Hi thank you for your question hope this helps.
P0670 = Glow Plug Module Control CircuitP0683 = Glow Plug Control Module to PCM Communication CircuitP0540 Intake Air Heater "A" Circuit
OK lets start with the glow plugs. This is the test steps, But I have found this problem solved by changing the GPCM. (Glow Plug Control Module)DTC P0683 is set when the PCM detects an open or short to ground or power diagnostic line. Possible causes: open diagnostic line diagnostic line shorted to ground or power faulty PCM circuit faulty GPCM circuit Perform KOER On-Demand Self Test and retrieve/clear continuous DTCs. Is DTC P0683 present? Clear DTCs and retest, make sure there is no DTC P0670 present. If DTC P0670 is present, perform DTC P0670 pinpoint test sequence and fix problem before performing DTC P0683 pinpoint tests. Yes GO to QB12 . No END test if no further glow plug DTC detected. QB12 CHECK FOR OPEN IN DIAGNOSTIC LINE Key off. Disconnect wiring at PCM and GPCM. Connect breakout box; leave PCM harness disconnected. Check resistance between PCM test pin 8 and GPCM test pin 9 at GREEN connector. Is the resistance greater than 5 ohms? (Typical resistance value is less than 1 ohm.) Yes REPAIR open in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST. No GO to QB13 . QB13 CHECK FOR SHORT TO POWER IN DIAGNOSTIC LINE Key on, engine off. Measure voltage between PCM test pin 8 and ground. Is the voltage greater than 5 V? (Typical voltage should be 0 V.) Yes REPAIR short to power in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST. No GO to QB14 . QB14 CHECK FOR SHORT TO GROUND IN DIAGNOSTIC LINE Key off. Connect PCM and GPCM. Key on, engine off. Measure voltage between PCM test pin 8 and ground. Is the voltage a constant 0 V? (Diagnostic signal that is not shorted to ground is a pulse train; 0 V - 12 V.) Yes REPAIR short to ground in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST. No GO to QB15 . QB15 CHECK GPCM CIRCUIT Install breakout box. Make sure PCM and GPCM are connected. Set up scan tool as follows: choose digital measurement system choose frequency meter choose level and change to 6-V scale connect only the scan tool signal to PCM breakout box pin 8 (GPCM Diagnostic input) Disconnect EOT sensor connector, (this will maximize the glow plug ON time). Key on, engine off. Measure frequency at PCM breakout box pin 8, (take the measurement when frequency stabilizes, approximately five seconds). Is the frequency in the range of 5 to 10 Hz? (Typical frequency for diagnostic signal with all plugs functioning is maximum 7 Hz. Typical frequency for the signal with one or more glow plugs failed is maximum 15 Hz.) Yes INSTALL a new PCM. RESTORE system. CLEAR DTCs and RETEST. No INSTALL a new GPCM. RESTORE system. CLEAR DTCs and RETEST.
With the P0540 I would check all the wire connections and wires for brakes.
I am looking at the wire diagram it shows the Intake Air temp sensor bit not seeing the heater.
Are you getting power to the heater the?
Let me know
this would help if vehicle had the heater for the 540 code it does not
i agree that replacing gpcm helps but has older or federal style that looks like a ford stater solenoid
and have verified that this starter solenoid works please read entire original text
codes are for equipment that does not exist on a 99 expecialy the 540 code which should not even be able to come up
this vehicle does not have a miah (mainfold intake air heatet)
it has a fuel heater