1999 7.4 Low sensor voltage P1122 P0122

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Christian Steffen

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My scanner does not show voltage. It does give me a linear graph showing what is going on. For the TPS, everything starts out ok and timing seems to be in the mid to high 20’s. After a few miles, the computer sees a 20% down to 0% and back and forth for the TPS. Like I’m letting off the gas and back on. It will do this a bunch up and down.

When I compare timing during when that happens, it is also pulling timing down to about 3-4 degrees. When the TPS starts acting normal again, the timing is still being pulled as the computer I believe is going in to limp mode. The shifts are hard and at higher RPM than normal.

Thanks
The timing is being pulled because of the knock sensor code, pretty sure it just pulls the max value allowable in the tables for it (10*?). The tps being sporadic is strange.. i really want to say its a physical connection issue but it really seems like you checked all of that. All the plugs into the computer are clean and connected properly i assume?

As far as new compute goes, a junkyard 98-2000 computer from a truck that has the same specs as yours should make it run (i assume yours is 454/4l80e? you'll want to match gear ratios too). For the sake of troubleshooting this is what i would do. If you would rather buy new, you'll have to get a tuner (or dealership?) to flash the correct tune onto the computer. A mail order tune (from Blackbear) would accomplish this as well, but he probably won't have it to you by new week.
 

rdomeck

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Christian,

I figured as much on the knock sensor. I believe that code is also a low sensor voltage. I guess my next test is to check voltage on those sensors while I am driving down the road. I have good voltage while sitting, just not after driving. Sometime its after a few miles, sometime it will give me 50-60 miles.

Thanks for the input and once I get this figured out, I will update this thread. I have found a few threads on the web, bit they are years old and not updated. They name the exact symptoms as I have, but no update on what the cure was!
 

Christian Steffen

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Christian,

I figured as much on the knock sensor. I believe that code is also a low sensor voltage. I guess my next test is to check voltage on those sensors while I am driving down the road. I have good voltage while sitting, just not after driving. Sometime its after a few miles, sometime it will give me 50-60 miles.

Thanks for the input and once I get this figured out, I will update this thread. I have found a few threads on the web, bit they are years old and not updated. They name the exact symptoms as I have, but no update on what the cure was!


Man, that's too bad no one finished their threads. The fact that it works as it should for a period of time really makes me doubt it's the computer.. but I could be wrong.
 

rdomeck

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Yep. It does make me think that the ECM is not the cause, but I’m out of other ideas.

I also have two 1996 computers with this engine and transmission combo, I plugged one in and it would barely run. Didn’t investigate past that as I figured the pin outs are different because it has an extra ply on it. Almost like an internal relay.

I have also found a very low mileage 99 Suburban about an hour from me that I am considering buying, swapping out parts to find the issue, fixing it, then selling it.

Thanks for input.
 

Christian Steffen

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Yep. It does make me think that the ECM is not the cause, but I’m out of other ideas.

I also have two 1996 computers with this engine and transmission combo, I plugged one in and it would barely run. Didn’t investigate past that as I figured the pin outs are different because it has an extra ply on it. Almost like an internal relay.

I have also found a very low mileage 99 Suburban about an hour from me that I am considering buying, swapping out parts to find the issue, fixing it, then selling it.

Thanks for input.

I know, I also don't really have more ideas. I bet this is pretty frustrating. Just for the sake of eliminating it as a cause, it would be nice to swap in a different ecm.
 

rdomeck

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I measured again this morning and I do not have a 5v reference voltage out of the ecm. I guess it’s time to buy and ECM.

I did have it. Now I don’t. This may confirm that the ECM is really the problem....

More soon
 

rdomeck

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Anyone know the difference in the 1996-1997 computers and the 1998-1999 computers? I see one extra connector that has two wires in it. I have two of those for this engine/transmission combo, not to mention an extra wiring harness....

The only pace that I can find a flashed ECM is AutoZone. Every other place that sells them are not flashed and that would warrant a trip to the dealer. Nope to both of those. I'd rather get an ECM from BlackBear in a week or two!

Thanks for the input so far!
 

Christian Steffen

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Anyone know the difference in the 1996-1997 computers and the 1998-1999 computers? I see one extra connector that has two wires in it. I have two of those for this engine/transmission combo, not to mention an extra wiring harness....

The only pace that I can find a flashed ECM is AutoZone. Every other place that sells them are not flashed and that would warrant a trip to the dealer. Nope to both of those. I'd rather get an ECM from BlackBear in a week or two!

Thanks for the input so far!
Aside from the obvious plug difference, I think the operating system is different. The 98-00 computers are tuneable with hp tuners or efi live whereas the 96-97 aren't. When I've hooked up to 96-97 trucks the number of PIDs available in the scanner is greatly reduced.

It's probably possible to delete that 5th plug but imo a junkyard of blackbear ecm is the best route.
 

rdomeck

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Thanks for the input so far. I have not had any luck with the local yards. Any good source for wiring diagrams? I have found some pinouts and the Hanes manual shows some stuff, but the Hanes doesn't;t show what connector and pin the wire terminates in to the ECM. I'm going to pull the engine harness out and go through it to make sure I don't have any bad wires or short to grounds.

I am leaning towards the 5 volt reference shorting ground rather than the ECM being bad. Once I get that sorted out, I will be buying an BlackBear ECM and having them tune it.
 

Christian Steffen

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Thanks for the input so far. I have not had any luck with the local yards. Any good source for wiring diagrams? I have found some pinouts and the Hanes manual shows some stuff, but the Hanes doesn't;t show what connector and pin the wire terminates in to the ECM. I'm going to pull the engine harness out and go through it to make sure I don't have any bad wires or short to grounds.

I am leaning towards the 5 volt reference shorting ground rather than the ECM being bad. Once I get that sorted out, I will be buying an BlackBear ECM and having them tune it.
Unfortunately, I don't have anything. All i have is my lextech pinout from the 0411 conversion.
 
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