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Old 08-20-2009, 05:41 PM   #1 (permalink)
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12 valve cummins tech thread

I bought this truck a couple of weeks ago.

http://pirate4x4.com/forum/showthrea...ighlight=dodge

specs are

98 12 valve CTD
Auto
3.54 gears
245/75/16 tires
125,000 miles

Couldn't be more stock.

After a couple of weeks of driving it, I've found some things that aren't right with it. Namely fuel mileage sucks. got a whole 13.333 miles/gallon, hand calculated.

2nd fuel gauge seems off a little

seems like the convertor locks up too early. It's hit's overdrive and locks the convertor right about 45mph. I've read this is normal but.......

Won't down shift when going 50 mph +. Holds the gear and gradually pulls. I had my slide in camper in for a weekend, went up I-70 over the pass and the thing plain and simple wouldn't downshift until it was, well I don't know when, I hit the OD button to get it down to third so I wouldn't lose to much momentum.

Track Bar, (ordering soon). Common problem on these.

Gauges, (ordering soon). Don't have any now.

There's a lot I want to do to this thing. I want to start by bring the truck back to 100% stock operation and learn as I go. For having owned 4 diesel trucks now, I'm not really up to speed on them. As much as I should be anyway. I just sold my other truck and have a little money to play with so.

A lot of you guys have referred me and others to the numerous "other" sites. I appreciate that and go there often. I hope this thread will be able to gather information on the 12v CTD for this site though. So I'll be bringing info from the other sites to here as I find it.

I plan to keep this truck for A LONG time and want it to be a reliable multi purpose vehicle serving as a tow rig, daily driver, and weekend camper.

Try not to clutter it up too much in here. If you have useful information, links you want to add, anything of help, please feel free.


Later

Links

BHAF part numbers and suppliers. Also has a heat shield template to download
http://mopar.mopar1973man.com/person.../bhaf/bhaf.htm

TPS Adjustment procedure
http://www.dieselbombers.com/1st-gen...ensor-tps.html

TPS bypass trick
http://www.dieseltruckresource.com/d....html?t=142641

TV cable adjustment
http://www.dieselbombers.com/1st-gen...djustment.html

Installing Fuel PSI gauge
http://www.cumminsforum.com/forum/94...-12-valve.html

Fuel Pre-Filter Cleaning/Replacement Procedure
http://dodgeram.org/tech/dsl/filter/pre-filt_clean.htm

Fuel Shutdown Solenoid linkage adjustment (can cause low fuel psi)
http://www.dieselbombers.com/12-valv...djustment.html

Idle speed adjustment
http://dodgeram.org/tech/dsl/adjustm...d_Gen-Idle.htm
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Last edited by Repost; 08-21-2009 at 07:41 AM.
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Old 08-20-2009, 07:41 PM   #2 (permalink)
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First things to do are to verify that fuel pressure is good. 18-26psi at 2000rpm with no load. Next, check your valves, .010 intake, .020 exhaust. Third, timing. It has most likely slipped, causing some lousy fuel mileage. You will need a timing tool kit to check it, or have a reputable shop check it. For a totally stock, no mods truck 13-14 degrees is good.
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Old 08-20-2009, 07:50 PM   #3 (permalink)
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IF you want a forum for diesel question, this is not the ideal place to ask. You are better off going to Cumminsforum.com or Dieselram.com. I'm a member of both forums. Or you pay to become member of TDR.

With that being said, I think that if you're getting 13mpg UNLOADED, then you should verify entire air intake integrity from turbo all the way to intake manifold (no boost leaks), make sure turbo is not dragging, make sure the transmission is not hanging up, make sure the fuel pump timing is set correct and make sure the rack is correct, make sure there is no restriction in exhaust and all that. there is a long list of possibilities of whats wrong with it.

You have to provide more specific explanation you are experiencing. Such as, color and size of smoke, boost pressure, transmission line pressure, water/oil temperature, fuel pressure to pump, operating altitude, driving styles and so on. Sometimes, its hard to diagnose a performance issues over internet without taking a look at it. So, some of the tests I recommend you do requires some of the tools you have to acquire.
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Old 08-20-2009, 07:58 PM   #4 (permalink)
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1. Mileage isn't too far off of what I would consider normal. I would consider checking/adjusting your timing and checking your injectors. That should help economy.

2. All the fuel gauges are off. Diesel fuel affects their accuracy over time.

3. Tranny operation sounds about normal. You might consider playing with the TPS adjustment to get things a bit more to your liking.

EDIT: A complete timing kit is not needed to check and set your timing. All you really need is a dial indicator and a delivery valve socket.

