Towing an Off Road / Overland FMTV

simp5782

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Don't worry. There are some of us that can rescue you from anywhere. Woods, beaches, desert. Two 55,000lb winches. Plus two 70 ton snatch blocks. It can drag you over the mountains and thru the trees if it needs to.
 

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Ronmar

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From what I have read and understand so far, a code reader is necessary to retrieve engine codes on the 3126 (I'd be delighted to learn that I'm wrong). I read a technical manual for the 3126 that was discussing buses, but what it described for buses was the same on our truck. The manual said that codes can be read out on a 3126 if the vehicle is equipped with cruise control. I do not think LMTVs have cruise control. If they do, mine seems to be missing. :) Thanks for your insight. I will look into getting a code reader.
Supposedly, you can read active codes on the 3126 using the check engine light. I believe this is done by closing the cruise control set/accel circuit with the cruise control circuit open/not enabled. Cruise control is native to the 3126 engine ECU. Now wether that feature was enabled on the engines delivered to the military is a question. The pins that need to be jumpered would be accessed in the ECU connectors. Broke overland over on youtube just detailed how to install a cruise control and he shows very well how to access the appropriate pins on the ECU. I do not know if he had to have his ECU programmed to enable cruise control, but he has cruise control:) You can find the pins on a 3126 ECU wiring diagram as they are listed/labeled. there are also some youtube vids showing guys reading codes in civillian trucks with the cruise switches on their dash...

yes it would be good not to need a code reader out in the boonies. One reason I like the 3116, especially given the 3126 ECU’s track record of just loosing its brain when it’s internal battery craps:)
 

simp5782

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Supposedly, you can read active codes on the 3126 using the check engine light. I believe this is done by closing the cruise control set/accel circuit with the cruise control circuit open/not enabled. Cruise control is native to the 3126 engine ECU. Now wether that feature was enabled on the engines delivered to the military is a question. The pins that need to be jumpered would be accessed in the ECU connectors. Broke overland over on youtube just detailed how to install a cruise control and he shows very well how to access the appropriate pins on the ECU. I do not know if he had to have his ECU programmed to enable cruise control, but he has cruise control:) You can find the pins on a 3126 ECU wiring diagram as they are listed/labeled. there are also some youtube vids showing guys reading codes in civillian trucks with the cruise switches on their dash...

yes it would be good not to need a code reader out in the boonies. One reason I like the 3116, especially given the 3126 ECU’s track record of just loosing its brain when it’s internal battery craps:)
Cruise control is in the ECM on the 3126 Adding a simple switch to power it up and put a circuit break off the brake light switch.

 

Mullaney

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Supposedly, you can read active codes on the 3126 using the check engine light. I believe this is done by closing the cruise control set/accel circuit with the cruise control circuit open/not enabled. Cruise control is native to the 3126 engine ECU. Now wether that feature was enabled on the engines delivered to the military is a question. The pins that need to be jumpered would be accessed in the ECU connectors. Broke overland over on youtube just detailed how to install a cruise control and he shows very well how to access the appropriate pins on the ECU. I do not know if he had to have his ECU programmed to enable cruise control, but he has cruise control:) You can find the pins on a 3126 ECU wiring diagram as they are listed/labeled. there are also some youtube vids showing guys reading codes in civillian trucks with the cruise switches on their dash...

yes it would be good not to need a code reader out in the boonies. One reason I like the 3116, especially given the 3126 ECU’s track record of just loosing its brain when it’s internal battery craps:)
.

So, I have to ask...

You mentioned "3126 ECU’s track record of just loosing its brain when it’s internal battery craps:) ".
Stupid question I guess - but is there an internal battery (somewhere) that needs to be replaced? On some schedule?

AND if this battery is keeping some electronic component powered with important information...
How do you (we) store that data while the battery is replaced?

.
 

coachgeo

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.

So, I have to ask...

You mentioned "3126 ECU’s track record of just loosing its brain when it’s internal battery craps:) ".
Stupid question I guess - but is there an internal battery (somewhere) that needs to be replaced? On some schedule?

AND if this battery is keeping some electronic component powered with important information...
How do you (we) store that data while the battery is replaced?

.
would assume one would clip in a (for sure charged) aux battery to keep power to the unit while remove old/replace new battery. Though info storage as backup for plethora of purposes would be good. Maybe on old eprom??
 

Ronmar

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.

So, I have to ask...

You mentioned "3126 ECU’s track record of just loosing its brain when it’s internal battery craps:) ".
Stupid question I guess - but is there an internal battery (somewhere) that needs to be replaced? On some schedule?

AND if this battery is keeping some electronic component powered with important information...
How do you (we) store that data while the battery is replaced?

.
The battery is not replaceable, that is the problem. You must replace the ECU, and cat will program it based on your engine s/n...
 

