older style sending unit and gauge

fmalott

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1983 thundercraft 470 merc i posted previously on my fuel gauge and sending unit problem i need to ask one last question is it possible i have bought the wrong fuel sending unit meaning it reads 30 - 240 ohms would my gauge be set up for a 0 - 90 ohms sending unit. would this higher resistance sending unit peg out the gauge
 

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fmalott

Chief Petty Officer
Joined
Jun 28, 2010
Messages
561
Re: older style sending unit and gauge

i have pulled the gauge out of the boat i took a fully charged battery and i ran a ground to the neg on sending unit and the gauge i then ran a positive wire to the I terminal so it had a constant 12v i hooked up the pink wire to the sending unit and it peg out to full so please tell me what is wrong? i have totaly bypassed all wires so there was nothing to short out the only things attached to battery was the sending unit and the gauge i have done the test on the gauge grouned out the S teminal and the gauge pegged out when the key on so it passed, the new sending unit went to 30 full 240 empty so as far as i know it works but my auto mechanic said the sending unit has to much resistance for that gauge please help this is getting &%^# frustrating
 

Silvertip

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Sep 22, 2003
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28,771
Re: older style sending unit and gauge

I don't know what the instrumentation/sender values are for Canada - however, "International" standards have used 0 - 90 ohm fuel gauges and senders. What I suggest you do is go to any electronic store (Radio Shack for example) and buy one 90 ohm resistor. This will cost you in the neighborhood of about 50 cents. Disconnect the pink wire from the sender. Connect the resistor between the pink wire and ground. If you have a 90 ohm gauge it should register EMPTY proving you do indeed have a 90 ohm gauge. Have you actually measured the sender resistance. If you have a 0 - 90 ohm sender and a 33-240 ohm gauge, zero ohms will peg the gauge since it expects to see at least 33 ohms or more. Until you measure the sender you don't know what you have. If you had a 0 - 90 ohm gauge and a 33 - 240 ohm sending unit, the gauge would neither reach full nor empty. 240 is too high a resistance for empty (90 ohms) and 33 ohms is too high for full (0 ohms). So my thoughts are you have the wrong sender for the gauge, not the other way around. Finally, while you may have done this already (sorry but I don't go back to re-read old threads from top to bottom when someone starts a new one) but have you connected things like you have and moved the float arm through its travel. If the gauge does not move at all, that sender may be shorted internally. I posted a picture in the last couple of days of a sender and how it can happen.
 

fmalott

Chief Petty Officer
Joined
Jun 28, 2010
Messages
561
Re: older style sending unit and gauge

the sending unit is definitly 33-240 i did that test my problem is the old sending unit was not functioning to see what type it was i bench tested it it read nothing no matter where the float is. so it looks like i have to match up the gauge and sending unit. first i will try that 90 ohm resister test and post THANKS
 

fmalott

Chief Petty Officer
Joined
Jun 28, 2010
Messages
561
Re: older style sending unit and gauge

bad fuel gauge replaced all's good now
 
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