault in the tractor wiring or switch will cause a No Spark condition. To make sure it is the ignition that has failed, disconnect the kill switch wire at the engine and see if the engine has a spark or will run. If the engine runs, the ignition switch is in the run position and the engine dies when you touch the kill switch wire to the kill switch terminal, then the problem is in the tractor wiring. The location of the kill switch terminal is shown in the figures below.Figure A, SSI under.jpg and Figure B, OH140Stator2.jpg

Also, it would be a good idea to measure the resistance between the spark plug terminal and ground. If the reading is high or erratic, see the page on Pulse Transformer Repair.

Tickle Test:

The easiest way to check if the charging coil is working is to touch the kill switch terminal while the engine is cranking. You should feel a slight tingle as the charge coil puts out a few pulses on each revolution. On the stator configurations, you may need to use a small pointed object like a nail to make contact with the center pin of the connector.

..Repairing the older 610759, ..

Added Sep 06;

In the figure below , note the mounting plate and the Electronics Module mounting stud. There should be zero resistance between the center of the stud and the mounting plate. If you detect some resistance with a good ohm meter, disassemble, clean the mounting plate and nut and put it back together. If you don’t have a good meter and the unit is not working, just take it apart, clean and reassemble. Then test to see if it works. Note that there is an adjustment for the air gap at this point which should not be disturbed.

Repairing the older 610759, Configuration A.

This part comes in two different internal configurations, one has the charging diodes located on the Printed Circuit Board, PCB in the Electronics Module, Configuration A and the other configuration has the charging diodes located in the Charging Coil, Configuration B. So far, several of the Configuration A’s have been repaired by replacing the main capacitor and none of the Configuration B have been repairable. So, how do we determine the Configuration.

How to tell the difference between Configurations:

The circuit diagrams of the Charge Coil for the two configurations are shown below in Figures C and D. ChargeCiolA.jpg and ChargeCoilB.jpg

To determine the configuration, the SSI unit needs to be removed from the engine. Be sure to scribe a mark around the mounting bolts so the ignition timing will be correct when it is replaced. There is a wire that goes from the Charge Coil to the Electronics Module labeled DC or AC Input in Figure E. The kill switch wire goes here also. Remove the nut to disconnect the wire from the Charge Coil. The measurements below are made between this wire and the mounting bracket.

Measure the resistance in both directions by reversing the red and black meter leads. If the meter reads between 925 and 960 ohms in both directions, then the configuration is A, no diodes in the Charge Coil. If you have a diode test function, and measure it, you should get zero volts in both directions.

If your resistance measurement on a digital meter is open / 7 meg ohms, either the coil is open or Configuration B. If you test the coil on a diode test and get open in one direction and about .9 Volts in the other, then the diodes are present and you have Configuration B. ( What you are measuring is the .48 junction voltage across each of the two silicone diodes. And they would both be good.) There is a terrible tragedy here. I and I am sure others have interrupted an open or high resistance measurement as an open, failed coil not realizing we had a diode inside, and destroyed a good coil.

Removing the Wrapper:

To replace the main capacitor in 610759 configuration A, we need to remove part of the potting . This is done by first removing the potting material around the edge of the PCB to expose that edge. See Figure F, unwrap.jpg.

The potting can be removed using a file or a grinder working at an angle of 45 degrees to the surface. The potting can also be removed by heating it with a Butane or Propane torch and carving it off with a Stanley knife. As soon as the edge of the PCB is located, stop. Next, apply some heat to the surface of the potting material and after 10 to 15 seconds, insert the edge of the Stanley knife between the PCB and the potting at the terminal to the Pulse Transformer and it usually pops right off. On the units I have done, I put my finger on the PCB; it was warm but not too hot to touch. One could place a damp rag on the unit to cool it off. The exposed part is shown in Figure 1. DeckerCfix.jpg

The components to be replaced ( C1 and D6) are abandoned in place by disconnecting them at one end. This is done by making a cut in the foil and removing a thin slice of foil.

We need to make use of the existing connector for the wire that goes to the Pulse Transformer, so a Jumper in soldered in as shown.

The new main capacitor, C1 is too large to fit under the flywheel, so we will put it at the Pulse Transformer where one of the connections goes anyway. The repair technique has evolved to where I mount a terminal lug on the PT mounting bracket for the connections. Figure 2, PTMod.jpg The capacitors for an earlier repair are shown in Figure 3, NewCap1.jpg. The circuit diagram is simple but shown in Figure 4, PTMod1.jpg

The new capacitor is two NTE MLR474K630, .47MF, 630Volt in parallel . If you go to http://www.nteinc.com/, you can find the dealer closest to you. They will probably have to order it for you. You could also use a single NTE MLR105K630, 1.0 MF, 630Volt,$6.62, but it will be a little bit bulkier. I used E-6000 adhesive to fasten the new capacitors to a sanded area of the Pulse Transformer. The E-6000 came from a craft store.The diode D6 is a 1N4007.

I want to thank John Decker for offering the failed SSI module for me to play with and I am glad I could return it in working condition. I would also like to thank Mike Cummins who provided failed parts that did not survive the autopsy that was necessary to recover the actual circuit design and component location.