Circuit Diagram & Explanation

Engines&Magnets

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1 Autopsies, Schematics and How it Works

General Rewritten 28 October 05, updated February 28, 2006

This page includes autopsies and the actual circuit diagrams of the older 610759 Configuration A ( under the flywheel) Tecumseh Solid State Ignition and the newer 610906 ( above the flywheel) SSI with explanations of how the ignitions work. The actual circuit diagram was derived from the autopsy of actual units.

1. Older, Under the flywheel Solid State Ignition ( PN 610759 )

Added 29 Oct 05, This part comes in two different internal configurations, one has the charging diodes located on the Printed Circuit Board, PCB in the Electronics Module and the other configuration has the charging diodes located in the Charging Coil. This complicates the reverse engineering of the part and makes it necessary to divide this presentation into two parts:

Part A - Configuration A

Part B - Configuration B

How to tell the difference between Configurations is covered under the Repair page.

Part A -Configuration A:

I have a real problem with the circuit diagram of the TECUMSEH Solid State Ignition in all the manuals, Figure 1. If one follows the path for the current to charge the capacitor, it is open at the electronic switch or Silicon Controlled Rectifier, SCR. Thus this circuit will never work.

Figure1 .............techschema.jpg Figure 2........ Aoldssickt.jpg

Figure 2

Figure 1

The complete actual circuit diagram for PN 610759 is shown in Figure 2 and is divided into three sections. At the right, set off be green Xs is the Charge Coil. In the center is the Electronics Module. On the right is the Pulse Transformer set off by red Xs.

The process starts when a magnet speeds past the Charge Coil and generates a series of pulses. See oscilloscope photo Figure 3. This charging process is taking place typically 180 degrees of crank shaft revolution before the spark. The time between the two positive peaks in Figure 3 is that for one crank shaft revolution.

Figure 3

The wire for the Kill Switch is connected to the output of the charge coil to short out this charging voltage, thus no spark. In some configurations, the wire is directly connected to the charge coil. In another configuration, ( PN 610965 ), there are diodes in the circuit. See Figure 4. In yet another configuration, ( OH 140 Stator , PN ? ) there is a 40 olm resistor in the circuit. Kohler uses a 220 Ohm resistor in series with the kill switch. Chargcoil965s.jpg

Figure 4

Figure4a

A simple easy way to test the Charge Coil is to touch the terminal when the engine is cranking to feel a tickle from the pulses. The multimeters we use are made for either a constant DC voltage or a sine wave alternating voltage. The pulses don’t fit either case.

Moving on to the Electronics Module, this voltage pulse passes thru a 820 Olm resistor, R1 and diode D4 to charge the capacitor, C1. In the circuit diagram, the diodes D3 and D4, act like a check valves, and pass the positive part of the pulses. ( I think, diode D3 provides a current path for a magnetic field to develop in the charge coil for the negative pulses when the magnetic field is reversed.) The other end of C1is essentially grounded by the primary winding of the Pulse Transformer, .2 Olms. Initially the electronic switch or SCR is open and may as well not be there as the capacitor is charging. When the capacitor is fully charged, it contains a lot of electrical energy. Once the capacitor is charged, it just sits there until the SCR fires / spark event, unless it is defective. A leaky capacitor will loose its charge thru internal defects. The wave form at test point TP2 is shown in Figure 5. 2DeckerAyes.jpg

Figure 5

Figure 6

When a trigger pin or magnet , depending on the configuration, passes the trigger coil, Ls (start) or La (advanced) , a tiny trigger voltage is generated that switches the SCR from an open state to a closed state instantly. The wave form at the SCR gate, TP3 is shown in Figure 6, Gate.jpg. The relative amplitude of the negative pulses change when the RPM changes.

Figure 7

When the SCR fires, it dumps all the energy stored in the capacitor directly into the primary of the Pulse Transformer, PT. Figure 7, PTspark.jpg. The very high voltage is simultaneously produced in the secondary winding of the Pulse Transformer which fires across the spark plug. As you can see, this is a very short duration event. When the capacitor voltage decays to zero, the SCR resets to the open state. At every opportunity, I remind all to always have a grounded spark plug connected to the Pulse Transformer when turning the flywheel.

