[Alvy]

Hello everyone. I'm a member of a few forums and feel that this one will help me solve my problem. My 94 Sierra crew dually with 6.5 TD has no overdrive until it reaches around 200 degrees on the temperature gauge and then it will engage O/D. After that I have O/D all day as long as the operating temp stays warm, works flawless. Have it sit for a few hours or get cold, back to no O/D until warm. I have been told that it should do this and also that its torque converter lock up that I'm not getting and not O/D. I know the difference between the two and also know that this is a problem and NOT what it SHOULD do. I've read that I should have O/D between 130 and 170 degrees which is normal but mine won't engage until around 200 and sometimes I have to turn truck off and then back on to get it. I have changed the coolant temp sensor for the ECM, T-stat, and coolant with no change. I've also been told that there is a TTS (transmission temp sensor ) in 96 and later models that would do this also but mine is a 94. Is it possible that a 96 or later trans was installed in my truck before I bought it or does mine have this sensor? I've heard of other possibilities being the speed sensors, shift solenoids, or TPS. I don't have any stored DTC's. If anyone could help me here it would be great. I've had no luck with any other forums and I don't want to throw parts at my truck. If anyone has resistance specs for the sensors I've mentioned that would be great too. Thank you, Mike

 

[gmctd]

What's your mileage on the odometer?

If no DTC's the problem will likely be mechanical, as in the 4rth clutch, or the valve body - when PCM demands 4rth, a pressure switch indicates that range is activated.

No switch = DTC - if switch closes, but no 4rth the clutch associated mechanicals are not engaging, without setting DTC.

High temp prevents 4rth and TCC, but gives DTC.

Sounds like time for a rebuild - pull the pan to check that.

 

[chrisk1500]

coolant temp sensor may be bad......cts controls whether or not ECM allows trans to shift into OD....

 

[Dieseldreamr]

I have changed the coolant temp sensor for the ECM, T-stat, and coolant with no change.

Yeah it sounds like its just startin to wear, how many miles on the ole girl and what does it do for a living?

 

[96GMC6.5L]

Mine does that every once in a while when the trans is real cold, it redlines for a bit before it shifts again.

Jacob

 

[chrisk1500]

When you changed the CTS did you change the harness as well? Most sensors will come with a new pigtail....just a thought..

 

[Tracy]

My 93 has a trans temp sensor that is in the wiring harness in the pan. It needed to be replaced because my lockup converter would engage then disengage for no reason going down the highway. I would get a low temp or high temp code can't remember the number off hand.

 

[Alvy]

Thanks for all the input guys. Dreamer and GMC TD, truck has 140K but previous owner had reman trans installed at 96K. He pulled big 25 ft boat with it which was about 10k lbs. before the trans swap. Did not change the harness Chris but checked it for continuity and OK. Goes into high idle when unplugged from ECT sensor and then back to normal when plugged in. Thanks Tracy, I have decided that is my next step to look at that sensor but really need specs to check it with ohms, ins and outs, etc. I really think its temp related because it's so routine when it happens. I guess I'm just optimistic about not having to go through a rebuild. I could live with this problem but its dangerous sometimes when I pull out into traffic and can't get past 50 ish. I think I have 410's in the back, haven't researched that yet.

 

[CanadianDiesel]

whats the transmission fluid like? still red? smells burnt? you might have to pull the pan and look for any traces of clutch filings

 

[Alvy]

Changed fluid and filter about 1k miles ago, nothing out of the ordinary in the pan. Previous owner said this problem has been since reman trans installed.

 

[CanadianDiesel]

your truck is a 1 ton it has 4.10's in the rear, all gm trucks that were 1ton had the only option of 4.10 the 3/4 and lower had two options for rear axle gearing.

btw put your truck in your signature and any and all modifications you did to it, helps us help you.

what does your temp sensor read when cold? and when hot on the transmission for ohms?

 

[Tracy]

I believe you can check the trans temp by using a OBDI, if there is anything erratic it should show up by having high or low temps. The sensor is in the internal (just a fat looking wire) wiring harness and can't be bought seperate, after market harness I think around 100$ or GM 500$ guess which one I choose.LOL

 

[Turbine Doc]

Alvy I'm bumping this over to the transmission section , we got some trans gurus that hang out there, take a minute to also update your signature with truck info, it helps with diagnosis assist, follow link how to in my or any other 6.5 moderators signature

 

[Alvy]

Does anyone one have any other ideas for me? Any help would be fantastic!

 

[marcdeluca]

You really need to get access to a scanner to look at realtime data. The trans has a temp sensor that will show fluid temp as well as being able to see what the coolant temp sensor is reporting to the computer. My '91 will not engage lockup until the fluid reaches 75°F, but OD will engage even when really cold. You are shooting in the dark w/o scanner data.

 

[Tracy]

You really need to get access to a scanner to look at realtime data. The trans has a temp sensor that will show fluid temp as well as being able to see what the coolant temp sensor is reporting to the computer. My '91 will not engage lockup until the fluid reaches 75°F, but OD will engage even when really cold. You are shooting in the dark w/o scanner data.

Exactly, I went to GM when my trans temp sensor went south, it is simply amazing (eliminates the guessing)all the info they can get on whats going on in the trans from the comfort of your drivers seat.

 

[Alvy]

Hello again,
A friend of mine is letting me borrow his Vertronix scanner this week. I have already driven the truck with the scanner on it and have some info. and a question. I actually had a stored code of 87 in the truck, no MIL on however. I need to know from someone exactly what happens during overdrive. I am viewing the 1-2, 2-3 solenoid sequence, trans temp, input/output speed, etc. but am confused about what to look for electrically when I'm not getting overdrive when I should. In park, the 1-2 is on which doesn't make sense to me and the scanner tells me what gear I'm in which is corrrect. Then, shifting from first to second, 1-2 off, 2-3 on and then second to third 1-2 on, 2-3 on. But what about overdrive? It stays the same 1-2 on, 2-3 on for overdrive as well (when it finally works after 10 minutes) I heard about a pressure switch that tells the computer to demand O/D. Is there an O/D solenoid? Also, TCC works perfect all the time, locks and unlocks with the brake pedal. Yes, I am to blame for not having the service manual which I desperately need but can get things one at a time lately it seems. MIKE L. PLEASE HELP WITH THIS!!!

 

[Rich H.]

only 410 in 1 ton

your truck is a 1 ton it has 4.10's in the rear, all gm trucks that were 1ton had the only option of 4.10 the 3/4 and lower had two options for rear axle gearing.

btw put your truck in your signature and any and all modifications you did to it, helps us help you.

what does your temp sensor read when cold? and when hot on the transmission for ohms?

Not true on the 1994 1 tons, Mine came from the factory with a 373 and when I complained they told me, "no option on that truck for that year"
Now that was mine I have no idea how others were setup.Also I had a sticker on the door "Do not pull with trans in OD" now I was mad when I seen that. So I had a new 1 ton that had no pulling power & no highway speeds with a trailer on it. The next truck I bought was a Gas job and it did not have that sticker on it. I think it was just that year "94" it had more electronics on it then the "93" trucks did.

 

[Alvy]

Mike L. Please read my last post, you're my only hope!!

 

[Tracy]

ALVY found this out for code 86, "undefined gear ratio error (transmission pressure control)" at another site it said "analogue/digital electronic module(ECM) error or shift soleniod B stuck on"

I don't know which solnoid B it is referring to. Your getting a little closer to tracking this gremlin down, hopfully someone with a manual will come along.