Last edited by mondtster; 08-20-2009 at 07:59 PM.
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Old 08-20-2009, 08:55 PM   #5 (permalink)
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my 96 4x4 regular cab long bed automatic truck gets about 18-19 mpg unloaded on the freeway, towing it gets around 13-15 depending on the load......Keep it out of OD unless you are on flat level ground keeping a constant speed about 55 mph, lugging it down in OD is a great way to kill the tranny quickly......Start with the basics though, change the fuel filter, which can be a pain if it dont have a Prime-Loc filter relocation kit on it, also check the output pressure of the lift pump.......I havent seen too many of these trucks have injectors go bad at such low mileage, did the truck set ALOT before you got it?? cause it may just need the old diesel ran out and the tank needs a fresh fill and fresh fuel filter and that may help......
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Old 08-20-2009, 09:09 PM   #6 (permalink)
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As an ex 12 valve owner I'll give you a few pieces of wisdom that I paid a whole lot of cash for...

You can't make yourself a fuel pressure gauge setup fast enough. Pull the banjo bolt out, drill and tap it and get a short flex hose then slap a 60psi gauge on the end of it. That alone will tell you a ton about what the thing is doing, the 12 valve Cummins is so simple it's funny but the little things will get you. I'd replace the fuel filter no matter what the previous owner told you, I'd also pick up a BHAF as soon as you can and get it on the truck... you'll notice a difference. I would remove the fuel pre-filter completely since you don't need it and clean out that screen when you do the fuel filter because it will collect junk and cut down on flow. Once you get fuel and air to her, then start messing with timing.
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Old 08-21-2009, 07:21 AM   #7 (permalink)
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Thanks for the input guys. I edited the original post and started adding links for some of the stuff you mentioned.

Sounguy, can you elaborate on the fuel pre-filter?
You say remove it? I found the cleaning/replacement procedure http://dodgeram.org/tech/dsl/filter/pre-filt_clean.htm

Mondtster. Can you elaborate on checking and setting your timing with just a dial indicator and delivery valve socket?
I found this link. http://dodgeram.info/tsb/1994/18-10-94a.htm Is that the method you were talking about?
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Last edited by Repost; 08-21-2009 at 07:35 AM.
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Old 08-21-2009, 08:13 AM   #8 (permalink)
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Quote:
Originally Posted by Repost View Post
Mondtster. Can you elaborate on checking and setting your timing with just a dial indicator and delivery valve socket?
I found this link. http://dodgeram.info/tsb/1994/18-10-94a.htm Is that the method you were talking about?
Yes, measuring plunger lift at TDC is the most common way to determine your injection timing. The link you posted details how to do it with the whole kit (which I have but it is not needed). You do pretty much the same thing, but the dial indicator holder can be substituted for a mag base or some other method of holding the dial indicator and measuring the plunger lift.

EDIT: By the way, here is a link to a better plunger lift chart: http://www.competitiondiesel.com/for...ad.php?t=55821

I would not use the timing pin as an accurate measure of TDC unless you verify it first. The timing pin is adjustable (for machining tolerance variances) and can be off by quite a bit it if has been removed or was never adjusted properly. I always use the drop valve method of finding TDC.

You also could pin the pump and pin the motor. That would get you close to stock timing, but I stress close. It is by no means accurate and the pump pin is completely adjustable too.

As for fuel pressure, every 12v motor that I have worked on has had low fuel pressure due to age. The piston style lift pumps do wear out, but you are more than likely going to need a new overflow valve. I personally have never found fuel pressure to be a factor in fuel mileage however. My truck got the same mileage (both city and highway) with low fuel pressure and fuel volume as it did after I fixed it. I picked up power and the injection pump will thank you. Unfortunately for me, my truck had been run with a lack of fuel for too long (prior to my ownership) and it killed the injection pump (yes, you can kill a p7100 with a lack of fuel just like you can kill a VP). The truck still ran, but not well.

Last edited by mondtster; 08-21-2009 at 09:24 AM.
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Old 08-21-2009, 08:15 AM   #9 (permalink)
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Quote:
Originally Posted by LOGANSTANFORTH View Post
I havent seen too many of these trucks have injectors go bad at such low mileage, did the truck set ALOT before you got it??
"Going bad" is a relative term. In a mechanical injection system they may still appear to work fine but are far from optimal.
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Old 08-21-2009, 09:04 AM   #10 (permalink)
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fuel gauge reading is a notorious issue on these... the arm on the float gets loose effecting the contacts... you can take it out and jack with it using a new nut and bolt, or try and find a dealer selling just the float arm assembly for the whole unit.
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Old 08-21-2009, 04:03 PM   #11 (permalink)
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when i first got my 96, (ca) truck with egr and cat, i only got 14 mpg empty.
put in 4" exhaust, lost the cat. then i put in a 3000 gsk and 10 plate, star wheel full in and afc full forward. then i removed the egr.
bumped the timing to 16 degrees.

now i get 17 unloaded with my foot in it, but if i drive easy i can get 18-19.

