prestolite ignition question

[dirktoesup]

I would appreciate any feedback on this ignition question. I have a 1989 Force 125hp o.b. which seems to run acceptably, however in reviewing the prestolite wiring diagram I note the following:
1. the diagram indicates that the ignition coils need to be grounded(black wire). the four ground wires coming off the four ignition coils are attached to the mounting plate(bracket) and this is not grounded to the block since it is rubber mount isolated. The two CDI units are grounded to the block(not the bracket) via two black ground wires but are not electrically connected to the mounting plate(bracket) since the bracket is isolated from the CDI units via their own rubber mounts. The two black with yellow stripe wires are connected to the terminal strip and seem to function as they are supposed to. It seems to me that the six black coil ground wires and CDI unit ground wires should be attached together probably to the mounting plate(bracket) as commen ground. If I simply attach a ground strap between the mounting plate(bracket) and the block, I lose spark to #3 and#4 cylinders. I find nothing specific in any manuals or on line to either support or otherwise answer my question. Everything I've seen simply shows all 4 ignition coils and the two CDI units need to be grounded(without being specific as to being grounded to the block or to the mounting plate. Can anyone clarify this for me? Thanks in advance

 

[pnwboat]

Re: prestolite ignition question

The coil mounting plate and engine block should be at the same ground potential. In other words, it shouldn't make any difference whether you ground you CDI units to the coil mounting plate or the engine. Having said that, on my 1989 125HP motor, there is a ground wire that runs from the coil mounting plate to the block. I added a second heavy gauge wire between the plate and block just to be sure. Also, the coils and the CDI units on my motor are all grounded to the mounting plate.

I bet if you add the jumper wire between the mounting plate and block, it shouldn't make any difference where you place the CDI grounds. They could go either on the block or the plate.

 

[Frank Acampora]

Re: prestolite ignition question

That particular ignition has only one itty-bitty ground wire running from the mounting plate to the bypass cover. I never liked it.

I use crimp connector rings and "daisy-chain" all four coils and both CD boxes with some #18 wire to the ground connection on the terminal strip at the exhaust cover side.

 

[dirktoesup]

Re: prestolite ignition question

thanks for the reply,in this particular case, the CDI(s) are grounded to the block and the coils are grounded to the plate(independent and insulated from the block). Although it runs satisfactorily I don't think it"s correct in that measured dc voltage between plate and block is 95-115 vdc in present configuration and as stated if a jumper to ground is installed between plate and block, I lose spark to #3 and#4 cylinder. I will try grounding CDI units to plate and then installing ground between plate and block.Thanks again for the guidance.

 

[dirktoesup]

Re: prestolite ignition question

Thanks for the info. along with the info from the next post see my response. Thanks again

 

[pnwboat]

Re: prestolite ignition question

Something doesn't sound right. Adding a jumper between the block and mounting plate should not have any affect on cylinders 3 and 4. Sounds like something is not wired correctly. I don't see how the coils are operating correctly without the mounting plate grounded to the block.

 

[Frank Acampora]

Re: prestolite ignition question



Yeah! Mounting plate should be at ground potential. There should be no voltage between it and the Block. Something is not correct with the wiring. Pics would help.

Try grounding everything to the plate and then running a ground to either the block or the terminal strip on the exhaust side. If you have the time, connect all grounds to the wire in addition to the plate. If i get a chance i will take a pic.

Notice the blue ring connectors attached behind the mounting plate to everything? You can't see the wire that runs around the block to the terminal strip though.

 

[dirktoesup]

Re: prestolite ignition question

to Frank and pnwboat--Thanks for the help. Today I properly grounded the CDI boxes to plate as well as installed additional grounds from the four coils to the plate and block. This solved the runability problem and eliminated the "hot" plate. Thanks again.