I disconnected the wire harness from the map sensor to check the voltage coming to it through the wire harness. I had the key on and engine off. I had 4.8 volts between the voltage supply wire (orange) and the ground wire (black). I also had 4.8 volts between the signal wire (green) and the ground wire. Is it normal to have voltage in the signal wire when it is disconnected from the sensor? It is a 1987 f-150 4.9 liter straight six.

 

I don't think there should be any voltage on the signal wire, but I can't remember for sure. What is the signal voltage whe the MAP is plugged in?

 

It is a new map sensor. When it is hooked up the voltage is a constant 2.5 on the signal wire no matter what vacuum is applied. My old map acts exactly the same. So does a used map I got from a junk yard.

 

That's what is supposed to happen when it is plugged in. Sounds like it is working properly. These have to be tested using a tachometer or an oscilliscope as they are frequency varying MAP sensors, not voltage. A Haynes manual would have the procedure to follow.

 

It is a useful check to disconnect sensors and check VREF and ground.
However, checking the voltage on the sense line "looking back into the ECM" from any disconnected sensor is generally not particularly useful.

The voltage you will get on the sense line depends on the design of the ECM. In the absence of a connected sensor, the ECM may pull the signal to VREF, to ground, or leave it floating. Any of these techniques can be used in the design of an analog sensing circut, for voltage, current, or frequency. The choice is really an arbitrary accident of how the ECM designer went about it, and has little to do with how the sensor functions when connected. For this reason, the troubleshooting guides generally don't suggest or have anything to say about this sort of measurement.

If you have a known good truck, you could try comparing what you see there against your back-probing. Alternatively, you could disconnect the battery, the sensor, AND the 60 pin connector at ECM, and then use your ohmeter to check the resistance of the sense line to VREF and to ground. It should be infinite. Check the resistance from the sensor end to the MAP input on the ECM connector. This should be zero or close to it. (It probably is, given the 4.8 volts that you measured).

As Mr. EPNCSU2006 said, the sensor is a variable frequency generator, not a variable resistor like your TPS.

Your 2.5 volts regardless of vacuum is a result of your voltmeter averaging the ~4.5 volt high and 0.5 volt low of the variable signal being produced. This is exactly what it should be, and the voltage will not vary with vacuum (but the frequency will).

As an alternative to the suggestion of a tach or frequency meter, you could use an oscilloscope to check the frequency of the signal. It is fairly slow, by electronic standards, at about 100 hz.

For that matter, if you have none of the above tools you could hook up the MAP sensor output to an audio amplifier (or a junkyard radio speaker in series with a 1K ohm resistor).

It should play a cheery bass tune which will vary in pitch as you apply vaccuum to the sensor.

 

wow,,very well put.

 

A tach showed the map to be responding to vacuum. The PCM shows a pass on all codes. I put in a junkyard PCM and had the identical driving problems: (1) hard start/no start when cold (2) very bad hesitation on hard acceleration when cold, especially going up hill. Gradual acceleration is smooth whether truck is cold or warm. Hard acceleration is fine when the truck is warm. It always idles smooth, with 17.5 inches Hg manifold vacuum. The junkyard PCM showed a continuos code 95, which the Haynes manual says is the fuel pump - a connection between the PCM and ground. The fuel pump pressure and fuel flow tests are a pass. Is it possible to pass the flow and pressure tests and still have something wrong with the fuel pump? I had the injectors serviced and tested.

I put in a new throttle position sensor, egr valve and regulator. The coolant temp sensor and manifold air temp sensor test o.k. I can't find any vacuum leaks in the hoses or around the manifold gasket.

 

You say the coolant temp sensor and IAT sensor test OK, but your truck is showing all the signs of failing to recognize that the engine isn't warmed up. Perhaps the signals from the temp sensors aren't making it back to the PCM. I suggest you disconnect the battery and the 60 pin connector, and check the resistance from the corresponding pins back to the sensors. Should be zero or close to it.

Other "shotgun" ideas: Check all your grounds. Check the TFI modlue.

This is one of those cases where a scanner would be nice. The PCM can tell the scanner what temperature it thinks everything is at, and what fuel enrichment value it is using.

 

Fefarms, I want to say thanks. I now read all your posts. I put in a new TFI. I wonder if my problem is similar to Meck1, although mine has the additional problem of being a hard start when cold. When Meck1 says it misses on high vacuum I think he means on low vacuum, because when you press the throttle the vacuum goes down. I am going to check my grounds and resistances.

 

When I said thanks I forgot to mention EPNCSU2006. Thanks!

 

It was the coil. I was getting spark, so I had not focused on the coil. It turns out the secondary resistance was 15,000 ohms. The specifications were 8,000 to 11,500 ohms. The truck runs great with a new coil. I was reading a book about engines, and it was talking about how you need a stronger spark to ignite a lean fuel mixture. I think when my truck was cold the mixture was more lean, and it got even more lean when I accelerated, and my spark wasn't quite strong enough to ignite it. When my truck warmed up the mixture probably got less lean and my weak spark would work.

 

Do you still get 2.5 volts at the sig str ( black wire) at the MAP sensor? That was to the the battery ground correct? Pluged and unpluged. I get 0 volts both ways and get 0 volts at all sig rtns at all sensors. Just woundering. Also some else here with a same truck got 0 volts also. Is you engine light on?

 

In that post I was calling the green wire the "signal" wire, and the black wire the ground. I am certain that is correct, that the black wire is the ground. In the wiring diagram Ford calls the black wire the "signal return." I don't know why.

 

Read somewheres in the Haynes manual that the black- white striped wire is the SIGNAL RETURN/GROUND wire. Should have less then 1 volt on this wire. Weird.. ground but not a ground??? 87 f150 4.9

 

Can a truck run without the MAP sensor connected?