front end, i suggest doing a track bar conversion. it replaces the old one with the newer style, getting rid of the tre that wears out.
a dss will help a ton also by supporting the steering box.

the 45 lockup is normal, but the not downshifting doesnt sound normal.
probably just needs a tps or kickdown cable adjustment.

and yup the fuel gauge issues are common. common enough that i watch my miles more than the guage, never know when it might give me a bad reading.
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Last edited by derek; 08-21-2009 at 04:04 PM.
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Old 08-21-2009, 06:18 PM   #12 (permalink)
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Quote:
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the 45 lockup is normal, but the not downshifting doesnt sound normal.
probably just needs a tps or kickdown cable adjustment.
You can adjust the TV cable all you want, but it will never do a thing for OD and lockup shift points.
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Old 08-21-2009, 07:05 PM   #13 (permalink)
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dieseltruckresource.com is another good one.
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Old 08-22-2009, 11:31 AM   #14 (permalink)
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You can adjust the TV cable all you want, but it will never do a thing for OD and lockup shift points.
wasnt saying for the od lockup. the cable adjustment should help with it not kicking down when he floors it.
guess i should have worded it better.
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Last edited by derek; 08-22-2009 at 11:32 AM.
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Old 08-22-2009, 12:21 PM   #15 (permalink)
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Quote:
Originally Posted by derek View Post
wasnt saying for the od lockup. the cable adjustment should help with it not kicking down when he floors it.
guess i should have worded it better.
Actually I think all it does is adjust line psi between shifts.

Correct me if I'm wrong
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Old 08-22-2009, 12:23 PM   #16 (permalink)
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Quote:
Originally Posted by derek View Post
wasnt saying for the od lockup. the cable adjustment should help with it not kicking down when he floors it.
guess i should have worded it better.
Per the original post, it doesn't kick down out of OD when he floors it at anything over 50 mph. Again, that screams TPS adjustment or leave it well enough alone. I personally don't feel that adjusting the TV cable will help him with his complaint.
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Old 08-22-2009, 12:30 PM   #17 (permalink)
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Quote:
Originally Posted by Repost View Post
Actually I think all it does is adjust line psi between shifts.

Correct me if I'm wrong
Not exactly.

The TV cable does adjust pressures, and the 1-3 shift points are controlled by the pressure rise created from the TV cable and the governor solenoid.

OD and converter lockup are 100% electronically controlled, and the TV cable will do nothing for their functions. Vehicle speed, TPS, tranny temp, etc. all factor in to the equation on when the PCM decides to allow you into OD and lockup. Overdrive typically engages around 45mph and lockup comes shortly after, as you have already determined.

You can mess around with things a bunch trying to get the shift characteristics to change, but you only have so much that you can play with that is external to the PCM. If you don't like the way it works, your best bet is to just control OD by using the button on the shifter.
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Old 08-23-2009, 07:35 AM   #18 (permalink)
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I see you have already been to DieselBombers.Com from the links over there. Go post up some of you questions on that site and they will hook you right up with out alot of bullshit.
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Old 08-23-2009, 07:52 AM   #19 (permalink)
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I see you have already been to DieselBombers.Com from the links over there. Go post up some of you questions on that site and they will hook you right up with out alot of bullshit.
Well HELLLLLLLOOOOO there sunshine......
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Old 08-26-2009, 08:56 PM   #20 (permalink)
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Save it, use it. The owner is a retired engineer that worked at Cummins. His name is Mark, ask for him he will spend the time with you. He will NOT steer you wrong or hurt your truck. Best mpg, power, torque. Mark
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Old 08-26-2009, 10:51 PM   #21 (permalink)
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kdp taken care of yet?
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Old 08-29-2009, 11:23 AM   #22 (permalink)
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you guys rock !!!!! i just did the tps bypass thing and it works great!i was ready to burn my truck in the driveway.i love my dodges been a dodge guy all my life.mine is a 96 ram 2500,it's my first diesel and i have owned it for about 4 years now.at about 170,000 i had to have the trans rebuilt,it worked fine for about 4 months then it started to act up, did'nt want to stay in OD kept going in and out at 50 mph,even my trans guy did'nt know what to do,started looking on pirates and found this thread.priced a new tps, wow 220.00 what a rip off! that thing is'nt worth that.keep up the threads guys dumbass like me need all the help we can get.
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Old 09-07-2009, 05:26 PM   #23 (permalink)
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Finally looked at my TPS voltage.

1.02vdc at idle
3.74vdc at wot.

Same problems? still won't downshift from 4-3 above about 50 mph.
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Old 09-08-2009, 12:27 PM   #24 (permalink)
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check your TPS on an old style analog ohm meter. Look for flat spots or spikes/dips. It should be a smooth progression. Values at any given point don't tell the story.
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