The Kohler version of their CDI has the capacitor physically located with the Pulse Transformer ( Ignition Coil), Kohler PN236836. Thus one might be able to replace the capacitor ( and Pulse Transformer) with the Kohler part. For Configuration A, some provision should be made for diode D6. The value of the Kohler capacitor was measured by Klaus Wolter at 4mf and by me at 6 mf, which is some what higher than 1 mf for Configuration A and 2 mf for Configuration B, but it should work. The current for the SCR may be higher with a higher value capacitor.

Part B - Configuration B

Part B - Configuration B

I need failed units to do an autopisy here. So far, I know the Electronics Modules are different. Configurating A has the charging diodes on a PCB and Configuration B does not have a PCB ( it uses point to point wiring ) and the charging diodes are in the charging coil. Configuration B probably fewer components. The good news is that, to the extend I can tell at present, both ends of the main capacitor are availabe at the external connectors so we don't need to open it up to fix that.

2. Autopsy of Older, Under the flywheel Solid State Ignition and actual schematic. ( pn 610759) Revised 28 Oct 05

Configuration A:

The original part looks like the Electronics Module in Figure 8,SSI.jpg. The naked parts are in figures 9, NakedEM.jpg. and 10, PCfoil4 , the latter being the Printed Circuit Board, PCB, with the connections of the various components labeled. Figure 2 is the schematic of the assembly which was generated from these naked parts.

Figure 8

Figure 5

X Ray View

Figure 9

The actual circuit is essentially the same as what I expected with a few interesting surprises. The presence of Diode 6 across the Pulse Transformer primary might have two functions. The first is make sure that C1 does not already have a negative charge when it comes time for the positive charge. This could diminish the power stored in the capacitor. The second is to it limit the negative voltage across the Silicone Controlled Rectifier, SCR. SCR’s have a limit on the Peak Inverse Voltage which when exceeded ,turns the device from a semiconductor into a resistor by rearranging the molecules in side. This is why it is always a good idea to have a grounded spark plug hanging on the secondary of the Pulse Transformer. Also, the SCRs made 30 years ago are not as nearly as forgiving as those made today.

The presence of Resistor R1, and likely R3, would limit the peak input current charging the capacitor. I am not sure why.

You can see the trigger coil for starting, Ls, is connected directly to the gate of the SCR. The advanced trigger coil, La, has the resistor R2 in series with it to reduce the voltage going to the gate of the SCR. As the RPM increases, the voltage will increase and when it gets up to the trigger level, it will fire the SCR ( turn it from the open state to the closed state) some what ahead of the arrival of the voltage from Ls. ( Kohler has the resistor in parallel with the trigger coil.) There is also a second capacitor, C2 in the trigger circuit. This is a polarized type of 1.6 mf. It is

probably a Tantalum type which are prone to failure and self healing.

Armed with the actual circuit diagram, Figure 2 and the map of the component leads, Figure 10, I have started to repair the units. One principal I think we should follow is to maintain the original circuit design and not modify it. When replacing the original capacitor, I mount the new one on the pulse transformer with a new D6. This maintains the original circuit design but makes the repaired parts not interchangeable with the original parts. With a little configuration control here, the parts of the repaired units would be interchangeable with other repaired parts. So far all the failures I have found are the main capacitor C1. I can make this repair for less than $35. Not with standing, I have tried to provide the information for a good mechanic with a little electronics experience to change the capacitor. We really need more statistics on failures to just assume the failure is always C1, but the equipment required , (Oscilloscope and a way the run the ignition without a running engine ) would put the diagnostics out of reach for most. So, with no other choices, one might just try changing C1. See SSI Repair page, LINK

SSI Repair

The units are repairable, so please don’t through these parts away.

