I have a new problem this week. The liquid fuel stop solenoid 20FD-1 did not open when the logic L20FD1X is forced to ONE. This is done on weekly basis to prime up the liquid fuel line.

With the solenoid not connected, I could get the 125Vdc supply when the logic is forced but once I connect the solenoid and measure the voltage in parallel, I could get only 24Vdc when the logic is forced. I have tried to replace the solenoid with a new and the result is still the same. I did try on the other unit which works well (so far ) and the solenoid operates without any problem.

The solenoid is connected on SO17 on the DTBC on <QD1) core. The DTBC module is relatively new (was replaced recently due to short circuit of 20PG2-X and all the jumper setting (M17 and P17) are in place correctly.

There were no current flow to the solenoid as well when it is forced. The valve was able to be operated smoothly by hand. Any one has any idea on what may gone wrong? Your advice is highly appreciated.

 

Refer to Appendix E of the Mark V Application Manual, GEH-6195, Fig. E-5.

There is a fuse, mounted in the <PD> core, which is probably NFW (Not Flippin' Workin'), or, blown.

 

Can i remove and replace the fuse when the unit is online?

 

I wouldn't suggest replacing the fuse if you're forcing 20FD or if you're running on liq fuel, but if you're running on another fuel it probably shouldn't be a problem.

The real problem is: Why did the fuse blow in the first place? The typical Liq. Fuel Fwd. Stop Valve has a high pull-in current and a low holding current, and the fuse is a slow-blow fuse that's intended to "ride through" the high current pull-in condition (which should be only a split second).

If the fuse is blown, that's usually an indication of a problem with the solenoid coil circuit that switches from pull-in to holding current, or a stuck solenoid that didn't actuate the change from pull-in to holding current, or an electrical problem with the coil or the circuit. I've also seen people quickly toggle this output which can cause the fuse to blow.

But, in any case, if you don't know why the fuse blew, you should be examining the solenoid, the actuator, the valve to see if there is any mechanical binding or other visually obvious problem, and be prepared for another blown fuse if you don't see an obvious problem. You did say you've replaced the solenoid, so that should not be a problem.

One last thing, some of the older solenoids were polarity sensitive. Sometimes the only indication was a small red "sleeve" on one of the coil leads.

Let us know what you find out, please.

 

Firstly I couldnt locate the FU28 fuse on TCPD module. Is it the fuse below the 2 red LEDs CR26 and CR27? If yes, I believe the fuse is still ok since the red LEDs are not lighted. The LEDs should light up when the fuse blew right? Is there any possibility that the red LEDs are not lighted even when the fuse is blown?

I have tried to reverse the polarity and the results are the same.
BTW let say the fuse is blown, will I still get the 125Vdc supply to the solenoid? This is because whenever the solenoid is not connected I could get the 125Vdc at the junction box when L20FD1X is forced. When I connect the solenoid, the voltage drops to about 24Vdc when L20FD1X is forced. I don't think there is anything wrong at local (solenoid, actuator or valve) since it all performed well when tested).

As we are running on gas, what I did for the time being is that I have manually lifted the valve and tight it to the solenoid body with the manual isolation valve before the solenoid in closed position. Should we have the instruction to run on distillate, then the manual isolation valve is to be opened.

 

I believe the fuse you are looking for is one of the two fuses below the J15 and J16 connectors along the top edge of the TCPD card at the upper left corner. Yes, there should be some LEDs there, and early versions of the TCPD card had the LEDs on when the fuses were good and off when they were open. They were changed at some point to be on when the fuses were open and off when the fuses were good. I'm *only* referring to the two "special circuit" fuse LEDs for outputs 16 and 17 of the <QD1> DTBC card.

There should be a paper drawing of the TCPD card in a plastic pouch on the back of the <PD> core door and you should be able to locate the fuse there. It's been said before on control.com: GE manuals are not always error-free; it could be a typographical error on the drawing of the fuse circuit. The LED on the drawing in the appendix for output #`17 is labeled CR26. so I would presume the fuse below CR26 would be the one you're looking for. Just be very careful, because if you remove the wrong fuse when the unit is running, you can open the compressor bleed valves.