Mullaney

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would assume one would clip in a (for sure charged) aux battery to keep power to the unit while remove old/replace new battery. Though info storage as backup for plethora of purposes would be good. Maybe on old eprom??
.
For sure if that is how it is stored. An extra eeprom is a small easily stored insurance policy.

Maybe before I die I can read enough to know about something BEFORE it breaks or needs repairs.
.
 

Mullaney

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The battery is not replaceable, that is the problem. You must replace the ECU, and cat will program it based on your engine s/n...
Okay, so does that mean that you should have a new ECU sitting in a sealed box waiting for the day you need it?
.
 

Ronmar

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or this one:)


some say cruise off, set/accel on, some say cruise on and set/accel on, it seems to work both ways, so you at least need to make that set/accel contact between the 2 ECU connector pins. The gentelman in the first vid was nice enough to list the 3126B codes.

From what I have read, this method will only display current active codes, not codes stored in memory, so it is not necessarily good for troubleshooting intermittent problems.
 
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Reworked LMTV

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The battery is not replaceable, that is the problem. You must replace the ECU, and cat will program it based on your engine s/n...
Is the board potted ? (Sealed in epoxy or ?) I wonder if it could be back fed or tapped in to.
 

Ronmar

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Okay, so does that mean that you should have a new ECU sitting in a sealed box waiting for the day you need it?
.
You could. You could also pre-emptively replace it. I have been dealing with ECU’s for decades now and have never encountered an ECU failure. It supposedly does happen, but I have never personally experienced or heard from anyone who has had an ECU issue, working on a lot of Toyota, Honda and Mitsu... Normally removing power only wipes out stored codes and learned data, not the base program, This is what appears to happen to the cat, as the ECU failures I have read about and helped troubleshoot usually occured after a loss of vehicle battery power.

From what I have read(probably mostely here on SS), cat quietly admitted it made a design error and supposedly changed the design to be more foolproof(main program in non-volatile memory?) like other manufacturers apparently do...
 

Mullaney

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You could. You could also pre-emptively replace it. I have been dealing with ECU’s for decades now and have never encountered an ECU failure. It supposedly does happen, but I have never personally experienced or heard from anyone who has had an ECU issue, working on a lot of Toyota, Honda and Mitsu... Normally removing power only wipes out stored codes and learned data, not the base program, This is what appears to happen to the cat, as the ECU failures I have read about and helped troubleshoot usually occured after a loss of vehicle battery power.

From what I have read(probably mostely here on SS), cat quietly admitted it made a design error and supposedly changed the design to be more foolproof(main program in non-volatile memory?) like other manufacturers apparently do...
Thanks Ronmar !

Loss of (Main Batt) power for a couple of weeks didn't appear to have harmed my truck when it went through the battery theft and trying to figure out how to properly connect it back. It still doesn't appear to be the way it was - but now it has been so long ago - who knows.

Seems that if the MainBatt was a problem there would be all kinds of problems on the "sale yards" too.

Does the ECU have a port that receives programming? And if so, is that information CAT confidential? Maybe reading and pushing it onto an old ECU with a repaired battery? Just fishing. I would like to help if there are parts to play with...

THANKS for the info.
 

Ronmar

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Thanks Ronmar !

Loss of (Main Batt) power for a couple of weeks didn't appear to have harmed my truck when it went through the battery theft and trying to figure out how to properly connect it back. It still doesn't appear to be the way it was - but now it has been so long ago - who knows.

Seems that if the MainBatt was a problem there would be all kinds of problems on the "sale yards" too.

Does the ECU have a port that receives programming? And if so, is that information CAT confidential? Maybe reading and pushing it onto an old ECU with a repaired battery? Just fishing. I would like to help if there are parts to play with...

THANKS for the info.
unpredictability is part of the problem. Cumulative time without vehicle power may be a factor, age of the internal battery may be a factor, who knows but the person who designed it... I once worked on a radar system that used a keep alive battery on the main processor board. To change the battery you wrote the programming parameters specific to your installation(a lot of info) to a non volatile location, then replaced the battery, then you loaded the parameters back into working memory. But the basic program Operating System was in non volatile memory so the processor would reboot after the battery change. Why they didn’t make this an automatic process and skip the battery altogether is not clear...

You can access the programming with cat et software on the j1939 data bus if the processor is operational(OS running). This allows you to change parameters and view sensor data, but I don’t know if you can access or alter the basic OS. If it bricks itself and wipes the OS in the process of this failure, you might not be able to even access it, and it would probably be something cat specific to reload/force feed the basic OS... This is known as a bootstrap process where you send a specific set of commands in the blind to prep the processor and memory followed by the OS program data, all without any acknowledgement untill it gets rebooted and either runs the loaded software or it doesn’t...
 
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