MEP-803A Low Voltage

[jmpratt]

That's very nice! It shows that what I'm measuring is normal in Start, that is K1-7 voltage goes up to K16. I thought that voltage was supposed to be present during Start on A1-5 & A1-6 (F+, F-), but according to the schematic, it's not.

 

[jmpratt]

Ok, more progress! Normally, I've had the panel open during start to measure various voltages. I started with the panel closed and noticed that I have full (240VAC) voltage on M1 when S1 is in Start. I guess my DMM didn't respond fast enough to catch that when I measure A1-5 and A1-6 before.

So now my problem is that the voltage drops off when S1 is in Prime/Run.

 

[kloppk]

You're getting there!

With the set running and S1 in the START position do you get AC voltage across A1 terminals 7 & 8? What is the voltage?

With the set running and S1 in the START position do you get 120 VAC voltage across A1 terminals 3 & 4?

 

[jmpratt]

Fixed it. Turns out the Quad fuse upgrade fuse was burnt out. I'm sure I checked it when I started. When I first tried to start this unit, the MPU was disconnected and the fuel solenoid would not stay engaged. I think through the process of diagnosing and fixing those problems, the fuse might have blown. Or I just plain missed it.

Either way, I learned a lot about the genset. Thank you everyone for your help!

 

[kloppk]

Nice! Glad you got it straightened out.

 

[Guyfang]

If the fuse blows, then you may have a problem with the Volt regulator. That's what its there for. So don't just jump it.

 

[Ray70]

Looks like I have a 400 hr. tier 2 reset 802A with the same "low AC output" issue. I get 240 on the meter while holding S1 in the crank position, but output drops off to about 56VAC ( 28V per leg ) when I release S1.
The machine has the quad fuse mod, which tests good.
I plan to work on this machine this weekend and was wondering from Kurt's comment above about A1 pin 7 and 8, what voltage should I expect to see there while its running and S1 closed?
Also, what should I see at A1 pin 1 through the quad fuse for voltage with no load? ( assuming this is an important place to measure )
Lastly, since my starter interrupt is working and I get 240V with S1 closed, is it safe to assume the magnetic pickup is working correctly?
I have other good running machines available to swap out the VR, but want to do some basic troubleshooting first, to rule out the silly stuff.
Any further diagnostic steps beyond this thread's prior content are appreciated.
Thanks
Ray.

 

[Guyfang]

Looks like I have a 400 hr. tier 2 reset 802A with the same "low AC output" issue. I get 240 on the meter while holding S1 in the crank position, but output drops off to about 56VAC (This is exciter voltage) ( 28V per leg ) when I release S1.
The machine has the quad fuse mod, which tests good.
I plan to work on this machine this weekend and was wondering from Kurt's comment above about A1 pin 7 and 8, what voltage should I expect to see there while its running and S1 closed?
Also, what should I see at A1 pin 1 through the quad fuse for voltage with no load? ( assuming this is an important place to measure )
Lastly, since my starter interrupt is working and I get 240V with S1 closed, is it safe to assume the magnetic pickup is working correctly? (yes)
I have other good running machines available to swap out the VR, but want to do some basic troubleshooting first, to rule out the silly stuff.

A1-1&2 are voltage adjust, from R1
A1-3&4 are the sensing wires. They need to see 120 AC voltage
A1-5&6 go to the F1 and F2 exciter. 24 VDC. Terminal #6 is positive.
A1-7&8 are the input, (Q1 and Q2) to the G1. Voltage is AC.

 

[Ray70]

Great, thanks Guy. Sorry about possible confusion calling it Pin 1, It was Pin 7 and 8 I was thinking of, but pictured it backwards in my mind.
I did some testing last night. I was getting 120V on the sense lines and seemed like 67 VAC? on the 7 &8 quad lines which seemed odd and 24V on pins 5 and 6.
During testing it suddenly came to life and ran for a couple hours under load with no problems other than it prematurely trips the overload, so I need to clean the AM/VM and AC select switches. Once that is done I will do some extensive load testing to determine if there is an intermittent problem somewhere or not.

 

[Guyfang]

Great, thanks Guy. Sorry about possible confusion calling it Pin 1, It was Pin 7 and 8 I was thinking of, but pictured it backwards in my mind.
I did some testing last night. I was getting 120V on the sense lines and seemed like 67 VAC? on the 7 &8 quad lines which seemed odd and 24V on pins 5 and 6.
During testing it suddenly came to life and ran for a couple hours under load with no problems other than it prematurely trips the overload, so I need to clean the AM/VM and AC select switches. Once that is done I will do some extensive load testing to determine if there is an intermittent problem somewhere or not.

You might also want to check your wires hooked to the A1 and assorted devices. Several times I have found the wire eyelets cracked, due to the crimping tool being too tight. The vibrations do not help.

Pins 5&6 are reading right.
Pins 3&4 are right.
Pins 7&8, well that's something I never measured. Maybe someone here can check out what an operational unit reads there.

Mistakes happen. I have made several in the last few days. Getting old. Need to look at the books more often, and trust my moth eaten old brain less.

 

[kloppk]

I just measured my 802 a minute ago with no load and got these measurements.
Terminals 1 to 2 1.07 VAC
Terminals 3 to 4 120 VAC
Terminals 5 to 6 5.0 VDC
Terminals 7 to 8 69.1 VAC

With 45% load the only change was..
Terminals 5 to 6 went up to 6.8 VDC

 

[Guyfang]

I just measured my 802 a minute ago with no load and got these measurements.
Terminals 1 to 2 1.07 VAC
Terminals 3 to 4 120 VAC
Terminals 5 to 6 5.0 VDC
Terminals 7 to 8 69.1 VAC

With 45% load the only change was..
Terminals 5 to 6 went up to 6.8 VDC

Kurt,
So my question is why would 5&6 be 5.0 VDC? Initial is 24 VDC. I seem to remember that a CECOM guy told me it took about 4-6 volts to light off the excitation. If so, that figure would be right. So why would the S1 send 24 VDC? Why not put a resister in there to lower the voltage?

 

[kloppk]

Guy,

There is a resistor in the Flash circuit, it's R14. That and the diode CR2 after it will drop the 24 volts to something lower when flashing.
I'm going to do another start and see what the flash voltage is at Terminals 5 & 6.

 

[kloppk]

Just measured again with S1 held in start.
All measurements with respect to chassis ground.
K15 to R14 voltage is about 22 VDC.
Other side of R14 to CR2 is 8.4 VDC
Other side of CR2 to F+ is 7.6 due to CR2 voltage drop.

So 24 VDC is never present across F+ & F-

 

[Guyfang]

Just measured again with S1 held in start.
All measurements with respect to chassis ground.
K15 to R14 voltage is about 22 VDC.
Other side of R14 to CR2 is 8.4 VDC
Other side of CR2 to F+ is 7.6 due to CR2 voltage drop.

So 24 VDC is never present across F+ & F-

Your right! Your right!! I forgot about R14 and CR2!!