Fuel Tank light and Check Engine both flashing in diagnostic mode

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GeoffBart

GeoffBart

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Anyone seen this before? Have just got my truck back together after major drivetrain overhaul (engine & gearboxes removed, recoed and back in) and have it driving again now.

All good after the drivetrain transplant and truck is driving great but on setting the ignition timing while in diagnostic mode I noticed that BOTH the Check Engine light and Fuel Tank lamp (beside it) are flashing?? They are both flashing at an even speed (but are not in sync). Check Engine is not showing any "codes" and is just flashing like it should when everything is working fine.

If the Fuel Lamp was not flashing I would say everything is working perfectly and there is no problem whatsoever with the control systems. Had all wiring harnesses from the ECU to engine and gearbox out while doing the mechanical work and everything is back where it should be now. Have tried disconnecting and reconnecting the battery to clear ECU but issue remains.

Anyone seen this flashing Fuel Light in diagnostic mode before?
 
WCBlueSky

WCBlueSky

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Do you mean the low fuel "dummy" light? This low fuel light is a pretty simple circuit: battery voltage is connected to a lightbulb in the gauge cluster, which connects to a thermistor in the fuel tank, connected to ground. If the fuel level drops below the thermistor, the thermistor heats up, its resistance drops, and current now flows through the light bulb. Since it is lighting up without low fuel, it must be shorting to ground. My guess: in diagnostic mode, the ECU periodically grounds the check engine light to make it flash, so that wire path must be shorting the thermistor wire to ground as well (since they both flash). Can look in the EWD to see which wires to check for broken/missing insulation.
 
GeoffBart

GeoffBart

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Thanks for the reply WcBlueSky,
Yes, low fuel light immediately to the left of the Check Engine light. Only thing is the two lights are flashing at an even speed but they are not in sync. One is flashing slightly quicker/slower than the other. Thinking that if the fuel light was getting a common ground from the Check Engine light it would be flashing at exactly the same time. It's not, so thinking it's being controlled independent of the Check Engine feed? It's got me stumped, fuel tank over half full, everything is operating as it should, truck is driving great and there is no indication of a fault code on the Check Engine light. Not a show-stopper and thinking l might just ignore for the moment and try it again in diagnostics mode in week or two to see if things have normalised?
 
WCBlueSky

WCBlueSky

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Thinking that if the fuel light was getting a common ground from the Check Engine light it would be flashing at exactly the same time.
Maybe there is some capacitance in one of the circuits, which causes a delay? 🤷‍♂️
 
CJF

CJF

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Thanks for the reply WcBlueSky,
Yes, low fuel light immediately to the left of the Check Engine light. Only thing is the two lights are flashing at an even speed but they are not in sync. One is flashing slightly quicker/slower than the other. Thinking that if the fuel light was getting a common ground from the Check Engine light it would be flashing at exactly the same time. It's not, so thinking it's being controlled independent of the Check Engine feed? It's got me stumped, fuel tank over half full, everything is operating as it should, truck is driving great and there is no indication of a fault code on the Check Engine light. Not a show-stopper and thinking l might just ignore for the moment and try it again in diagnostics mode in week or two to see if things have normalised?
Was your rig originally a diesel?
 

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