adjusting fuel level sender with resistor

Hello. First off, I have a 1998 Oldsmobile Regency which has a regular analog instrument cluster. I have purchased and installed a digital cluster (which includes a trip computer which couldn't be equipped on my particular car) from a 1994 Oldsmobile Ninety-Eight. The wiring and connectors for the cluster are all the same, but my problem is with the fuel gauge. My '98 Regency's fuel level sender (in the tank) uses the following values: 248 +/- 3.3 Ohms when Full, and 40 +/- 2.5 Ohms when the tank is empty. The problem is that the new digital cluster was programmed for the Ninety-Eight, who's fuel level sender uses different resistance values: 90 Ohms when Full and 0 Ohms when Empty. I know I can add a resistor in parallel to the wiring that goes to the fuel level sender, but I'm not sure which type of resistor or what value to use to reduce my full tank resistance of ~250 Ohms to 90 Ohms. What if I wired in a 160 Ohm Resistor in parallel. That would bring my full tank resistance of 250 Ohms down to 90 Ohms, BUT, would that math hold true for the empty resistance? Would that give me negative resistance? (40 Ohms - 160 Ohms = -120 Ohms???) Or would the resistor still just me 0 Ohms when empty? Any help would be awesome.

I uploaded the wiring diagram for my car's fuel gauge and level sender. Would I run a resistor across the F9 and E3 terminals on the back of the instrument cluster connector? Could I use a variable resistor to "dial in" the correct resistance after getting the tank empty, or would it be better to use a regular resistor? Thanks, Nick

And, for good measure (mods, combine posts if you'd like), I've also added the schematic for the digital dashboard's fuel gauge circuitry.

All you can do resistor-wise is wire a 141 Ohm = 150 Ohm resistor in parallel to the wiper arm and return (ground). That drops full from 248//150 = 93.5R (Ohms) & empty 40//150 = 31.6R - ie, probably indicate ~ 1/3 full. But it may not be linear between the ends.

A resistor won't work to rescale your sender, you need to just replace the sender.Kevin Pierson

Or a converter.... But for mere mortals that means replace the sender as KP suggested. (Converting is thru electronics & level shifters, else mapping via programmable devices etc.)

oldspark wrote: Or a converter.... But for mere mortals that means replace the sender as KP suggested. (Converting is thru electronics & level shifters, else mapping via programmable devices etc.)

Have you ever had any luck converting?  I tried it once using op amps and it didn't work because the system needs actual resistance to calculate value.  Simply shoving the desired voltage in the circuit in place of the resistor didn't give accurate, consistant results.  I looked in to some digital pots but never actually messed with them.

Kevin Pierson

To summarise - I'm with you on this one KP! Transconductance amplifiers, negative impedances... The biggest problem is using with thermal gauges where sender impedence is in series with the meter, and its pulsed bi-metal 8V regulator.... Electro-mag meters are easier as are electronic displays. But all tend to be non-linear and getting the correct transfer function is difficult - though it is possible because the meters themselves tend to display linearly. That makes it sound so easy.... The last time I attempted it, I modified the sender that matched the new meter (ie, bent & kinked the arm appropriately). Actually it worked well and had te bonus of the low-fuel warning (light) which I set accurately enough for 1/11th tank (so remaining distance is 10% of the tripmeter (which gets zero'd at fuel fills). I gave up on my transy condy and negative resistorancy thingy. Admittedly I was also inverting - ie, say 20 to 150R to 130 to 7R etc. But analog transfer functions are just too forgotten for me. (Though I reviewed them for anticipated stiffening cap & amp-PSU arguments...) I was considering a PIC etc, but it didn't justify the learning curve (I was saving that for ignition & EFI systems.) But a PIC or uPC mapping that drives a voltage/current source is probably how I'd do it (if a sender swap wasn't possible).

Thanks for the replies. I don't really care much about the gauge being linear, or for that matter, accurate when full, as long as I know when the tank is empty. The problem with replacing the sender is that the 90/0 Ohm sender for the Ninety-Eight is not the correct part for my Regency. I *may* be able to get the resistor and/or float arm from a junkyard-ed Ninety-Eight, and rig it up to my sending unit, but I have no clue as to the compatibility of the two parts. I should also mention that my sending unit was recently replaced with an aftermarket (non-OEM) one, and it is not nearly as accurate as the original factory sending unit. When the gauge reads EMPTY and the LOW FUEL light comes on, I still have about 8 gallons left in the tank. Also, when I fill up the tank completely, I *never* can get the gauge to register right on full. It's always about 1/16th or 1/8th of a tank less. Perhaps the aftermarket sending units use incorrect resistor values? Nick