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Old 02-05-2017
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VREF issues

Hi guys,

I believe either the ground on my PCM is bad or some sort of reference circuit to many sensors is bad. I have a 1994 2.3L ranger 4x2. Let me explain.

I bought an OBD1 scanner, and it spat out a ton of codes even ones that don't necessarily make sense. But the gist is that my MAF, ETC, ITC, TPS, EGR etc etc are all bad (which seems unlikely). So I believe the wiring common to all these is an issue somewhere. I was doing the Key On Engine Off test first and some codes pertained to the engine running. The issue is very similar to this thread:

"Bad Ground/Power Connection; PCM? - The Ranger Station Forums"

So, I started with the TPS. In his case splicing in a new ground worked. However, I'm not receiving any high idle, high rev problems. The truck is sluggish without power (maybe normal - this is my first 2.3) and does this surging thing.

I tried back probing the TPS following the instructions in this thread (Ford Explorer - Ranger TPS Test Procedure | Ford Explorer and Ford Ranger Forums - Serious Explorations) and let me tell you that's FUBAR. The TPS cycles from -0.1V to -0.56V, yet the truck is still driveable. Pulled the plug and VREF (brown/white wire) reads 5.0V when grounded to the battery. Now this other thread (tps ground problem | Ford Explorer and Ford Ranger Forums - Serious Explorations) on the next to last post by X-North titled "The Solution" suggest measuring the voltage between the VREF (brown/white wire) and wiper (grey/white) wire. I got 4.85V; he's saying you need 5.0 and if you don't 'get 5.0 then it's the grey/white wire that's the problem. The grey/white wire to battery is roughly 0V and the red/grey wire (signal return) is 5.5V grounded to battery. X-North, then suggest checking the connection behind the alternator to see if the same 4.85 V exists between brown/white and grey/white. If so, he says replace grey/white back to PCM.

One question I have is which connector is he talking about. The one directly behind the alternator looks difficult to get to and unplug, especially the PCM side.

I have also checked the different grounds by placing a probe on battery terminal then touching rings/wires/bolts both on the ground by the battery and the one by the computer. Both showed continuity, but on the OBD1 reader I had to jumper the small plug to the battery because there was no ground there and the reader wouldn't get the codes without a proper ground.


Any suggestions? Why is the TPS so far out of range? What do all these sensors have in common? Is the grey/white really bad? Is it the ground? Thanks for any help, and please let me know if I can be more clear or provide more info.

Last edited by jcastilow; 02-05-2017 at 07:24 PM. Reason: make model
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Old 02-06-2017
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Welcome to the forum

Your 1994 PCM will use 3 digit OBD codes


Look here for PCM(EEC-IV) wiring: Ford EEC-IV

TPS does use 5volts from PCM pin 26, as do several other sensors
Usually the top wire on TPS will be that 5volts
Center wire is the Return Voltage, so with key on it should show under 1 volt(.69 to .99)
And at wide open throttle it should be at or above 4.5v(4.5 to 4.9)
Lower wire is a ground, but not vehicle Ground, it is Grounded via pin 46 at PCM

PCM has several Grounds to vehicle and they are not all shared internally, pins 16, 20, 40 and 60 all need to have good grounds.

Yes, TPS, IAT(air temp), ECT(computer engine temp), share the 5volts(pin 26) and ground(pin 46)
MAF doesn't but internally it does use the 5volts on pins 14 and 15, so could be effected if internal 5volt power supply is the issue
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Old 02-06-2017
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Thanks Ron,

That's useful information and a useful link.

So, when I check the top wire on TPS plug (not the TPS itself) I see that 5 volts when grounded to the battery - red probe in plug; black probe on battery. Hence, I expect the 5V supply in the PCM is good.

I think the real problem is the lower wire on the plug reads 5.5V when grounded to the battery - not the 0V I would expect of something that's grounded. I think this is also why the TPS reads -0.1V to -0.5V when the sensor is back probed. I think if the ground read 0V then I would get the usual 0-5V reading that's normal.

I have two questions (maybe three): I'm going to check that pin 46 at PCM reads 0V when compared to the negative post on the battery - is that the right thing to do? If it DOES, then would an open sensor somewhere cause this issue with the ground? If it DOESN'T, would this mean I need to splice in a new wire at pin 46 and attach it to the vehicle grounds?

Thanks for any help.

Last edited by jcastilow; 02-06-2017 at 12:45 PM. Reason: negative sign
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Old 02-06-2017
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Here are some better diagrams

2.3l should be the same as far as these sensors go

Thing is the other sensors sharing this voltage just pass it thru, so that "ground" is not really a Ground as in 0volts, it is a reference ground, so for example if the older PCM was really at 4.8volts, reference "ground" would be offset the same, so data is still acurate
Attached Files
File Type: pdf 94 4.0 eng 1.pdf (57.3 KB, 4 views)
File Type: pdf 94 4.0 eng 2.pdf (39.1 KB, 4 views)
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Old 02-06-2017
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OK... so 5.5V at the reference ground is not a big deal? Do you have any idea why the TPS would only read between -0.1V and -0.5V when ignition is on and the sensor is being backprobed on grey/white and grey/red?

Should I check the TPS itself - do you just check it for continuity between grey/white and grey/red?

EDIT: I would also say that since the top wire reads 5V and the TPS is suppose to go from 0 to 5 volts that the reference ground should read 0V when grounded to the battery. Otherwise the TPS is never going to read 0-5 volts. I see what you're saying - that the difference between the top wire and bottom wire just has to be 5V, but if one reads 5Vs then the other should read 0 volts. right?

Last edited by jcastilow; 02-06-2017 at 04:10 PM.
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