Logo
FULL SITE MENU

1988 Cobra 50 GM missing low coil volts

D

dwlog

New member
"I have been wrestling with a

"I have been wrestling with a power loss with this engine for some time. See earlier posts. I have determined that the voltage at the + coil terminal is about 5 volts (ignition on, points closed). I believe it should be 8 to 9 volts in that mode. I am checking for bad resistor wire and/or bad connections. I have miss-firing at all revs and 2000 is all she will put out (laboring heavily). Less than 10 knots at 2000 RPM. Any other ideas?"
 
David...you could run a jumper

David...you could run a jumper around the resistor wire right to the coil for test purposes. This would feed a full 12v to coil just like when starting and see how it runs. If the resistor has gone bad then you may have to substitute a ballast resistor in it's place. I'm assuming that you have a fully charged battery and a working alt putting out around 13.5 or more volts and that you have cleaned and tightened all battery to engine connections.
 
"Thanks Bob for responding. I

"Thanks Bob for responding. I checked the resistor wire and it is 2.1 ohms = within spec.
Again, with points closed, all primary wires connected and ignition switch on: the voltage at the + coil terminal is 5.5 Volts +/- and very close to that at both the S (sensing) and EXC (exciter) terminals on the back of the alternator. I believe the voltage at the + coil terminal should be at least 10 volts under these normal engine running conditions.
I disconnected both purple and red/yellow wires from alternator and the voltage at all three alternator terminals is = battery voltage. When I disconnect the distributor wire from the - coil terminal, but leave the ignition wires connected the voltage at the + coil terminal = 10.7 volts.
When I reconnect all the wires, but disconnect the condenser clamp from the base plate I get no + coil voltage.
I am thinking there is something haywire in the distributor system that is draining off the voltage: feed wire, points, condenser or the base mounting plate. A partial short to ground?
Again, my symptom is mis-fire at all speeds and no engine revs above 2000.
What do you think?"
 
"I would replace the condenser

"I would replace the condenser, something just doesn't sound quite right, when were the point's replaced last? You could have a high resistance build up on the contacts, at least run a nail file or some sandpaper through to see if it makes a difference or just replace them."
 
"Anyluck solving this? I have

"Anyluck solving this? I have a 1988 with a 5.7 liter. I was out with my wife and kids on the weekend and it all of a sudden started loosing power. When I pulled back on the throttle it stalled and I could not get it working. I have replaced the coil, cap, rotor, condensor and wires. Have not changed points as my local dealer was having diffuculty matching them. I did clean, but I am still not getting any spark. I can check voltages etc. but am not completely versed in the elect layout. Any advise? I did check the + of coil with ignition on and was getting 5.5 volts. Battery is fully charged."
 
"I am still struggling with th

"I am still struggling with this ignition miss-fire problem, making several tests on the various components and circuits. Right now I get 11+ volts to the red/purple wire at the EXC terminal at the alternator. With the coil wire disconnected from the (-)terminal I get 11+ volts at the (+) terminal. With the coil wire connected I get 5.5 +/- volts at the (+) coil terminal and 0.5 +/- volts at the (-) coil terminal. The resistor wire tests at 2.1 ohms, within OMC specs.
All above tests done with switch on, engine not running and points closed.
Under these conditions I measure 5 amps from alternator to coil and through coil to distributor. OMC factory manual says 1.0 amp maximum.
Coil resistance measures 1.6 ohms primary and 10.6K ohms secondary. OMC spec is 1.43 to 1.58 primary and 7.5K to 8.7K secondary.
The SOLEC manual for Volvo with same vintage GM 5.0 with Prestolite distributor and external 1.6 ohm ballast resister is much more detailed and informative than my OMC factory manual. Solec says there should be 9 volts minimum at the (+) coil terminal under the above test conditions. On the other hand, many 12 volt Delco coil/point systems on 1960's and 1970's cars and trucks did just fine with 5 to 6 volts at the coil (+). Is my OMC happy with the 5.5 volts instead of 9 to 10 and 5 amps instead of 1 amp current?? I just found my alternator belt tension was low. I have not noticed a lack of battery charge while running the boat, but could a loose belt cause lower than battery voltage at the red/purple wire alternator connection and on down the line to the coil, causing weak spark and miss-fire?
I am doing all testing at home with boat on the trailer. Hose and earmuff test run does not creat engine load and so will not tell me if problem is solved. The nearest launch is 30 minutes away. I have had her out 4 times in the last 6 weeks and on each trip I got the same miss-fire condition under load, starting at about 1200 RPM and topping out at a very rough and ugly 2200.
The points are Sierra new I have substituted two used condensers I have kept as spares. I believe they are both good because they were removed for routine tune-ups over the years. In no case have I experienced this type of miss-fire before.
All wires from alternator through coil to distributor and the points all test for continuity and I have found no shorts to ground."
 
"I have finally solved the pro

"I have finally solved the problem: Low coil volts was a non-issue. I replaced the very old carb mount gasket with some resulting improvement, but still not right. Convinced this was still an ignition problem I trashed the old coil and Prestolite distributor and I installed a new Pertronix electronic distributor and 40KV coil. Still not right. The only thing left was the plugs. These were NGK V power plugs that looked "as new" with very little electrode wear, although they were several years old (installed by the previous boat owner). They all tested 4K to 5K ohms which I believe to be normal. I installed 8 new Autolite 24's gapped at 0.040 for the hot coil and now she growls like a champ.
Go figure."
 
Back
Top