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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2007700R4 Ambient Temp Issue w/ holding 4th (OD) - Why?
I've got an '84 Pontiac Firebird (130K) w/ a 700R4 overdrive automatic. I noticed this once or twice last year and again the other day when it dipped to about 30F locally. The transmission wouldn't hold or maintain 4th (OD). Basically I could be driving along on flat expressway with only the occasional overpass and the thing would almost constantly jump back and forth between 3rd and 4th for the entire drive. It just seems that the problem of jumping down to 3rd and not holding occurs specifically when it's cold OUTSIDE. Now on any cold morning for a few minutes it can be a little balky about shifting but this quickly goes away within just a few minutes. But yesterday even after 40 minutes of driving to work it doesn't seem to get any better as far as OD specifically. I would think that within a 40 minute drive of mostly 65ish cruising that the transmission would be completely warmed up and immune to outside temps. Maint background: Every 2 or 3 years I drop the AT pan and replace about 5 qts and change filter. I don't use any special synthetics - just standard Dexron that meets the owner's manual spec. Level is between full and add last I checked. Transmission works just about flawlessly 362 or so days out of the years. It's just those 2-3 days a year we get down well below freezing that it 'hunts' and can't hold 4th (OD). I'm wondering if anyone has seen this or has an explaination of why it would not 'hold' 4th (OD) at cruise when it's below freezing outside. Would temperature exacerbate a low fluid condition? loadhawg Wrote: > I've got an '84 Pontiac Firebird (130K) w/ a 700R4 overdrive automatic. > I noticed this once or twice last year and again the other day when it [quoted text clipped - 23 lines] > would not 'hold' 4th (OD) at cruise when it's below freezing outside. > Would temperature exacerbate a low fluid condition? Transmission fluid level good? Any external transmission oil coolers? Is the engine holding good full operating temp when it does it? Also it might be interesting to check the rad tempt when it happens If cold that may explain some of it. The transmission cooler runs threw the rad and also warms up transmission in cold weather. Any engine power lose? Remember engine has to be running good for transmission to work good.  SignatureMT-2500 http://www.automotiveforums.com > Any external transmission oil coolers? I'll need to check tomorrow - it is purely stock 84 Firebird w/ 2.8L (Chevy) V6 if that helps. > Is the engine holding good full operating temp when it does it? Temp guage is rock steady year round. AC nice and cool in summer and heater is toasty warm in winter. > Also it might be interesting to check the rad tempt when it happens > If cold that may explain some of it. Again temp is rock steady. Replaced Rad perhaps 5 years ago and replaced TSTat while I was replacing hoses couple years ago. > The transmission cooler runs threw the rad and also warms up > transmission in cold weather. OK I didn't realize that. Again I'll have to check and see if factory installed a cooler on 84 Firebird V6. > Any engine power lose? Not that I've noticed (not that it ever had a lot of power in the first place! ;-) ) > Remember engine has to be running good for transmission to work good. >> Any external transmission oil coolers? > [quoted text clipped - 5 lines] > Temp guage is rock steady year round. AC nice and cool in summer and > heater is toasty warm in winter. For what it's worth, I had an 84 Olds with a three speed torque converter lockup transmission that wouldn't kick into lockup until the engine was fully warmed up. While driving to work in the morning I could see on the tach when the TC lockup occurred due to a small drop in engine RPM. And as the thermostat opened up and the engine cooled during the first 5-10 minutes, the RPMs would bounce back up till the engine warmed up again. While the temp gauge and heater output sound like your engine is holding a constant temp, there is a second engine temperature sensor for the engine computer. If that is out of range it could be telling the engine computer I'm not warm enough and kicking the transmission out of OD, since it's the engine computer that tells the trans when to kick into OD. IF your transmission uses a solenoid to engage the OD. (Not a mechanic, but an ex-electronics tech.) Just a thought. Bob >> Any external transmission oil coolers? > [quoted text clipped - 24 lines] > >> Remember engine has to be running good for transmission to work good. Hey loadhawg, Bob (ex-electronics tech--with obviously sound electronics background AND advice!) got close but didn't exactly say this, but are you sure it's jumping in and out of OD? or could it be jumping in and out of lockup-mode (torque convertor mode, that is) ? There IS a difference. A mechanic should be able to show you where and which wire(s) to disconnect under the hood that send signals to TC telling it when to go in and to go out. IIRC, in '84 there was a cluster of ~3 wires, only 1 specific wire which should be TEMPORARILY disconnected--not the entire cluster. With this inoperative, test drive it again. If it stays in "gear" as you've been feeling, then likely the TC/TC circuit (including a brake-disconnect switch, which must be adjusted properly) could be the source of mal-function. One can live w/out the TC-lockup feature functioning (as when that wire is disconnected) but I wouldn't want to live w/out the OD gear working. Both will hurt gas mileage; but no OD could keep engine revved much higher, approx. 