Problem with my 1999 Chevy Malibu V6. Code for Reference A high code.
Repair work done prior = Initial customer concern was a racing idle. Code existed for  MAF. IAC
motor was frozen and unable to be commanded with the Tech 2 scanner.
Work done to repair this problem = Replaced IAC motor and MAF.  

Question = Replaced IAC and idle no longer raced but still was too high. MAF code for  reference
voltage. Back probed sensor and found 4.2 V at the MAF. Disconnected
MAF  and reference pin
was at 5V. Replaced MAF, code cleared. Idle still erratic. Code set  for Reference A voltage high.
Back probed TP, Fuel Tank Pressure Sensor, MAP, and  EGR and found 5V reference either
connected or disconnected. Noticed that upon key  on, Reference A will go to 6.2V. Took fuel
pump out of the circuit and Reference A  stays at 5V with key on. Jumping in the fuel pump
causes the Reference A voltage to go to 6.2V with or without the sensors connected. All grounds
have been checked good   and tight. Any ideas of what next? I can't imagine a voltage regulator in
the PCM failing in such a way to cause this behavior, but I'm at a loss as to what's causing it.
Maybe a back EMF on the ground from the fuel pump? In any case, any help/insight would be  
greatly appreciated.

What is the code you are getting? Is it P1635?

When you say you took fuel pump out of circuit, volts go to normal 5V, did you actually just
disconnect the fuel pump at the pump, or the entire sending unit harness elsewhere.

Have not heard back from you in while, but anyway. The fuel pump power and ground circuit
would have nothing to do with the 5 volt reference. Although that relay also has something to do
with the A/C compressor control in my schematics.

You need to test for 5 volts on one of the sensor with everything back together, and disconnect
the sensors on that circuit 1 by 1 until volts drop back to normal 5 volts.
A/C pressure sensor, TPS, MAP, MAF, Fuel tank pressure sensor. (See attached schematic)
Have had either of these sensors or the wiring to them cause the 5 volt reference circuit to have

Also, check the harness from under the body going to the fuel tank module. A pinched wires
above the tank for the tank pressure sensor can cause this. As well as the tank pressure sensor
itself. Lower the tank enough the access the sensor and unplug it only. Not the pump connector.
Then cycle key, check to see if 5 volt circuit is normal.

Thanks, already had the schematic. Found a corrosion bridge between Reference A and fuel
pump supply in the pass through connector to the fuel pump. This was causing the change in the
reference when the fuel pump circuit was energized. Cleaned the pass through connector and
repair checks good. Thanks for the response.
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1999 Malibu Engine Light Code- Reference Circuit High