When I was troubleshooting my air horn circuit I decided to map the MCFC connector that runs through the firewall. This pinmap is only truly valid for my truck but is probably very similar for most Volvos. MCFC pin Description Circuit Comment 1 12v FB01 from battery 17A 2 trailer left turn signal 112C 3 12V F81 18E VECU 4 trailer right turn signal 113C 5 12V F22 fuel heater 953 6 12V F56 air dryer 952 7 12V F61 ABS1 8 12V F62 ABS2 9 electric horn 261 from horn relay 10 gnd for primary solenoid G 264 air horn solenoid - 11 fuel level sensor 183 12 12V F47 263 primary solenoid F, air horn solenoid + 13 driving lamp 37 14 fog lamp 35 15 Right high beam 33R 16 Right lo beam 32R 17 Left high beam 33L 18 Left lo beam 32L 19 differential switch 426 20 air filter restriction 199 21 oil temp, divider axle 469 22 oil temp, rear axle 468 23 left turn signal 112A 24 rear left turn signal 115 25 rear right turn signal 116 26 windshield wiper motor 382A 27 backup lamps 410 28 Cummins A:25 556 29 fuel level sensor gnd 30 instrument cluster 774 31 instrument cluster 778 32 EMS to sleeper thermostat 958 33 buss 34 buss 35 right turn signal 113 36 A/C compressor 301 37 Not Used 38 primary solenoid D 431B suspension dump 39 primary solenoid E 422B interaxle lock 40 instrument cluster 779A 41 secondary solenoid E 426B differential lock 42 primary solenoid C 424B fifth wheel 43 Not Used 44 Not Used 45 Not Used 46 VD12 engine control 555A 47 Not Used 48 trailer brake light 72 30A relay 49 trailer aux 40A fuse 50 Not Used 51 Not Used 52 Not Used 53 Not Used 54 ground 55 ground 56 fuel cooler fan 991 57 sleeper thermostat contl 598B 58 Not Used 59 Not Used 60 Not Used 61 alternator I post 14 62 engine brake cnx 628D 63 pyro + 190 64 pyro - 191 65 sleeper thermostat contl 598A 66 starter relay 284 67 Not Used 68 Not Used 69 Not Used 70 Not Used 71 ambient air temp 699 72 12V F21 Cummins engine 73 RLY05 trig + 910 74 RLY05 trig - 910A 75 Not Used 76 Not Used 77 Not Used 78 wiper motor high 383 79 windshield washer pump 387B 80 12V F26 windshield wiper 81 12V key 385 82 washer fluid level 387A 83 buss 84 buss 85 Not Used 86 Not Used 87 Not Used 88 Not Used 89 Not Used 90 Not Used 91 engine brake cnx 628E 92 Not Used 93 Not Used 94 fuel cooler switch 992 95 parking lights 566 96 trailer marker lights 53 20A relay 97 rear tail lights 90 98 trailer parking lights 51 20A relay 99 ground 100 ground 101 ground