97 OHV; No start; wrong voltages at TPS

[jkk9]

Engine cranks fine, 40 PSI at fuel rail, no start. Using 2000StreetRod's Engine No Start Procedure post, I found the PCM reference voltage to be 0.5 volts instead of 5.0.

Is a bad PCM the only possible cause?

 

[Turdle]

No, there could be a shorted sensor in the circuit-

You might try disconnecting and re seating the PCM connector first, then if that fails to bring the voltage up, start eliminating the sensors one by one until the voltage comes up. Have you checked all the fuses under the hood and in the interior panel?

 

[jkk9]

No, there could be a shorted sensor in the circuit-

You might try disconnecting and re seating the PCM connector first, then if that fails to bring the voltage up, start eliminating the sensors one by one until the voltage comes up. Have you checked all the fuses under the hood and in the interior panel?

Fuses are ok. Wiring diagram shiws just TPS, DPFE and fuel tank assembly drawing from the reference voltage. Disconnecting dpfe did not help so i'm looking to disconnect fuel tank assembly.

 

[imp]

I once DROVE an EEC-IV equipped vehicle 2,100 miles with NO TPS on the engine. It seems debateable the TPS would cause no-start totally. At the very least, I would think a few farts & misses would occur.

The TPS is "at rest" during starting, with no pressure exerted on the accelerator pedal.

imp

 

[budwich]

the original post and title don't necessarily line up ..."exactly"... ??? The title "indicates" a TPS voltage issue BUT the text suggests nothing about TPS and instead refers to the PCM voltage with no mention of the actual TPS readings that may have led the poster down the path to the PCM reference voltage test. Of course, not knowing what exactly is in the PCM (ie. internal circuitry), BUT measuring the "reference voltage" as suggested in the "sticky" referred to in the post would appear to be incorrect. It suggests moving the red test lead from the TPS test to the reference voltage of the PCM... BUT this measurement is "highly suspicious" IF your black test lead IS NOT on a "known ground" unless you are trying to measure some sort of "floating reference" which I don't think is the case.
IF the poster did the PCM measurement exactly following that procedure, it quite possible, depending on what's in the PCM circuitry that his PCM reference voltage measurement is "meaningless" although it is more likely that he is just measuring some "floating voltage" which likely indicates perhaps a broken lead going to the TPS OR maybe a lost/weak ground (doubtful). Hopefully, the OP can clarify things.

Note: I use "float" some what incorrectly ... the voltage across the TPS "moves" about depending on the resistance of the TPS wiper... it doesn't really "float"... however, IF the same "unknown" black test lead point is subsequently used to measure a voltage elsewhere is the "system", then "float" is somewhat more accurate... because it is likely that there is no direct path to a ground for the test.

 

[jkk9]

Hopefully, the OP can clarify things.

I appreciate your attention to detail. I assumed that the sticky was (is) accurate; there are other posts that refer to voltages of ~1 to 5 volts.

The reference voltage is 0.5 volts (measured to chassis ground); the sticky says it should be ~5 volts. The TPS voltages (ranging from 0.098 to ~0.5) are also off by aproximately a factor of 10.

So, since the PCM is the source of the reference voltage, the problem is either the PCM or something pulling down that voltage (mentioned in my other post).

 

[budwich]

sorry I must have been on "bad beer"... what drdoom said.

 

[drdoom]

So is that the 5V measured at the harness, unplugged from the TPS, probing the two outermost pins, w/ key on.

 

[jkk9]

So is that the 5V measured at the harness, unplugged from the TPS, probing the two outermost pins, w/ key on.

Well, it's supposed to be that 5V. It currently measures ~0.5V.

The PCM supplies ~5V to the TPS, DPFE and the fuel tank pressure transducer (but not pump assembly). Either the PCM is putting out low volts or there's something lowering that 5V to 0.5V.

So, trying to isolate the cause, disconnecting the TPS and DPFE connectors, the supply is still 0.5V. So, I need to disconnect the fuel tank assembly to see if that's pulling down the 5V. But, I can't find an easy way to disconnect the fuel pump assembly - short of lowering the fuel tank.

Does anyone know how to disconnect the fuel tank assembly w/o lowering the tank?

 

[jkk9]

Does anyone know how to disconnect the fuel tank assembly w/o lowering the tank?

I found the connector to the fuel tank assembly (i.e. fuel tank pressure transducer, pump and sender). The connector is accessible between the tank and the frame.

Disconnecting it does not give me ~5V for the reference voltage on the PCM, so I'm guessing the PCM is bad.

It's possible that the PCM is not supposed to give 5V under some condition, but I can't imagine what that would be given that the voltage is 0.5 volts.

So, I'll try a used PCM, probably ebay.

 

[jkk9]

Pcm was bad

Problem fixed. Got the PCM from ebay. Measured +5V at TPS so I now have the +5V reference voltage from PCM.

Truck starts. We'll see if there are issues with used PCM...