trans tech help

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daredevil

daredevil

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45rfe is intermittently downshifting with a bang like releasing a trans brake when cruising in od. Started out like a slip taking off and has changed to this.Wish I understood computerized systems more. Where should I start.OBD2 code is p1698
 
daredevil said:
45rfe is intermittently downshifting with a bang like releasing a trans brake when cruising in od. Started out like a slip taking off and has changed to this.Wish I understood computerized systems more. Where should I start.OBD2 code is p1698
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What kind of car is it, a Grand? What year? What engine? I can look it up for you. Might be a TSB.
 
The slip taking off is probably limp mode (2nd gear). The code says you have intermittent power loss from the tcm (trans control module). Could be bad tcm, or shift solonoid pack. Wiring could also be a problem. The input speed sensors are also problematic and are a cheap shot in the dark.
 
j5dart said:
The slip taking off is probably limp mode (2nd gear). The code says you have intermittent power loss from the tcm (trans control module). Could be bad tcm, or shift solonoid pack. Wiring could also be a problem. The input speed sensors are also problematic and are a cheap shot in the dark.
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Do you think the trans bang downshift is due to the speed sensors not keeping the tcm the correct info? I keep hearing the term limp mode. Can you explain what this term means please?Are the speed sensors a dealer item and where are they located?I apologize for all the questions but to afford to fix this I must do it myself. If not I will be driving my beater s10 all day with no ac in 100 degree tx. heat. I,m a residential plumbing superintendent and my truck is my office all day. thanks.
 
P1968 is a bus communication problem. It's likely the wiring somewhere between the TCM and the rest of the modules causing an intermittent condition where the TCM can't communicate. A wire is probably chafed/corroded etc. and when it rubs or makes contact, the bus drops out. The TCM can't communicate and you get your condition.

Limp mode is a fail-safe kind of deal where the trans only has 3rd gear. It can be driven but only in a limited sense. It's so you don't destroy the rest of your powertrain driving with a problem.

On older cars/trucks with a PCI bus, the crank sensor can cause a no start and a 'no bus' message from the instrument cluster. Does not sound like what you have though, just some information.

Also, did you check your fluid level? Burned/low? Simple first. Regular service with filter change? Any leaks? Any repairs? Any other symptoms? Air bag, SKIM key, cluster, ABS etc?

I wouldn't start throwing parts at it until you figure out the communication thing first.
 
It has all its gears just the intermittent down shift bang. Scares the bejeebers out of me everytime it does it. I can be driving with cruise on at 55-65 and it feels like I got rear ended. I can stop and take off from a standing start full throttle and it shifts crisp. fluids red not dirty on a paper towel.160k on the clock. Fluid was last changed at 85k.I know its past time to do it again but would like to get this sorted out first. I think youre right about a chafe or burned wire. I have had the truck since new.Not a glitch till now. Now that you mention it it does feel like a 3rd gear take off.
 
BTW, the TCM should be at the right/rear of the engine bay. The PCM has three large connectors, the TCM I think has one or two. Might be tucked behind a few things. Take a look, maybe you can find something. Check fuses too if you have a test light, a blown fuse can bring the bus down.

You'll probably need a wiring diagram at some point to see where all the connectors are and what goes through where. If you have to take it somewhere, make sure they can access wiring diagrams before they start otherwise they'll start throwing sensors at it and will likely not solve the issue.

A sensor could be dropping out but generally you get a code for a particular circuit if that is the case.
 
I have located the tcm and pcm.Will try and check it out in the morning. everythings wet outside now. thanks for your help
 
daredevil said:
I can be driving with cruise on at 55-65 and it feels like I got rear ended.
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Does it only happen with the cruise control on? If so, that could possibly be your issue, maybe a common wire/connector between the two modules. Not real likely but if the CC has a fault, it potentially can bring down the bus. Just a thought.
 
daredevil said:
no luck cleaning the plugins
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A chafe spot could literally be anywhere in the harness. There have been times where I had to take out an entire interior and take all the tape off a harness to find a split. Sometimes I don't find it. Intermittent faults can be really tough to diagnose. Could be water, rubbing, heat, etc. There also may be another fault somewhere that causes this too, just can't know without a scan tool. Probably something simple but these things don't always reveal themselves at first.

Unfortunately you might have to take it to someone who can monitor the in/out data as it occurs. That would give better insight as to exactly what is happening.

I suggested looking at the TCM harness just to see if there were was any obvious red flags.

I looked for TSBs for you, didn't find any that matched your concern.

Just as an example, my 01 Cherokee had a weird problem like you are having. Every time I would pull away from a light, the car would go into limp mode unless I was really light on the throttle. I'd constantly have to restart the car, was driving me insane. At the same time, I was also getting an overcharging condition. I replaced the alt. and the PCM, no change. We had a trans service done prior to the condition appearing. For whatever reason, they decided to zip tie the main harness to the trans dipstick. Every time the engine would torque over, it would pull a connector apart just enough to cause the over charge condition which put the car into limp mode. They overfilled it too so it was a mess. I just happened to notice the the fluid around the harness one day and that it was tied to the tube. I pulled it off the tube and it never happened again. :dontknow:
 
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