Hey all,

I'm not new to the site, or wrenching for that matter, I just never post anything. However, I recently picked up a 1999 F350 with 135k miles on it, and I'm working through some problems - mostly sluggish throttle response and low boost pressures. The truck has a PHP Hydra chip and I'm reading the values with a SCT X4 scanner through the OBD2 port.

I've read on here that your MAP, Baro and Exhaust back pressure values should all match. Unfortunately, mine aren't anywhere close.

KOEO
Baro reads 17.37 inHg
Map reads 29.82 inHg
EBP reads 95.75 inHg

I decided to take off the EBP sensor and tube to clean them. The tube looks like this:




I'm pretty sure you're not supposed to be able to see right through it. I'll be replacing it this afternoon when the parts store gets it in. However, that doesn't fix the sensor issue. I tried cleaning the EBP sensor with some brake cleaner, but it didn't help.

So, I tried a new MAP sensor, and it still shows basically the same pressure ~30 inHg. I also installed a new EBP sensor, and it shows 108 inHg.




And in PSI if you'd rather:





So, what gives? Is this a scanner issue, or is this a PCM issue? I'm kinda stumped, so any help would be greatly appreciated.

Thanks!
Kyle


Edit: after a little more research, it appears that it's more likely that I have a bad baro sensor. What do you all think?

 

The MAP sensor is reading about right if you're at or near sea level. Is it an early 99 or late 99? The early 99 has a BARO sensor external to the PCM; the later ones are internal.

 

Yes, sorry, its an E99. And I am at sea level essentially. I found the barometer sensor under the dash. I'll try a new one. From what I understand, all the sensors base their readings off the baro sensor. Maybe this is why my readings are off? Its just bizarre that the EBP sensor is so far off, and both the old and new are reading the same.

 

No, they don't base readings off of one another. They're all independent. There are times when a sensor goes bad that the PCM will report an inferred value, but monitoring the sensor voltage instead of the value can sort that out. As pressure increases, the voltage from the EBP sensor also increases. With the reading you're getting from the exhaust back pressure sensor, I'd have to guess there's a short somewhere. The reading equates to roughly 4.75 volts, so it's mighty close to the VREF voltage.

 

Ok. So, most likely a bad baro sensor and a short in the EBP sensor wiring? I just checked voltage. On the EBP sensor, I'm getting 5.07 volts on the reference wire, and .07 on the other two wires koeo. At idle, I'm getting .04 volts on the other two wires. What should I be getting?

Edit: I didn't have the sensor plugged in on the numbers above. With the sensor plugged in, koeo I get:

Brown/white wire - 5.04 volts
White/red wire - .03 volts
Purple/blue wire - 1.11 volts.

And that equates to ~ 54 psi on my scanner.

 

Okay, so I'm answering my own questions. I found this - ECSD - Exhaust Back Pressure (EBP) Sensor - 1994-1997 Power Stroke FAQ.

According to the diagram, from A to ground there should be less than 5 ohms. I am at 1. B to ground should be greater than 1000 ohms. I'm at 323. C to ground should be greater than 1000 ohms, and I show an open circuit. So, pin B is shorted to ground. What does an open circuit on pin C mean?

The next test was voltage with koeo. Pin A should be 0-.25 volts. I showed .08. Pin B should between 4.5 and 5.5. I'm at 5.09. Pin C should be 0-.25 volts, and I'm at .07. So, those tests all looked good.

The next two tests say to check with breakout box installed on engine harness only. What's a breakout box? And, any ideas of where to look for the short on the VRef?

 

A breakout box would be ideal, but they're kind of uncommon and somewhat expensive. The one the article talks about plugs between the PCM and its harness and gives you a test point for every pin in the harness. Sometimes it's the only to really see what's going on since you can be probing circuits with everything hooked up and the engine running.

104 Pin EEC V Breakout Box 014-00950

It's hard to give advice where to look for a short because it could be anywhere. I'd probably start by keeping the meter hooked up and start at the EBP sensor moving harnesses around to see if the resistance changes.

 

So, I spent the afternoon looking for the short. No luck. After thinking about it for a bit, all 3 sensors, MAP, Baro and EBP use the same brown/white VRef wire. I'm guessing all 3 sensors should have the same specs, so the same test I did to the EBP should work for all 3. Voltage tests were all within spec. However, the brown/white wire showed too little resistance in all of them - about 400 ohms at the MAP, 300 ohms at the EBP, and 200 ohms at the baro. I'm thinking all my problems can be traced back to this. Now I just have to find that stupid short. The only correlation I see is that as I get closer to the PCM, there is less resistance. Could it be a short in the PCM?

