Harry Flatters Posted April 2, 2007 Share Posted April 2, 2007 I have a SPA digital fuel guage which is completely accurate with just the ignition on but when the engine is running it can over-read by as much as 6 litres which, if I forgot, could get very embarassing . I have isolated the problem to be the fuel pump. By just havimg the ignition on and shorting the fuel pump relay, the guage starts over-reading, stop the pump and all is OK again. I am assuming that the fuel pump supply line is generating some noise which the fuel tank sender line is picking up. Both lines run in the same loom from fron to back and I am loathed to have to run more/seperate lines at this time. Is there some form of suppression I could use on the pump to try and eradicate this interferance 🤔 *arrowright*Harry Flatters *arrowright* AKA Steve Mell - Surrey AO and Su77on Se7ener Link to comment Share on other sites More sharing options...
rj Posted April 2, 2007 Share Posted April 2, 2007 Steve, I would think this is due to a poor earth. When the pump runs the the negative side of the sender's potential rises, hence higher reading. Try to measure the voltage between the earth on teh sender and on the battery - or even better the gauge! /regin Link to comment Share on other sites More sharing options...
Harry Flatters Posted April 2, 2007 Author Share Posted April 2, 2007 Earths are good Regin, both the pump and the sender have their own discreet earths. *arrowright*Harry Flatters *arrowright* AKA Steve Mell - Surrey AO and Su77on Se7ener Link to comment Share on other sites More sharing options...
rj Posted April 2, 2007 Share Posted April 2, 2007 Have just tried to call you but you're on your mobile... I am fairly certain that if you measure the earth between the sender and the gauge you'll find something interesting. Link to comment Share on other sites More sharing options...
rj Posted April 2, 2007 Share Posted April 2, 2007 I have to leave the desk now, but I'll try to give you a buzz later. /r Link to comment Share on other sites More sharing options...
Paul Deslandes Posted April 2, 2007 Share Posted April 2, 2007 Steve, I know its a hell of a long shot but the fuel return to the tank isn't hitting the float and affecting its position is it? My initial reaction was the same as Regin's and that its an earth problem. Just a thought Paul Link to comment Share on other sites More sharing options...
Harry Flatters Posted April 2, 2007 Author Share Posted April 2, 2007 An intriuging thought Paul but I'm pretty sure this is not the case. Theoretically all injections cars could have that as a potential problem I guess, provided the senders are mounted in the same orientation. If I hadn't remade the loom myself and provided discreet earths for the pump and sender I would be inclined to agree with the earth theory but I am 100% certain that those 2 earth are spot on. *arrowright*Harry Flatters *arrowright* AKA Steve Mell - Surrey AO and Su77on Se7ener Link to comment Share on other sites More sharing options...
rj Posted April 2, 2007 Share Posted April 2, 2007 Steve, Pauls theory is in fact quite good! You swapped the tank a while ago... Could you have oriented the sender differently than before? what happens if you disconnect the sender and with and ohm-meter measure the resistance with and without the pump running? /r Link to comment Share on other sites More sharing options...
Mick Day Posted April 3, 2007 Share Posted April 3, 2007 Steve We have the same SPA gauges & I've noticed this as well. I think it's more to do with battery voltage. With the ignition on the battery voltage is lower than when the engine is running. My fuel pump has a 15amp fuse so the voltage drop when this is switched on is enough to lose a few litres or vise versa. To confirm switch the ignition on & then switch the headlights on & see what happens to the gauge. Mick Link to comment Share on other sites More sharing options...
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