Mep 804 b having mpu issues

[Jackal63]

As per instructions , I disconnected 147c from terminal #16 and 148c from terminal #17, it only adjust to 776 ohms, it needs between 800 and 900. I bought an aftermarket mpu and it will only get to 286 ohms. Any suggestions ?

 

[Guyfang]

What instructions are you talking about? If the TM, please write a short blurb about what, exactly what, you did.

 

[Jackal63]

What instructions are you talking about? If the TM, please write a short blurb about what, exactly what, you did.

It is the TH , I disconnected 147c from terminal 16 and 148c from terminal 17, of the governor control unit. Then connected multimeter set to ohms, my highest reading was 776 ohms and 1 VAC. I then bought an aftermarket MP U and installed, my highest reading was only 286 ohms.

 

[loosegravel]

It is the TH , I disconnected 147c from terminal 16 and 148c from terminal 17, of the governor control unit. Then connected multimeter set to ohms, my highest reading was 776 ohms and 1 VAC. I then bought an aftermarket MP U and installed, my highest reading was only 286 ohms.

I believe that we check the MPU with an a/c volt meter, not an ohm meter. First, remove the MPU from the bell housing for inspection and cleaning. If it doesn’t show any signs of touching the flywheel, no damage to the tip, just clean it and reinstall it. Screw it in by hand until you can feel it stoping at the flywheel. Back it out 1 1/2 turns. Leave the MPU disconnected and connect your a/c voltmeter leads to the two terminals on the MPU connector. Use your dead crank switch to crank the engine over and note the a/c voltage output of the MPU. You’re looking to get as close to 2.8 vac that you can get without coming in contact with the flywheel. If the flywheel come in contact with the MPU while it’s turning, you’ll be buying another one. Screw the MPU in 1/4 turn at a time and recheck your vac output of the MPU each time. When you get close to 2.8 vac your done. Reconnect the MPU and attempt to start the engine again. I’ve learned all of the above from reading the TM’s and following the instructions from others on this forum

 

[Jackal63]

Thanks for the input, loosegravel, I was following the directions in the TH .

 

[Guyfang]

After you installed it, did you try and adjust it? After all, you bottom it out on reinstalling, and back it out 1 1/2 turn for a reason. It gives you room to adjust it, to the Correct AC reading, when you are turning over the engine with the S10, (dead crank switch). Get someone to use the S10, and you look at the meter. Stop cranking, adjust a hair, and test again. Better that you readjust 25 thousand times, then one time too far!

 

[loosegravel]

It's kind of interesting to me. I went back and read some of the information in the TM's about installing and adjusting the MPU. I did find that @Jackal63 is correct. It does mention in https://www.steelsoldiers.com/attachments/15kwmep-804tqg9-6115-643-24p-pdf.602128/ to use an ohm meter across 147C and 148C at the governor control unit. BUT...the wires do need to be disconnected first though when measuring! I would think that this test is simply checking the integrity of the wiring all the way back to the MPU. If the resistance reading is much lower than 800 ohms as @Jackal63 is indicating, that would suggest a potential short to ground on one or both of the two wires. If the resistance is much higher than 900 ohms, that would suggest an open on one or both of the two wires. Again Jack, just make sure that both wires are disconnected from the governor control unit before measuring your resistance. My apologies to you for suggesting that you were wrong.

The TM also says to set the adjustment of the MPU by measuring the ACV output of the MPU at 147C and 148C in the same location while using the S10 dead crank switch to turn the engine over. It says to start with one turn out from the flywheel and then turn the MPU in @1/8 turn increments to obtain between 2.00 VAC and 3.0 VAC. Then, lock down the jam nut. It seems to me that it's easier to measure the ACV right at the MPU connector because it's in the same general vicinity of the S10 dead crank switch.

 

[Guyfang]

I never measured at the GPU. Always at the MPU. and its always better to have a willing slave to crank, while you look. Do not adjust while cranking.

 

[loosegravel]

I never measured at the GPU. Always at the MPU. and its always better to have a willing slave to crank, while you look. Do not adjust while cranking.

Speaking from experience it seems like! Yes, those things are touchy from what I’ve discovered.

 

[Jackal63]

It's kind of interesting to me. I went back and read some of the information in the TM's about installing and adjusting the MPU. I did find that @Jackal63 is correct. It does mention in https://www.steelsoldiers.com/attachments/15kwmep-804tqg9-6115-643-24p-pdf.602128/ to use an ohm meter across 147C and 148C at the governor control unit. BUT...the wires do need to be disconnected first though when measuring! I would think that this test is simply checking the integrity of the wiring all the way back to the MPU. If the resistance reading is much lower than 800 ohms as @Jackal63 is indicating, that would suggest a potential short to ground on one or both of the two wires. If the resistance is much higher than 900 ohms, that would suggest an open on one or both of the two wires. Again Jack, just make sure that both wires are disconnected from the governor control unit before measuring your resistance. My apologies to you for suggesting that you were wrong.

The TM also says to set the adjustment of the MPU by measuring the ACV output of the MPU at 147C and 148C in the same location while using the S10 dead crank switch to turn the engine over. It says to start with one turn out from the flywheel and then turn the MPU in @1/8 turn increments to obtain between 2.00 VAC and 3.0 VAC. Then, lock down the jam nut. It seems to me that it's easier to measure the ACV right at the MPU connector because it's in the same general vicinity of the S10 dead crank switch.

