Engine Performance Problem with Main electrical switched to "On", runs better "Off"

[sirfinn]

Engine Performance Problem with Main electrical switched to "On", runs better "Off"

Hello,

Working on a new to me 81' M35A2. In fact, just drove it home this weekend after putting a new T-case in it and brake bleeding, windshield, and fluid changes. The previous owner complained that the engine power was very poor with the electric switch on, when it is shut off, it runs much better but will only rev to about 2k up or down hill(maybe the lack of in-tank fuel pump(off) or dirty filters), the it acts like it is limited on fuel. Everything runs fine at idle, and the charging system gage is in the green.

After disconnecting the power to the fuel pump and it had trouble starting, plugged it back in and started right up. During the drive home, running the headlamps and drawing down the battery, the belt was slipping when the power switch was turned on after running for an hour. It did not slip at the begining of the journey. The in-tank fuel pump whines when the switch is turned on "on", the batteries are setup negative to ground, it has the stock style 60Amp alternator, but it seems like the alternator is acting like a driveline retarder... maybe.

If it were a generator, my guess would be, it had reversed polarity. Is it possible the alternator was connected wrong and is acting like a reversed polarity or the regulator is wired up backwards?

According to the diagrams, it appears to be wired to the truck harness correctly. Is it possible the alternator field is backwards. Any thoughts on how to verify this or rule it out?

It could be a fuel pump, but it does not seem likely since it helps with starting.

The plan of attack is to solve the electrical issue first, then fuel filters and tune-up next. I will dig-in with a volt-ohm meter and isolate the alternator next. This is a very strange problem.

Thanks for any ideas!
-Sirfinn

 

[dmetalmiki]

Your truck does not have a dynamo fitted, and so can not be 'flash' alternate polarised. Negative earth is correct. The main switch does activate the in tank pump, so running performance issues with the switch turned on or off is showing up a fuel (supply) problem. That engine would happily 'turn' the generator seized solid, or rip the belts to bits trying. My 1/2 penneth, let us know the final solution.

 

[sirfinn]

Your truck does not have a dynamo fitted, and so can not be 'flash' alternate polarised. Negative earth is correct. The main switch does activate the in tank pump, so running performance issues with the switch turned on or off is showing up a fuel (supply) problem. That engine would happily 'turn' the generator seized solid, or rip the belts to bits trying. My 1/2 penneth, let us know the final solution.

Will do, it might be a few weeks before the time is available to work on it again... Best hypothesis at this point is the regulator is bad/wired to the alternator wrong. When the RPMs increase the voltage is backwards, the batteries win at low RPM and the alternator wins at high RPM and the electric fuel pump is trying to run backwards, hence the serious power drop. I will take the wire to the battery off, rig up a volt ohm meter between the alternator post and the wire and see if I get a less or more than 24volts at different RPMs... if this is the case I sould be seeing < 24 volts if it is charging correctly the voltage should decrease or even go negative on my digital VOM with the positive on the alternator terminal, negative to cable to battery positive. If the alternator is goofed up, it should increase the voltage > 29v. Did I get this right?

 

[gimpyrobb]

If charging correctly, you should see 28.8.

Have you hooked a v.o.m. to the slave port and watched the numbers while driving? That will help troubleshoot. What is voltage at rest, before you start it? Are the batterys new? Do they have water in them?

Ground to the - of the first batt, first batt + to the - of the second batt? + of the second batt to the starter?

 

[Jericho]

Ive found if the gauge is working its reasonable to use it to look at performance. If its in the red your out-put is high, any time ive had high out put it fried the alt any way, if its low(yellow and below, check it with the PSM 6 (multi meter, adj if you can ) make sure your ground is good and you belts are serviceable and properly adjusted. If all else fails null the alt and slave in 24 volts, does it run right now ?

 

[sirfinn]

The belt needs adjustment. It was slipping due to the load from running the lights off the new batteries. The voltage looks on target at the gage and the connector to the pump 24-28v. Thanks to all for reassurance. The problem seems to be the injector pump or govenor. I tried a quick test with no load. Full throttle with the pump off gives 2000rpm. Full throttle with the pump on gives 1,500rpm. This is the same as when under a load. Now to the TMs. This engine was replaced by the last owner twice. He said it was a M35A2C then he put the D engine in it... so I will need to figure out what it really has installed.

 

[DavidWymore]

Very odd...subscribing.

 

[F18hornetM]

For what its worth, I dont believe any dash mounted gauge in diagnosing an issue. They are good for everyday performance but not troubleshooting an electrical issue. If thats what you have.
Many years using Volt/ohm meter and current clamp prove me right. Good luck hope you find issue.

 

[gimpyrobb]

m35a2c means its a drop side bed, there is no indication for which turbo is on the motor.

The motors are (basically) the same, just different turbos.