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SAR v2 field output short?

PostPosted: Thu Oct 26, 2017 8:39 am
by johnkremski
I have a SAR V2 Dual serial # 4371 It is installed per SAR wiring diagram dated June 29, 1993 including the optional wiring contained in the Optional Wiring Diagram date March 26, 1993 including the remote mode connected to the EMON II (but switched off) TB2-5 EQL is not connected, TB2-7 B2 V is not connected, TB1-5 Lock is not connected. There is a single house battery bank and a starting battery connected to an isolator/eliminator model #EL512. The dual current limit option is not connected.

Last time I started the engines there was a fault indication one second on, one second off; and according to the EMON the B1 Voltage ran up to about 16 volts with 65 amp charging current (into a fully charge 500 amp hour bank). I shut off the engines, then turned off the switch to TB1-7 IGN and both switches S1 and S2 (wired as shown in the two engine/SAR wiring diagram in the manual) which turned off the coils in relays K1 and K2, causing the relays to open and the field wire from the alternator to TB2-10 to be disconnected.

Restarted the engines, EMON showed no charging from the alternators (as expected). With the engines off and with all three switches off (to TB1-7 and the S1 and S2 field relay coil) I am reading voltages as follows relative to TB1-6 GND: TB1-2 B+ is 13.5 volts (battery voltage per the EMON and also my multi-meter); TB1-8 B1 V 13.5 volts; TB1-10 AGND 0 volts; TB2-10 Field 13.1 volts,

When I disconnect the port and starboard field wires from TB2-10 Field (and reinstalled the screw so it is connected to the lower circuit board) the Terminal TB2-10 Field still reads 13.1 volts relative to TB1-6 GND; while the port and starboard field wires read 0 volts.

If I am reading the manual and understanding the operation correctly, this seems to indicate that there is a shorted MOSFET condition as described on page 15 of the Manual (dated March 28, 1993) and I will need to replace the unit. Are there any other tests I should run or can you make any other suggestions?

And yes, the unit has otherwise performed flawlessly for almost a quarter century!