Fuel sending unit wiring

paddyrk · Mar 15, 2019

I have a 1991 lx v8, and am trying to wire a custom fuel gauge. It is with the Holley hp efi system, but it’s not getting a reading. I have the yellow/white wire from the dash connector wiring into it and it is setup as a custom ohms input, but does not display a read out, meaning ♾ ohms, because it reads as 100% full no matter the scaling or boundaries. I have tried looking at wiring diagrams for the sender but I can’t figure it out. I assume the sender is grounded on its own away from the dash cluster connector, but I could be wrong. If anyone could help, it would be much appreciated.

a91what · Mar 15, 2019

does the input have a built in Vref?? if it does not have a pullup to 5v the ohmic value cannot be read it is still an ADC input.

General karthief · Mar 15, 2019

Just to be clear, are you wiring an aftermarket (holley) sender to the factory gauge?

paddyrk · Mar 15, 2019

General karthief said:
Just to be clear, are you wiring an aftermarket (holley) sender to the factory gauge?

No it is tthe 7” dash with the I/O harness (558-432) and it has a specific input for custom ohms reading

paddyrk · Mar 15, 2019

a91what said:
does the input have a built in Vref?? if it does not have a pullup to 5v the ohmic value cannot be read it is still an ADC input.

I’m not sure, how could I test to find out?

a91what · Mar 15, 2019

Post link to the document please. You may have to build a circuit.

paddyrk · Mar 15, 2019

a91what said:
Post link to the document please. You may have to build a circuit.

558-432 that is the dash I/O adapter, it says I/O 10 (pin 13) is for the fuel sending unit
553-106 that is tthe digital dash itself, not much info on the I/O configuration

paddyrk · Mar 15, 2019

a91what said:
Post link to the document please. You may have to build a circuit.

Maybe a better question is how would I wire a circuit through a multimeter to see if the sender is working properly? As in, wire a circuit and if I get a good ohms reading, I know it’s right.

a91what · Mar 15, 2019

paddyrk said:
Maybe a better question is how would I wire a circuit through a multimeter to see if the sender is working properly? As in, wire a circuit and if I get a good ohms reading, I know it’s right.

Simple test from the sender to ground

a91what · Mar 15, 2019

Should be a simple install, what values are you using?

paddyrk · Mar 15, 2019

a91what said:
Should be a simple install, what values are you using?

22-145 ohms, I got that number from the lmr website which is where I believe I got my sending unit from

a91what · Mar 15, 2019

Ok do some testing before dropping the tank.

paddyrk · Mar 15, 2019

a91what said:
Ok do some testing before dropping the tank.

Luckily I have to drop the tank in a week to redo the sending unit and pump seals since it does not click off at the pump since I changed them

paddyrk · Mar 15, 2019

a91what said:
Ok do some testing before dropping the tank.

I didn’t an ohm test, positive test on the yellow/white fuel sender wire, negative on the negative battery terminal, and it read about 56 ohms. Would me completely removing the factory gauge cluster and only jumping the green/red wire to a switched +12v source mess with the grounding of the sending unit?

a91what · Mar 15, 2019

yup that sounds to be correct, where do you have the ground for the holley dash coming from? you may want to consider re-grounding the dash and see if the signal comes up.

paddyrk · Mar 15, 2019

a91what said:
yup that sounds to be correct, where do you have the ground for the holley dash coming from?

The CAN bus has a ground that goes directly to the battery. It goes battery, can bus, dash, then back to the battery

a91what · Mar 15, 2019

ok... the canbus is usually reserved for communication with the ECU. let me look at the wiring diagram again.

paddyrk · Mar 15, 2019

a91what said:
ok... the canbus is usually reserved for communication with the ECU. let me look at the wiring diagram again.

Okay, not sure if you saw me edit my earlier post, the only cables I messed with from the factory dash connectors were the red/green that I attatched to a switched source, and the yellow/white fuel sending unit wire, I’m wondering if me not changing any ground messed it up

a91what · Mar 15, 2019

not sure without knowing how the holley is wired. trying to help on my phone is a pain. lol

paddyrk · Mar 15, 2019

a91what said:
not sure without knowing how the holley is wired. trying to help on my phone is a pain. lol

And I appreciate it greatly, you may have seen this already but I went and grabbed the wiring diagram for the system I am using

Fuel sending unit wiring

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wonder how much it would cost to ship you a pair

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