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TPS Question

Landcrusher

NAXJA Forum User
Location
Cascade Range
OK, this one has me stumped (not that that's never happened before, which is why I carry an axe in the kit with the other goodies!!)

94 XJ 4.0 5spd runs great with good responsive acceleration without a cough, hiccup, stutter or trouble code set - until you're on a long pull 5%+ grade at WOT and then the rpm's and mph fade (and again no trouble codes). On the flats, it runs like a raped ape. In the hills with the transfer in low range, it only stops when I've screwed up.

Exhaust is not plugged. Has had a recent tune. Compression and fuel pressure are good, fuel filter flows freely. No vac leaks found and cruise behaves as it should. MAP sensor voltages meet spec closely. And TPS is within spec but is only 3.0v at WOT.

Is the PCM possibly seeing the 3.0v max TPS output as acceleration at maybe 75% and not as WOT?

Any ideas?

Landcrusher
 
I don't know if this applies, but in the Renix the fuel pump gets power from the oxygen heater relay at WOT, not the fuel pump relay. This is to bypass the ballast resistor for full voltage.

I think the pump changed and the ballast resistor was eliminated in 94, but it still might be applicable to yours in some manner, such as a bad oxygen heater relay?
 
Joe,

You may be on to something. The 94 doesn't have a ballast resistor, but the fuel pump relay supplies common feed to both the fuel pump and the heated O2 sensor. These wiring diagrams can be a mind-bender at times.

Random thought here: With WOT supposed to be open loop, the O2 signal is supposed to be ignored by the PCM, but nothing is stated about the how the PCM does this. An open, of course, just means the circuit is turned off, no flow. So the common feed line from the fuel pump relay can now only supply the fuel pump module. However, one of the O2 lines goes to a common that is connected to two PCM power grounds at the #11 and #12 position connectors.

What fun !??!

Landcrusher
 
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