 

Attached you will find the pin out and wire colors for a DPC422. There are enough similarities between it and a DPC402 that it should work for you so that you can trace the grounds back to the PCM wiring harness/plug and check for a short there. It helps to remove the DS fender liner to access the plug on the e99 or you can detach the fuse boxes under the hood from the clips on the side which will give you a bit of room.

 

Thanks! I really dislike wiring. How exactly do I go about this? Obviously I unplug the harness from the PCM. I'm guessing I put one probe on the brown/white wire at the sensor and the other probe onto the ground pins at the plug? When I find a circuit that isn't open, then I have found the wire color I'm looking for? Then trace that wire until I find a break? Is that the process or is there another way.

Again, thanks everyone for all the help! Most of my wrenching experience has been on older vehicles.

 

You may want to look at your 42-pin connector (driver side, under and forward of the turbo intake hose) on the engine itself. Remove the air filter hose, pull the 42-pin plug (10mm bolt), squeeze the clip on the jack, pull toward you, lower it, and push it through the square hole. You can turn the jack over and look at the bottom of the wire bundle - particularly for the wires that match the color codes of the sensors in question. Rubbed insulators with exposed wire are not your friend.

One shorted sensor can throw other sensors out of whack - the forum has seen this before. You can unplug all your sensors and test each one, one at a time.

 

Yes! I am not a huge fan of wiring either. Pikachu and Tugly offer great advice.

For example. If you were to isolate pin# 30 and pin# 79 at the PCM side plug and do a continuity test, it should be open/there should be infinite resistance. If that is not the case then you'll have to start tracing wires which is where Tugly's advice will come in handy. I do not know if the color scheme on the wiring is contiguous from connector to connector or not; however, there are only forty two pins at the VC. Hopefully this will narrow it down a bit. My truck had some crazy wiring mods done to it prior to my ownership due to an aftermarket exhaust brake.

 

Thanks, it looks like I have some work ahead of me. I quickly checked the 42 pin connector this morning. Nothing obvious, and moving the harness around didn't help. I messed with every bit of harness I could find, both inside the engine bay and inside the cab. There was never any change in resistance. One odd thing I noticed was that the resistance at the EBP plug was less this morning than it was yesterday - 175 ohms vs 325 ohms. Also, if I unplug the MAP sensor, the resistance goes up from 175 to 230 ohms. If I then unplug the baro sensor, the ohms go up to 290 as measured at the EBP plug. I'm not sure what that's telling me, but I thought it was interesting.

I have plans with my wife today, so I might be able to work on it later this afternoon. If not, I'll tear into it tomorrow. I'll remove the inner fender so I can go to town on the PCM plug. If anyone has any other ideas, I'm open to suggestions!

Thanks!

 

I see some excellent advice. I have cut and pasted from other threads as well as some material I have. I have given credit at the page bottom. For some reason (mine I'm sure) I am having trouble posting pictures and the like. Not very savy.


The information here is from numerous sources. I can't take credit for but a small portion. Although, I can tell you it works IF you take time to plan an assault and not just jump right in.


Breakout Box's are a great tool. But, a DVOM and some patients along with some tricks of the trade work just as well. Just not as fast.


Remember, you're working with the 5 Volt Reference Circuit. So, DO NOT use a standard Test Light when troubleshooting PCM Damage will occur!

You must have a good wiring diagram and identify what sensors share this reference. All potentiometers, pressure sensors and some hall effects (cam, crank, and vehicle speed sensors) must be isolated to find the short. They've already covered that above.

Thermistors can be left plugged in and will never cause this condition due to internal design differences.

While measuring reference voltage with the key on, (usually at the TPS or MAP because of easy access) start unplugging the sensors one at a time.

Find the sensor that causes the reference voltage to return to 5v when you unplug it.

If you still have no 5v ref. with the sensors all unplugged, then you must disconnect the PCM and measure for a short to ground on the reference circuit. (Sensors still unplugged)

No short to ground then you have a PCM problem. Make sure you check all PCM powers and grounds before replacing the PCM.