3. Autopsy of Newer, Over the flywheel Solid State Ignition and actual schematic. ( pn 610906 )

Figure 10A

Updated 27 Feb 06

This is still in progress, but here is the current status. The SSI we are concerned with here is the one on the left in Figure 10A, Part number 610906. A Charge Coil is required to make a complete ignition system. On the older HH 120 engines with a Delco starter motor / generator, the Charge Coil is PN 610965 which I am in need of. The best I can tell, the 610906 is used on the OH140 and OH160 engines and the 20 Amp stator, PN 610902 which I used for the work here. ( For more information on the Charge Coil see Page 5 of this web site.)

Figure 10

Figure11

Figure 12

Figure 13

The circuit is fairly simple. To the left of the X is the Charge Coil which puts out an alternating current. If disconnected from the SSI, it looks like an ordinary sine wave, Figure 14. When measured with a multimeter set on the AC scale, I get 100 Volts @ 500 RPM, 200 VAC @1000RPM, and 300 VAC @ 1500 RPM. This measurement at 500 RPM is near cranking speed and looks like a good way to test the Charge Coil.

When the Charge Coil is connected to the SSI, the wave form changes to a series of negative going pulses because of the diodes D1 and D2, Figure 15. These pulses charge the main capacitor, C1 to a high negative voltage since the electronic switch, Silicone Controlled Rectifier, SCR is open. Note the high capacitance of C1, over 5 micro farads, mf. When a trigger pin goes by the trigger coil, a trigger voltage is generated which goes to the gate of the SCR and turns it on, changing it from an open switch to a closed switch in a millionth of a second. At low RPM, the large air gap of the short trigger pin generates a voltage too small the trigger the SCR. As RPM increases, this voltage increases until until the trigger level is reached and the short trigger pin fires the SCR ahead of the tall trigger pin.

I am trying to repair a unit, Figure 13, for the fun of it and so far have not succeeded. So far I have replaced the main capacitor, C1 even though it tested good. I also replaced the SCR and it still does not work. I disconnected the SCR and simulated its switching function by touching a test lead to ground and get a spark. So , right now it looks like the problem is in the trigger circuit. Later, I found that the trigger coil was open and I don’t plan to fool with that. I don’t have another failed 610906 to play with so I could use one. I was planning to repair one and run it off an 60 HZ inverter instead of a charge coil to test that concept. See the end of web page 5.

Spark Tester

4.Tecumseh Solid State Ignition Trouble Shooting:

( Questions on this topic are ask so often, that below is a canned answer for a place it start.)

The early version of the Tecumseh, Solid State Ignition, (SSI), actually a Capacitance Discharge Ignition, CDI, has part number 610759 with the electronics module is under the flywheel. The newer version has electronics module above the flywheel and part numbers 610906 or 610748. As a CDI, it has no points and never connects to the 12 battery. Part of it is a charge coil which generates voltages to charge the capacitor. There is one wire which has an external connection to ground to kill the engine by grounding out these charges pulses. There is more at the web site below including pictures.

The first thing to do is to disconnect the kill switch wire at the engine to separate the source of the problem being the ignition or the tractor wiring. If you have a
Delco Starter / Generator then there should be a ½ inch square nylon plug at the rear edge of the blower housing at about the 8 o’clock position. Disconnect the wire and recheck for a spark. If you have a Tecumseh starter motor and stator under the flywheel, then there is a connector coming out from behind the flywheel with three pins. The outer two are from the alternator to charge the battery and the center one is the kill switch. Just disconnect the connector , crank the engine and see if you have a spark. You may need to shut off the fuel to properly shut down the engine.

The doodad above the flywheel on the earlier version is the Pulse Transformer, pn 610760. On the later version, the pulse transformer is inside the SSI module. While we are at it, measure the resistance between the connector to the spark plug and ground. If the reading is open or erratic, cut off the spark plug boot and measure the resistance from the stranded wire. The resistance should be between 2 and 5 thousand ohms. Often, there is a problem with this connector and I would replace it with a Briggs & Stratton spark plug terminal.