The fuse in this circuit is only for the positive side of the solenoid supply voltage, the "switched" leg of the supply voltage; the negative side of the solenoid voltage is *always* "live". Refer to the drawing in the appendix for details.

The voltages below are for a system with no 125 VDC grounds, presumes the fuse for output circuit #17 is good, and that cables J15 and J16 are both properly connected and good. (WARNING: *DO NOT* unplug J15 or J16 at either end of the cables when the unit is running!) When measuring DC voltages with respect to ground, it is presumed the negative meter lead is connected to ground.

There are two hardware ("Berg") jumpers on the DTBC which should be IN (installed): P17 and M17. M17 supplies the -65 VDC to DTBC-65 and P17 supplies the +65 VDC to the NO contacts of the driving relay, and when the relay is energized, to DTBC-66.

Usually, for the Liq. Fuel Fwd. Stop Valve Solenoid you will find two wires used to power the solenoid (but not always). You should find a wire at DTBC-65 (the SOL terminal of output #17), where you should measure -65 VDC with respect to ground at all times. When L20FD1X (the *typical* name for the Liq. Fuel Fwd. Stop Valve Solenoid driving relay logic signal) is a logic "0", you should also measure -65 VDC with respect to ground at DTBC-66 (the NO terminal or output #17).

Because the negative side of the solenoid power is *always* applied to the solenoid coil (refer to the drawing in the appendix), if L20FD1X is a logic "0", you should be able to measure -65 VDC with respect to ground at both DTBC-66 (the "source" of the -65 VDC to the solenoid coil) and -65 VDC with respect to ground at DTBC-65 because you will be reading through the solenoid coil. When L20FD1X is energized and the NO contacts close, you should read +65 VDC at DTBC-65 and -65 VDC at DTBC-66, and if you measure from DTBC-65 to DTBC-66 you should read 125 VDC.

With L20FD1X forced to logic "0", you could first disconnect the wire at DTBC-65 (to be safe) and then the wire at DTBC-66. You should then take the meter and read the voltage at DTBC-65, and it should be -65 VDC with respect to ground, and at DTBC-66 you should read 0 VDC with respect to ground. Force L20FD1X to "1" and you should read +65 VDC with respect to ground at DTBC-65, and if you read from DTBC-65 to DTBC-66 you should read +125 VDC.

Any other voltages (presuming there are no 125 VDC grounds), would indicate some kind of problem with the cables between the DTBC and the TCRA card in Loc. 4 of <QD1>, or a problem with the TCRA in Loc. 5 of <QD1>, or a problem with the DTBC.

There was a recent post here on control.com about problems with a DTBC. The connector pins on the printed circuit cards for the ribbon cables between the DTBC and DTBD cards and the TCRA cards are *very* fragile and can be bent over very easily and bent pins can be difficult to spot visually. One must use extreme care when removing and replacing the DTBC, DTBD, and TCRA cards and replacing the ribbon cables as these connector pins can be bent very easily. If you find a bent pin, be very careful trying to straighten it; they snap very easily.

Also, the ribbon cables that connect the DTBC (and DTBD) and TCRA can *easily* be offset by one pin when being plugged in.

You stated in your original post that you had a new problem this week. If this is the same site that had the DTBC problem and this problem started after the DTBC was replaced, then it's likely that something is amiss with the way the new DTBC was installed or that the new problem is related to the replacement of the new DTBC. Like hardware ("Berg") jumpers not being installed properly, or bent ribbon cable connector pins on the DTBC, or a cable not properly installed (aligned) on the DTBC.

Good troubleshooting always asks first: What has changed most recently?

 

Thanks for the suggestions. I will check it out during the next outage and revert back here.
Wish me luck