Petersubaru Posted March 19, 2009 Share Posted March 19, 2009 (edited) '86 Carbed wagen..the fuel pump in order to make it work needs to have a wire directly taken from some other 12 volt source under the dash and the fuel pump is grounded directly to the body under the rear seat...Why is it that when testing for voltage, I get 12+Volts on the "original" fuel pump wire when "not" connected to the pump .. Then "when" it gets connected, the voltage drops to about 2-3V... the carb,coil,fuel pump, and the black box (fuel shut-off device) have all been exchanged, including even disconnecting the wires from the ignition switch and tried another one with no success... Edited March 19, 2009 by Petersubaru Link to comment Share on other sites More sharing options...
naru Posted March 19, 2009 Share Posted March 19, 2009 '86 carbed wagen..the fuel pump in order to make it work needs to have a wire directly taken from some other 12 volt source under the dash and the fuel pump is grounded directly to the body under the rear seat...Why is it that when testing for voltage, I get 12+Volts on the "original" fuel pump wire when "not" connected to the pump .. Then "when" it gets connected, the voltage drops to about 2-3V.... Because there ia a bad connection (high resistance) between where you are measuring and battery positive.W/o the pump connected,there is no current flow and the high resistance is irrelavant.With the pump in the circuit,most of the voltage is dropped across the high resistance rather than the pump. Could be dirty relay contacts inside the black box (fuel shut-off device),but,this seems unlikely as it has been replaced.I would check voltage being supplied to this unit. Link to comment Share on other sites More sharing options...
Cougar Posted March 19, 2009 Share Posted March 19, 2009 I would suspect the fuel pump relay as the problem. Link to comment Share on other sites More sharing options...
Petersubaru Posted March 19, 2009 Author Share Posted March 19, 2009 (edited) I would suspect the fuel pump relay as the problem. Not completely sure here, but I don't think there is a separate stand alone fuel pump relay on this carbed model ...but rather one of the functions of the "black box" is to "relay" when needed, which has been exchanged...I will try to double confirm this... Edited March 19, 2009 by Petersubaru Link to comment Share on other sites More sharing options...
Cougar Posted March 19, 2009 Share Posted March 19, 2009 You may be correct, I'm not sure either. Another possible area is the fusible links. It is pretty common to have connection problems there. Link to comment Share on other sites More sharing options...
Petersubaru Posted March 19, 2009 Author Share Posted March 19, 2009 You may be correct, I'm not sure either. Another possible area is the fusible links. It is pretty common to have connection problems there. I forgot to mention earlier that the fusible links were also exchanged..but thanks anyway... Link to comment Share on other sites More sharing options...
Petersubaru Posted March 19, 2009 Author Share Posted March 19, 2009 (edited) there is voltage going "too" the "black box"...but for me, as to what makes this problem more difficult, is my inability to actually know what wire coming "out" of the "black box" is for the fuel pump, for example, the wire connected to the fuel pump is of a different color then "any" wire coming "out" of the "black box"...also, any wireing diagram I have seen is different in color as to what I have, just maybe because it is a Canadian subi....Perhaps, what I should do, is run a new wire connected to any 12V output from the black box to the fuel pump until the correct wire provides the safety features of that box...who knows, I may get lucky Edited March 19, 2009 by Petersubaru Link to comment Share on other sites More sharing options...
naru Posted March 19, 2009 Share Posted March 19, 2009 there is voltage going "too" the "black box"...but for me, as to what makes this problem more difficult, is my inability to actually know what wire coming "out" of the "black box" is for the fuel pump, for example, the wire connected to the fuel pump is of a different color then "any" wire coming "out" of the "black box"...also, any wireing diagram I have seen is different in color as to what I have, just maybe because it is a Canadian subi....Perhaps, what I should do, is run a new wire connected to any 12V output from the black box to the fuel pump until the correct wire provides the safety features of that box...who knows, I may get lucky Better to be smart than lucky.Known wire colours are an unneeded luxury. ID the fuel pump output by putting your ohmmeter between the unplugged fuel pump wire and the unplugged FPCU plug.Once ID`d check voltage here w/everything plugged in. ID the ignition pulse line by allowing a test light to flash when connected to the FPCU plug w/engine cranking.Plugged in or not,no matter. Some FPCUs have 2 power inputs.One switched w/the key,one not. Some only have the switched input.ID the ignition pulse line first to save confusion. ID the ground wire w/an ohmmeter w/the battery disconnected. Link to comment Share on other sites More sharing options...
Petersubaru Posted March 19, 2009 Author Share Posted March 19, 2009 Better to be smart than lucky.Known wire colours are an unneeded luxury. ID the fuel pump output by putting your ohmmeter between the unplugged fuel pump wire and the unplugged FPCU plug.Once ID`d check voltage here w/everything plugged in. ID the ignition pulse line by allowing a test light to flash when connected to the FPCU plug w/engine cranking.Plugged in or not,no matter. Some FPCUs have 2 power inputs.One switched w/the key,one not. Some only have the switched input.ID the ignition pulse line first to save confusion. ID the ground wire w/an ohmmeter w/the battery disconnected. will do..hopefully it will be warmer tomorrow Link to comment Share on other sites More sharing options...
Recommended Posts
Please sign in to comment
You will be able to leave a comment after signing in
Sign In Now