Grounds may be shared between any inputs Signal wires are never shared between sensors The 5v reference can be shared (externally from the PCM) with pots, psi sensors, and Hall effects but never with thermistors
A shorted potentiometer, psi sensor, Hall Effect may pull down the entire 5v ref. <Circuit including all of the internal circuits that use this same reference voltage


A shorted thermistor will not pull-down the entire ref. circuit due to the internal resistor in the PCM for thermistor circuits


The PCM uses a 5 volt regulator for multiple purposes (communication and sensor inputs to name a few) If the 5 volt reference circuit gets shorted to ground the engine will NOT start (no injector pulse, no communication with scanner, possibly no MIL, no flash codes, possibly no spark.


KOEO check for 5 volt ref. at the TPS or MAP


No 5 volt ref:


Shorted sensor (any one of them that share this reference voltage except thermistors and switch inputs)
Short to ground in the wiring
An open in the wiring
PCM not sending out the 5 volt reference
Bad PCM
No power or bad ground for the PCM
You must have a good wiring diagram and identify what sensors share this reference.


All potentiometers, pressure sensors and some hall effects (cam, crank, and vehicle speed sensors) must be isolated to find the short.


Thermistors can be left plugged in and will never cause this condition due to internal design differences.


While measuring reference voltage with the key on, (usually at the TPS or MAP because of easy access) start unplugging the sensors one at a time.
Find the sensor that causes the reference voltage to return to 5v when you unplug it.


If you still have no 5v ref. with the sensors all unplugged, then you must disconnect the PCM and measure for a short to ground on the reference circuit. (Sensors still unplugged)


No short to ground then you have a PCM problem. Make sure you check all PCM powers and grounds before replacing the PCM.

All three (BARO/MAP/EBP) should indicate atmospheric pressure (14.7psi at sea level) and read within 0.5 psi of each other.

ICPv should read 0.20-0.25 volts.

Oil Temp should closely match the current Ambient Temp.

KOER


Note: with the engine running, MAP and EBP = Pressure. Baro = Atmospheric Pressure + boost".

i.e. BARO = 14.7 + 2 psi (boost) + MAP = 16.7 = 33.4

PID: MGP (Manifold Gauge Pressure) doesn't augment the BARO PID with the MAP Data. = BOOST


The EBP Sensor and EPV work in unison to warm the Engine Faster to OT. It is PCM but not to the extent that it "hurts" fuel mileage. There is additional fuel delivered to maintain an increased idle while active.


However, when the Tube is blocked, it does not allow the pressure transducer to fluctuate by the exhaust pressure. All this is controlled based on following parameters:

IAT (Intake Air Temperature) below 10°C (50°F) to activate EBP

EOT (Engine Oil Temperature) below 70°C (158°F) to activate EBP

A high pitched whine, in exhaust system, will be heard when EBP device is active.

At the turbo exhaust outlet is a butterfly that when actuated will restrict the exhaust flow, this causes the engine to work harder in cold ambient and generate engine coolant temperature quicker for cab heat.

The actuation of this device is controlled by the PCM and the EBP regulator located in the turbo pedestal. The EBP regulator restricts lube oil flow out of the turbo (center housing) and directs oil to the turbo pedestal which pushes the EBP actuator arm outward causing the butterfly to close or restrict exhaust flow.

The PCM uses the EBP sensor to monitor the exhaust restriction. The EBP sensor is connected to the right exhaust manifold using a pipe and fitting.

The exhaust back pressure device will open when the accelerator pedal is depressed and the exhaust back pressure increases. When the exhaust back pressure decreases or the vehicle is in a steady state, exhaust back pressure decreases per the EBP sensor, the EBP regulator can, and will activate the EBP device again even while going down the highway as long as the EOT temperature is below the specifications.

EPR: Exhaust Back Pressure Regulator, also EBP regulator Output: For quicker engine warm-up at cold temperatures. If the IAT is below 37°F (50°F some models) and the EOT is below 140°F (168° some models) the PCM sends a duty cycle signal to a solenoid which controls oil flow from the turbo pedestal. This causes a servo to close a valve at the turbo exhaust outlet. The PCM monitors the EBP input to determine if the EPR needs to be disabled to provide power for increased load, then reapplies the EPR as load demand decreases until EOT or IAT rises.

