R33 stock boost gauge/sensor

[jovial_cynic]

I've got a Frankenstein Datsun 510 that I'm wrenching on, and I'm having a bear of a time trying to figure out a sensor-to-gauge issue.

Quick rundown:

Car: 1971 Datsun 510
Power plant: SR20DET
Instrument Cluster: R33/RB25DET

Yes, I've got a skyline gauge cluster in my 510 that's pulling signals from the SR20DET motor. I've already swapped out most of the sensors that the gauge pulls from (oil pressure, water temp, speed sensor) so the gauge actually reads correctly on most of the gauge faces. The only one I'm having a problem with is the boost sensor, which takes a signal from a MAP-like sensor that only feeds the gauge; it doesn't send anything to the RB25 ECU. I've got the sensor, and I believe I've got it wired appropriately, but I'm having very weird issues with it.

When the gauge is powered and hooked up to the boost sensor, I get nothing at the gauge. Nothing at all. My volt meter reads correct voltage from the sensor (2.5v+).
When the gauge is powered and I take a 1.5v AA battery and connect to the gauge input, I get a signal at the gauge.
When the gauge is powered and I take a controllable voltage regulator and directly inject between 0 and 5 volts, I can control the needle at the gauge.

HOWEVER.

I took a random sensor (in this case, a pressure sensor) that takes in 5v and outputs between 0.5 and 4.5v. My volt meter reads correct voltage from the sensor.

So...

When the gauge is powered and I take the output of the sensor into the input of the gauge, I get nothing.
When the gauge is NOT powered and I take the output of the sensor into the input of the gauge, I get a correct reading!

This tells me that when the gauge is powered, I have some kind of increased resistance at the gauge that the signal source cannot overcome.

Bud oddly, when the gauge is NOT powered and I hook up the boost sensor (which we've already established provides a correct voltage output (2.5v), I don't get any reading from the sensor.


This is the strangest electrical puzzle. Anybody have any ideas?


For info, the gauge takes a 12v+ and a ground, and it also takes a (+) signal and a (-) signal. In the case of the two successful tests above (AA battery and voltage regulator output), the gauge works whether I connect the negative end of the power source to the (-) signal input or not.

I've poured over the wiring diagram and can't figure this out...