79 chrysler 70 hp voltage issue

[casper75]

I have a 1979 chrysler 70 hp. When trying to start up with 12volts the starter doenst realy want to turn and cant get it started. Now if I hook 24 volt it starts no problem. Any ideas

 

[Frank Acampora]

Re: 79 chrysler 70 hp voltage issue

Yeah! Make sure your fire extinguisher is functioning and handy! If you hook up the battery cables to 24 volts, the whole electrical system is at 24 volt potential. The components and wiring are not designed for that. Neither is the starter.

First check the 12 volt battery to be certain it is delivering its rated voltage. Normal fully charged voltage is around 13 volts and cranking voltage can drop as low as 9 volts. If fully charged voltage is less than 11 volts, or cranking is 9 volts or less, replace the battery.

What you need to do is to remove the starter and check it out. See if there is binding both in the starter and the engine. With the plugs out the engine should turn freely.

If that test is ok, then you need to disassemble the starter and check the condition of the brushes and commutator. Dirty or worn brushes or a dirty commutator will form a high resistance circuit that 24 volts may have enough potential to energise while 12 volts won't.

Lastly, you would need to have the motor tested on a "growler" to see if all the windings are intact. One or two broken or shorted windings will cause slow cranking. This requires rebuilding or replacing.

If you don't want to do all this, then look for a good used starter on the auctions. Any Chrysler or Force 3 or 4 cylinder starter (70 to 140) will fit.

 

[casper75]

Re: 79 chrysler 70 hp voltage issue

I am running to brand new batteries using a perko battery switcher. I even tried to start it with the switch set at both batteries. So low voltage is not an issue. The starter is only 8 mths old. motor spins free with all the plugs out.

 

[Frank Acampora]

Re: 79 chrysler 70 hp voltage issue

If you are sure the starter motor is in good condition, it is time to check ALL electrical supply to it AND grounding. Poor grounds caues many electrical problems. Here's a couple of ideas:

The starter motor is grounded through the mounting. Be certain that the mounting bolts and the mount are clean. Be certain the top cap of the motor is clean also. Be sure that it is making good contact with the body of the starter and the cast mount on the block. Try checking the black negative battery cable to be sure it is making adequate ground with the block. Use a lock washer on each side of it. They will "bite into the terminal and the block, breaking any corrosion. Do the same for both wires on the battery side.

Check both ends of both battery cables to be certain they have not partially broken, giving the capacity of a smaller wire. Since 24 volts will need half the amps to deliver the same power, smaller capacity wires or poor grounding will work whereas on 12 volts they will not.

If your solenoid has burned contacts inside, they can form a high resistance circuit. Test this by jumping directly from the battery to the positive terminal on the starter, If 12 volts now spins the starter and cranks the engine well, then replace the solenoid.

Remember to check the Perco switch also. It is possible that it is making poor contact in the single battery position or has corrosion on the terminals and thus not delivering the full 12 volts to the solenoid. Run the positive cable from the battery directly to the solenoid and try to start. If the problem goes away, the Perco switch is suspect.

Do not continue to try to start the engine with 24 volts. If you are indeed getting 24 volts, you run the very real risk of damage to the starter itself, the coil, CD unit, rectifier, and wiring.

 

[casper75]

Re: 79 chrysler 70 hp voltage issue

well I replaced all the cables still same problem. I unhooked the perko and went straight to the batteries still same problem. I checked the voltage on the silenoid it reads 12.76 on one side and 9.87 on the other side. I unhooked the starter and it read 12.76 on the side that was reading 9.87. I cleaned off all the terminals on the starter no help. I took the starter off hooked it up straight to the battery and it spins fine . any other ideas

 

[Frank Acampora]

Re: 79 chrysler 70 hp voltage issue

I know you don't want to hear it, but you just proved that it is time to rebuild or try another starter.

Under no load, a damaged starter can still spin "fine" because it is designed as a relatively low speed, high torque, high horsepower motor. So, for example, if say, 1/4 of the windings were either not getting voltage or shorted, or open, the starter would still spin at nearly the same speed with no load. However, under these conditions, under load it would not develop it's full rated horsepower and would crank slowly.

You have checked everything else, now it is time to check the starter even though it is only 8 months old.

 

[casper75]

Re: 79 chrysler 70 hp voltage issue

any other way to test the starter

 

[Frank Acampora]

Re: 79 chrysler 70 hp voltage issue

It is a standard Prestolite (or possibly Bosch) starter. A starter repair shop should be able to test it for you. Some auto stores test auto starters but I don't know if they can also test these marine ones.

 

[casper75]

Re: 79 chrysler 70 hp voltage issue

Frank

Well I finaly took the starter down and had it checked out and yup it was bad. Had them rebuild it and now works perfect