Troubleshooting preamp 75 HP

[WILL_GENT]

My 1983 Chrysler 75 HP (758h3f) stop running yesterday. I followed the "The Marine Doctor's" advice on troubleshooting, my CDI box appears to be good but the preamp in the disributer but may be bad. The preamp seems just to be an optical switch, is that the basic of it ? By turning the distributer with the blue wire attached it is putting out 2.4 volts - 0.0 volts to the w/b wire side as the optical wheel opens and closes letting the infrared light pass. Is there an ohms value test I can do ? I am on lay off right now and can't afford the "shot gun" effect in replacing parts. Does anyone know where I can get a used one at? Also the rotor seemed impossible to pull off, I was afraid of breaking it. The same my hold true for the optical wheel. Do these just pull off, or is there some trick in removing them? If anyone can help I will greatly appreciate it !

 

[Frank Acampora]

Re: Troubleshooting preamp 75 HP

Yes! The pre-amp is just an optical switch. UNFORTUNATELY, there is no practical way to test for a defective one. The only test is to replace it with a known good one and see if that solves the problem.

The rotor, shutter, and shaft is an assembly, epoxied together and will not disassemble. It can be removed from the distributor by taking off the top pulley but that's it.

Try to find a pre-amp at the Chrysler Crew, or franzmarine@aol.com. Franz is at the Chrysler Nationals in Oklahoma this weekend and may not answer email for a day or two. I would not recommend getting a used one since they can not be tested for function under load. New ones are a bit pricey but this is a case where you probably need to bite the bullet unless you can find a really cheap junk engine with one in it. I have one or two, but I an saving them just in case---Since I have two engines with electronic distributors, I would not part with these extras for any amount of money.

BTW: The pre-amp is the same for all three and four cylinder engines. There are two different colors and they work slightly differently but all are interchangeable.

Your alternate solution would be to find a used rotor and cam assembly from a points distributor and convert. The same CD box is used and the points signal it to dump the capacitors.

 

[WILL_GENT]

Re: Troubleshooting preamp 75 HP

Thanks, Frank. I am sure now that the pre-amp has failed..it is leaking voltage (2.2 V) from the blue power supply wire to the white/black control wire side. The CDI box will put out a nice spark thru the coil when I touch the white/black wire to ground. Strange this failed 30 minutes into a test run after rebuilding the carbs and fuel pump. The motor was running great until the pre-amp gave out ! By the way, the reason I rebuilt the carbs and fuel pump is that my fuel useage skyrocketed all of a sudden. Turned out my fuel pump developed a hole in it's diaphram dumping gas. I am searching the web now for the best price on that pre-amp...cheapest is $87 so far. Also looking for the used distributer with points alternative. Again, thanks for your help.

 

[chiquita]

Re: Troubleshooting preamp 75 HP

Thanks, Frank. I am sure now that the pre-amp has failed..it is leaking voltage (2.2 V) from the blue power supply wire to the white/black control wire side. The CDI box will put out a nice spark thru the coil when I touch the white/black wire to ground. Strange this failed 30 minutes into a test run after rebuilding the carbs and fuel pump. The motor was running great until the pre-amp gave out ! By the way, the reason I rebuilt the carbs and fuel pump is that my fuel useage skyrocketed all of a sudden. Turned out my fuel pump developed a hole in it's diaphram dumping gas. I am searching the web now for the best price on that pre-amp...cheapest is $87 so far. Also looking for the used distributer with points alternative. Again, thanks for your help.

Where did you find the preamp for $87