Sensor is above voltage spec?

[SMEGMA STENCH]

Let me first say that I'm an electricity noob. So I've never really heard of this kind of thing happening. I've only heard of things being below the minimum voltage.

The vehicle is a bone stock 1994 Mustang GT. I am having some trouble with the car running rough. It runs rough and cuts out and lacks power when you're driving it. So to get to the point, I scanned the codes and it came up with two codes. One said "Mass air flow sensor below minimum voltage". Ok. I've dealt with that before, and I know that you check the wiring over, but it usually means a dead sensor. But the next code, I'm afraid I don't really understand. It said "Intake air temperature sensor above maximum voltage". Hmm. So how does that work out? Somehow the sensor is getting more voltage then it needs, according to the code. So I can assume what... that the sensor is still good, but that I need to... look for... I don't know. I don't know how to go about dealing with that one. I was hoping someone here could tell me what to do about it.

[Strychnine]

Possible sensor failure, a short circuit to ground in the +5-VDC common supply, a short circuit from pin to pin, open return circuit in the harness / connector / sensor, or open signal circuit or shorted to voltage source.

[Diabolic]

I would clean the MAF meter first. The voltage too high I always interpreted as the voltage was too high for the temp it was registering. In other words at 90º it should read 3 volts, but it's actaullly reading 4.5 volts. Therefore it thinks the vehicle is warmer than it should be and is quite possibly dumping more fuel and pulling more timing than it should be. This is my understanding.

[SMEGMA STENCH]

Possible sensor failure, a short circuit to ground in the +5-VDC common supply, a short circuit from pin to pin, open return circuit in the harness / connector / sensor, or open signal circuit or shorted to voltage source.

So basically, I should thoroughly check over all the wiring that's leading to the sensor. And if turns out all that is ok, then replace the sensor. Sound about right? When it said "above max voltage" I was thinking "What... like a small power surge is getting here or something?" lol

I would clean the MAF meter first. The voltage too high I always interpreted as the voltage was too high for the temp it was registering. In other words at 90º it should read 3 volts, but it's actaullly reading 4.5 volts. Therefore it thinks the vehicle is warmer than it should be and is quite possibly dumping more fuel and pulling more timing than it should be. This is my understanding.

Ok cool. But this is a distributor car. I thought you set the timing by the distributor, and that was how the car knew what timing to run at. Then again I'm also a bit of an engine noob.

[orphan Shelby]

It might not be that its getting alot of voltage, but what the possibly bad sensor is perceiving as high. If it goes dumb, and reports to pcm that its cold,then pcm reacts by adj timng and fuel strategies which will screw up how its suppose to run. Cleaning maf is easy and super cheap also.

[TENGRAM]

Remove The IAT Sensor. Measuring across the terminals on the sensor, check that it corresponds with the values on the table below (ie. it should read around 20kohms today since it is 90*). Else the sensor needs to be replaced.

[orphan Shelby]

^exactly, cant have sensors lying to your PCM

[SMEGMA STENCH]

It might not be that its getting alot of voltage, but what the possibly bad sensor is perceiving as high. If it goes dumb, and reports to pcm that its cold,then pcm reacts by adj timng and fuel strategies which will screw up how its suppose to run. Cleaning maf is easy and super cheap also.

I went ahead and cleaned the MAF using the proper method. I just found it a little odd that both the MAF, and the IAT sensor were having these problems, when they are both connect to the exact same little wiring harness. It looks like they almost share the same wiring so I was a little unsure.

Remove The IAT Sensor. Measuring across the terminals on the sensor, check that it corresponds with the values on the table below (ie. it should read around 20kohms today since it is 90*). Else the sensor needs to be replaced.

Ok cool I'll break out the voltage/ohmmeter and check that. Do you happen to have the voltage specs for the Mass airflow sensor on hand? I cleaned it with alcohol and Q-tips before I started this thread. So apparently it being dirty wasn't the issue in this case.

[TENGRAM]

Do you happen to have the voltage specs for the Mass airflow sensor on hand?

This is the diagnostic procedure for a fox body... I imagine it would be similar for your 94.

[SMEGMA STENCH]

Dude you're freakin awesome. That is all.

[SMEGMA STENCH]

Sigh... well, it would seem that according to the chart you posted, the IAT sensor is within spec. So drawing on what little electrical knowledge I have, now I should test the IAT sensor's plug for... 12v? Yeah I don't know crap about this kind of stuff lol. I am thinking test for 12v at the sensor's plug, cause the car is on a 12v system. That's probably wrong though huh

[TENGRAM]

Sigh... well, it would seem that according to the chart you posted, the IAT sensor is within spec. So drawing on what little electrical knowledge I have, now I should test the IAT sensor's plug for... 12v? Yeah I don't know crap about this kind of stuff lol. I am thinking test for 12v at the sensor's plug, cause the car is on a 12v system. That's probably wrong though huh

Ignition in the 'on' position, with the IAT sensor disconnected, you should see 4-6v between the terminals on the harness connector.

The engine control sensors all utitize a "VREF" voltage of ~5v. The 'VPWR' voltage of ~12v is used for the actuators (injectors, emissions solenoids, IAC, etc)

[SMEGMA STENCH]

Makes sense. I just didn't know if it was like a cell phone charger or something, where it just toned down the power internally.

[SMEGMA STENCH]

Nice. The plug to the IAT reads zero voltage, even with the car running. Yet the engine code said the sensor was reading above maximum voltage. Replaced the sensors, and the car still runs bad but the check engine light went off LOL. Yeah I'm taking this to my trusted mechanic now.

[TENGRAM]

Nice. The plug to the IAT reads zero voltage, even with the car running. Yet the engine code said the sensor was reading above maximum voltage. Replaced the sensors, and the car still runs bad but the check engine light went off LOL. Yeah I'm taking this to my trusted mechanic now.

Sounds like the SigRTN circuit is open (broken). Ignition 'on', check voltage on the harness connector between each terminal and any good ground (or the negative post on the battery). One of those terminals (the IAT Signal terminal) should read 4-6v. If this is the case, your SigRTN circuit is toast. This is pretty common.




FYI, the SigRTN is the ECM's dedicated ground circuit and is shared by all of the engine control sensors.