I rechecked the new aftermarket mpu , the highest ohms , I could adjust to was 226 ohms and 0 volts ac. I then reinstalled old mpu , and was only able to get 752 ohms and 0 volts ac. I will try checking a.c. Volts with the mpu disconnected in the morning. I really appreciate the help with this problem.

 

[Jackal63]

I rechecked the new aftermarket mpu , the highest ohms , I could adjust to was 226 ohms and 0 volts ac. I then reinstalled old mpu , and was only able to get 752 ohms and 0 volts ac. I will try checking a.c. Volts with the mpu disconnected in the morning. I really appreciate the help with this problem.

I went back and checked the mpu disconnected with the volts meter, I got 0 volts a.c. , I then checked the ohms and got 726. I then pulled the mpu out and checked the ohms and got 732, while it was laying on the fender.

 

[Jackal63]

After you installed it, did you try and adjust it? After all, you bottom it out on reinstalling, and back it out 1 1/2 turn for a reason. It gives you room to adjust it, to the Correct AC reading, when you are turning over the engine with the S10, (dead crank switch). Get someone to use the S10, and you look at the meter. Stop cranking, adjust a hair, and test again. Better that you readjust 25 thousand times, then one time too far!

 

[loosegravel]

Hi, just a little clarification. The ohm check is only to check the integrity of the wiring to the MPU and also to check the windings inside the MPU. The proper way to check and set the a/c output voltage on the MPU is while cranking the engine over with the starter. (With good, fully charged batteries) The fact that you have 732 ohms resistance across the MPU terminals is good. Now, reinstall it into the bell housing until it bottoms out on the flywheel ring gear. Back it out 1 and 1/2 turns. Connect your voltmeter across the two MPU terminals. THEN crank the engine over using your dead crank switch, or have a friend do that for you. You’re looking to get as close to 2.8vac across the two terminals on the MPU without touching the ring gear. If you’re not getting any a/c voltage from the MPU while the engine is cranking over, then something isn’t right. If you see a lesser voltage reading than 2.8vac then turn the MPU in 1/8 of a turn and recheck again WHILE CRANKING THE ENGINE OVER. The voltage reading is not a static test. The MPU generates the voltage when the teeth on the flywheel ring gear go past the tip of the MPU.

 

[Guyfang]

1. Open up TM 9-6115-643-24.
2. Turn to TM page #2-306, (PDF page number 36.
3. Start reading at: 2-121.3 Installation.
4. Stop reading into it.
c. Set multimeter for ohms and connect to ends of disconnected wires 147C and 148C. Multimeter, should indicate between 800 and 900 ohms.

This say nothing about adjusting it. Its just to tells you, the wires 147C and 148C are good and run down to the MPU and back. Nothing more.

5. Continue reading.
d. Leave multimeter connected to wires 147C and 148C and set multimeter for AC volts. (I would not measure at the GCU, I would measure at the MPU. Its simpler)
e. Crank engine with DEAD CRANK switch and observe multimeter. Multimeter indication should
be between 2.0 and 3.0 VAC.
CAUTION
Do not adjust magnetic pickup inward more than one eighth turn each
time or damage to magnetic pickup may result.
f. To adjust output voltage in step e above, loosen jam nut and turn magnetic pickup in no more
than one-eighth turn at a time to increase or decrease output voltage. Tighten jam nut.
g. Repeat steps e and f above until proper output voltage is achieved.
h. Remove multimeter and connect wires to governor control unit.

You have been reading into the procedure. Just follow the yellow brick road. The closer you are to 3 VAC, the better. But not more. All I have done is repeat what Loosegravel wrote, by just posting whats in the TM.

 

[Jackal63]

Hi, just a little clarification. The ohm check is only to check the integrity of the wiring to the MPU and also to check the windings inside the MPU. The proper way to check and set the a/c output voltage on the MPU is while cranking the engine over with the starter. (With good, fully charged batteries) The fact that you have 732 ohms resistance across the MPU terminals is good. Now, reinstall it into the bell housing until it bottoms out on the flywheel ring gear. Back it out 1 and 1/2 turns. Connect your voltmeter across the two MPU terminals. THEN crank the engine over using your dead crank switch, or have a friend do that for you. You’re looking to get as close to 2.8vac across the two terminals on the MPU without touching the ring gear. If you’re not getting any a/c voltage from the MPU while the engine is cranking over, then something isn’t right. If you see a lesser voltage reading than 2.8vac then turn the MPU in 1/8 of a turn and recheck again WHILE CRANKING THE ENGINE OVER. The voltage reading is not a static test. The MPU generates the voltage when the teeth on the flywheel ring gear go past the tip of the MPU.

I did reinstall mpu , turned in until it contacted the flywheel then backed off 1and 1/2 rounds connected multimeter, checked with multimeter. It showed 0 volts a.c. and 752 ohms

 

[loosegravel]

I'm sorry, but I just don't know how to communicate this any differently. The a/c voltage test is not a static test. The engine has to be turning over to generate the a/c voltage signal. If you're not cranking the engine over while you are checking for ac voltage at the MPU, you will always get 0 voltage at the MPU.

 

[Guyfang]

Stop worrying about ohms. Its something you CAN NOT ADJUST. Like Loosegravel wrote, You have to use the S10, (dead crank Switch, to turn over the engine. The movement of the flywheel teeth, past the MPU, generates the INDUCTIVE AC voltage. The distance from the teeth to the MPU dictates the amount of ACV.