Mercruiser 4.3 no spark

[rapidrick]

I am new to boating and just got a 1997 Searay 175 with a Mercruiser 4.3 LX. took it out 4 times and then it ran rough for a few seconds and I turned it off, checked the engine, levels, etc. and all seemed okay, but it would not start. Had to get towed off the lake which was not fun. I know I have fuel and I can see a stream of fuel in the carb when I advance the throttle, but I get no spark from the plug wires or the coil.Here is what I have done: (after reading a lot of posts on this great site and talking to some freinds)looked thru the wiring diagram and checked all switches.Disonnected the tach - no changeReplaced the coil & coil wireReplaced the sensor assembly in the distributorReplaced the rotor and dist. cap as recommended by one of the Merc mechanics when I bought the sensor.I have checked the typical things like the run switch, shift switch etc. there is a new battery with good, clean cables. Engine cranks nicely but still no spark.I have 12.82 V at the battery and from + coil to block and from - coil to block I have 12.74 volts. Sorry to be so long, but I want to get all the info you guys might need to be able to help. As far as I can see the only ignition related part left is the ignition module which is a 384.00 part. I dont want to replace this unless I am sure it is bad and I dont want to throw in an expensive part if I still have some electrical condition that could cause a new module to fail.Any help is appreciated.Rick

[Bt Doctur]

sounds like the module at this point.you can substitute a v-8 module ,beg/borrow for testing only.you may want to double check the ground connection of the module on one of the rear mounting bolts. maybe dirty/corroded/loose.

[rbezdon]

or possibly the wire running from the - coil to the module. Your electrical readings show the system is OK up to the module. The full battery voltage at the -coil tells that the module is not switching on and off like the points do in a standard ignition. Check the )12 to the module (if you have two wires going intoi the distributor and make sure the distributor housing has continuity to ground.

[Don S]

The first question is, do you have the Thunderbolt IV or Thunderbolt V ignition.Different troubleshooting charts for each.You may have found that replacing good parts gets expensive real fast.

[rapidrick]

I will try the suggestions from vatter41.The ignition is the Thunderbolt V and yes replacing parts that dont solve the problem is expensive and frustrating. I am working from a small amount of knowledge and a large amount of suggestions like 'No spark - it has to be the coil', then when still no spark it just 'has to be the sensor'.....so I am having fun.

[rapidrick]

I just tried the suggestions that vatter mentioned. I have a wiring diagram in the engine install manual so I have been able to check a good bit of the wiring. I have power to the module and it is grounded also. +coil to module has continuity. I have continuity from the dist. housing to the block and to ground in general.I said in my last post that I had the Thunderbolt V ignition, but my cowl just says Thunderbolt Ignition - I dont see anything that says IV or V. I am really stuck here and appreciate the posts so far and anything else would be welcome.

[Don S]

Then the first thing we have to do is determine which distributor you have. Here are your two type, check them with what you have.Here is the Thunderbolt IV distributor. It only has the ignition module mounted of the distributor (item a) The Thunderbolt V shown below has a lot more going on. Ignition module and a knock sensor.

[rapidrick]

I have the Thunderbolt IV based on the drawing. There is nothing else mounted on the dist.

[Don S]

[rapidrick]

this will be a great help. Thank you very much, I will check this systematically tonight.

[rapidrick]

this is what I needed all along. I re-checked each item down the list and it checks as being the ignition module. I will order one tomorrow - a genuine Merc part. Is there anything I need to be aware of in changing this out? I have already removed it so I can match it up if I can buy it locally. Thanks and I will post my progress.

[Don S]

Yes, there is one more thing. It's the best tool you will ever own for that engine. Click Here

[bamadave]

The S in Don S. stands for super. I wish you were paid a nickel for everytime you helped someone. I only hope that I can personally meet you someday. The whole world could learn by example for all the unselfish time and knowledge you give us EVERY day. God bless and keep it coming so we can all enjoy the waters all over the world!

[rapidrick]

I got the ignition module today and installed it and it started up on the first try. It was a long journey, but its finally running. Thanks to Don S. - without your help it would have been difficult to do this, and thanks to everyone who posted here. As a note the new ignition module is not the same as the old one and some refitting is required. You have to cut off the old connector and put bullet connectors on the wires to attach to the new module. I chose to solder the connectors on as I didnt want to have any more problems. Signed - a happy boater.