The Exhaust Back-Pressure sensor is a variable capacitor sensor that is supplied a 5-volt reference signal by the PCM and returns a linear analog voltage signal that indicates pressure. The Exhaust Back-Pressure sensor measures the pressure in the RH exhaust manifold. This sensor is used in conjunction with the exhaust back-pressure regulator to form a closed loop exhaust back-pressure control system.

The exhaust back-pressure is controlled by the PCM to provide more heat to the coolant for cab heating when ambient air temperature is below 7°C(45°F) and engine oil temperature is below 75°C (167°F) during low load, low speed operating conditions.

An open or short in the Exhaust Back-Pressure sensor wiring will result in a low out of range voltage at the PCM, and the PCM will disable Exhaust Back-Pressure control.


Puncturing wiring insulation should only be done as a last resort. Once the insulation is pierced air and moisture are allowed in. Ever see that Green Stuff on wires? That USED TO BE WIRE. It occurred because of moisture and air react with copper and over time the copper gives way to the invading microbes.


So, if you must puncture a wire, cover the hole with an APPROVED SEALANT. There are many on the market. I personally use FINGERNAIL POLISH which works very well.


Keep in mind that testing circuitry is a tedious process. Done incorrectly, you can damage components such as PCM, ECM, GEN, etc. It’s important to have a plan, research it, and proceed.


The #1


Tip I can give anyone who is testing circuits is that if you don’t require the key in the on position REMOVE IT FROM THE VEHICLE!


The #2 Tip I can offer up is: When testing PCM, ECM, GEM, etc. Circuitry, KNOW IF WHAT YOU ARE TESTING IS (+) OR (-) ACTIVATION! Use an Automotive Logic Probe or similar tool to identify.


The majority of Todays vehicles are Ground-Switched Circuits. Be aware. Get a certified repair manual and wiring diagrams. You’ll thank your self later.


The #3 Tip: Know how to Back-Probe. Here is an example.


Tip#4: NEVER, EVER USE MORE THAN ONE “T” PIN ON A PCM, ECM, GEM, ETC. If you must, separate them with piece of card board or other material what will prevent them shorting. Be careful, these components are expensive!


IAT: 5vReference in, 3.897volts@32°F, 3.09@68°F, 1.72@122°F.


ICP: 5vReference in, 1.0volt@580psi, and 3.22volts@2520psi.


MAP: Frequency output; 111Hz / 14.7 psi, 130Hz / 20psi, 167Hz / 30psi.


EXBP 10-15psig w/ EBPV closed and not exceed 45psi at WOT Note: EXBP showing low (3-5psi) that barley rises with acceleration indicates bad sensor or plugged tube.


Credit to Paul Danner, James Danner, Dan Sullivan, and Thomas White. All great authors in diagnostics in their fields.

 

Thanks PFW! That's a lot of good info. Unfortunately, I'm still stumped.

I spent the last few hours looking for a short. I started at the 42 pin connector simply because it was easier to get to. All tests were negative for a short (i.e. open circuit from the VRef line pin 21 to any ground). I also tested to the individual signals for the ICP, CAMP, and EBP sensors (the only ones that run through the 42 pin connector). Still all negative.

Next, I moved to the PCM. I checked for continuity between the VRef pin 90 and a ground pin 25. I had around 1200 ohms resistance. Hmmm? I checked the resistance between the signal return (pin 91) and ground and AP (pin 89) and ground, and they had about the same resistance - around 1200 ohms. However, the EBP, MAP, CAMP, and ICP pins were all an open circuit. These are the 5 sensors that use the 5 volt reference signal. Why would the accelerator pedal circuit on the PCM have 1200 ohms of resistance when the other 4 sensors have an open circuit?

I finished with testing the harness side of the PCM connector. The first round of tests had one probe to the battery and the other going into the harness. The VRef pin, Signal return pin, EBP, CAMP, ICP, AP, and MAP were all open circuits. No shorts. Next, I tried going to ground pins (25, 51, 77, and 102) and the tests were again negative for a short.

If you're still reading my ramblings, thanks! Here's the summary. The only odd thing I found was the AP ohm reading on the PCM. I guess the VRef and Signal return ohm could also be to blame. Unfortunately, I have no idea what is considered out of spec. But, since all other tests were negative, I'm heavily leaning towards a bad PCM. Does that sound logical, or am I missing something else?

Thanks!