Project Lexus SC300 Road Racer: Part 9 – Begin Rewiring at the… Back?


Haltech’s fuel level conditioner

If you recall our article about installing the fuel tank, we opted for a Fuel Safe / Radium FCST 22 gallon fuel cell that has a fuel level sensor in it. Factory fuel tanks usually don’t have baffles and use simple floats to measure how much fuel is in the tank. This can result in wide swings in indicated fuel level at the gauge during cornering when the tank empties (not to mention possible fuel starvation, but our FCST fixes that).

Fuel Safe employs a bit of mechanical genius in the level sensor they offer for their fuel cells. The sensor has a probe inside of a long tube. The tube has a tiny hole in the top and bottom of it. If you have ever tried to pour oil out of a quart too quickly you know that it tends to start to gurgle/glug. This has to do with air trying to enter the bottle to replace the oil as it exits. If the air can’t enter fast enough, a vacuum forms, which slows the flow of oil, until the pressure differential is so great that it stops the flow enough to suddenly allow air in, which then lets the oil flow more rapidly again, and so on and so forth.

In the case of the Fuel Safe sensor, something similar occurs. Because of these tiny holes in the tube, the fuel can't physically leave the probe fast enough in turns, braking, or accelerating.  This is what prevents their sender’s value from fluctuating rapidly and helps to provide a consistent fuel level reading. But we wanted more, because why not?

Haltech happens to manufacture a fuel level conditioner that can be used to damp the fuel level signal. It provides cleaner information at the dashboard and in the data logs. Many classes in NASA and other organizations permit the use of factory fuel tanks, which means factory fuel level senders, so this little gadget is extra super useful in those conditions.

Looking at the harness schematic, you can see that there are actually four wires that go to the level conditioner:

  • +5v power supply
  • Fuel level (input)
  • Conditioned fuel level (output)
  • Signal ground.

Wait… Signal ground? What the heck is signal ground? We're glad you asked. Stop rolling your eyes. We see that!

If you managed not to quit reading earlier in the article when we talked about chassis ground, you’ll still remember everything about noise and references for sensors. One way that manufacturers try to ensure their electronics have really good sensitivity for measurements is by creating dedicated and conditioned sensor voltage reference and signal ground circuits.

In the case of the Haltech Elite 2500 ECU, they have upgraded from using chassis ground as a signal reference to providing a signal ground circuit at the ECU. They also have both a +8v and +5v reference output / sensor supply for use with sensors and other devices. These dedicated circuits help to isolate out the noisy automotive electrical environment, but remember that ultimately, the ECU is getting its power and ground from the car battery and the chassis- which means these reference levels are still dependent on “clean” chassis ground. Just because the ECU provides these dedicated conditioned circuits doesn’t mean that you can ignore all the other chassis ground information from before.

Back to the fuel level conditioner. 

The fuel level conditioner takes a 5v input for power and applies a voltage across the fuel level sensor. For argument’s sake, it measures the voltage drop across the sensor (fuel level input) and conditions that signal, creating a voltage on the output which is representative of the fuel level. This fuel level output signal is sent back to an analog voltage input on the ECU, and thus the ECU knows how much fuel is in the tank with some simple calibration.


The Radium FCST / Fuel Safe Pro Cell.

Our fuel cell is equipped with a surge tank from Radium Engineering that employs both a low-pressure lift pump (that fills the surge tank) and another high-pressure pump (that goes to the fuel rail). For the sake of simplicity and brevity, let’s say that liquid pumps tend to draw a lot of current, and their current demands are related to the amount of pressure they are generating. These current demands will influence two aspects of the wiring — the connector selection as well as the details of the wiring.

Since the low pressure pump really is not meeting any resistance to its fuel flow, the current draw on the AEM pump will be very low. We will only need to use a 10A circuit on the Racepak for the lift pump.

We have a Fuelab 51502 pressure regulator that is a 1:1 rising rate unit. For every 1 PSI of manifold boost above atmospheric pressure, the fuel pressure regulator will add about the same amount of fuel pressure. If we start with a static fuel pressure of 50 PSI (for example), and are running around 14PSI of boost, that means that max fuel pressure will be around 64 PSI.


AEM’s 50-1000 pump’s current vs pressure chart.

AEM suggests that at this fuel pressure we can expect just under 14 amps of current draw. Now that we have a ballpark figure for current, we need to look at the connector and the wiring. We also know that we will need to use a 20A output from the Racepak Smartwire.

Deutsch makes several different series of their gray and orange sealed connectors (although they’re also available in other colors). In order of “smallest” to “largest”, they are DTM, DT, and DTP. We have a single connector at the rear bulkhead for this trunk sub-harness, and there will be another electrical connector at the fuel pump. Radium already provides connectors and pigtails that are designed to handle lots of current, so we only need to worry about the rear bulkhead connection.

If we look at the specs for the DTM connector on MilSpecWiring, we find that these are rated at only 7.5 A on each pin. This could work for us if we ran two wires across two pins, which is very common. But, we wanted extra insurance.

The specs for the DT connector state a 13 A current capacity per pin. This is more like it. On a single pin we could just about handle all the current we need for our high pressure pump. But what about the wiring?

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