30% plus, causing excess engine wear and premature failure. Determine a definite diagnosis first. HTH. s Thanks for all the responses. Yes it could very well be it what you and Bob are referencing... First let me say I know little about automatics as all of my many cars have been manuals - this being my first auto ever. Now it's too warm (lows only getting to mid 30s) so it won't do it anymore. But when the RPMs jumped a lot when it was below 32F (outside ambient temp and engine FULLY warmed up) I would be cruising say at roughly 65mph and with the trans in OD position the RPMs would constantly jump or hunt between roughly 2100 and 2600 rpms as I recall. So roughly a 500 rpm change which is not exactly a full gear change and would likely be the torque converter in lockup or not ... ??? So if it's the torque converter and the engine is definately warmed up as shown on the guage (and believe me the heater gets HOT in this car and overal cooling system is in great condition (knock on wood) and works efficiently year round beit 35F or 110F) - what would cause this? After reading Bob's post above, I did recall seeing a 2ndary temp sending unit that feeds the ECU. NOt sure how to test it but if it's not too expensive I could just replace it outright. thanks again > >> Remember engine has to be running good for transmission to work good. Hey loadhawg, Bob (ex-electronics tech--with obviously sound electronics > background AND advice!) got close but didn't exactly say this, but are you [quoted text clipped - 12 lines] > approx. 30% plus, causing excess engine wear and premature failure. > Determine a definite diagnosis first. HTH. s Great insights here Bob - really appreciate it. It's 'roughly' a 500 rpm difference as it was 'hunting'. I need to figure out if my '84 (carbureted) V6 F-Body controls the TC electronically. While I'm an auto tranny neophyte I suspect it is the TC. I'll also need to figure out how to test the 2nd temp sender to the ECU (vs the one for the in dash guage) or perhaps simply replace it outright. Thanks - this has been helpful > >> Remember engine has to be running good for transmission to work good. Hey loadhawg, Bob (ex-electronics tech--with obviously sound electronics > background AND advice!) got close but didn't exactly say this, but are you [quoted text clipped - 12 lines] > approx. 30% plus, causing excess engine wear and premature failure. > Determine a definite diagnosis first. HTH. s > Great insights here Bob - really appreciate it. It's 'roughly' a 500 > rpm difference as it was 'hunting'. I need to figure out if my '84 [quoted text clipped - 4 lines] > > Thanks - this has been helpful Loadhawg, A quick search on google groups found the following: http://groups.google.com/group/rec.autos.tech/browse_frm/thread/99d5d890d6c64e4d /3a969bf277985158?lnk=st&q=gm+temp+sensor+resistance+values&rnum=1&hl=en#3a969bf 277985158 or this web page: http://www.diy-efi.org/gmecm/component_info/sensors.html If you can find the coolant temperature sensor a quick resistance check with an ohm meter should give you an indication of what it's thinking the coolant temp is. It's possible the sensor is opening up when cold, but I think (guess?) that it would trip the Check Engine light. Could be that it's old and out of range, IE, reading too high a resistance when cold or below 32F. Considering all the temperature cycles that coolant temp sensor goes through it could be out of range and while I like to find the exact cause of a problem, swapping the sensor could be a quick and inexpensive fix. Which just triggered another thought - have you checked for any stored trouble codes in the engine computer? With the 84 GM you can jumper a couple of connections in the diagnostic connector with a key on - engine not running and it will blink out any codes on the Check Engine light. It might give a clue to what the ECM is unhappy about. For what it's worth my 84 Olds had a coolant temp sensor problem, but opposite from yours. On real hot days (90's and above) while driving on the freeway for a while the Check Engine light would come on for a bit, and then go out. When I did the diagnostic test with the Check Engine light one of the blink codes pointed to a coolant temp sensor out of range. One new sensor later and the Check Engine light never came on in hot weather again. Hope this helps, Bob Thanks. Yes I've counted the blinks on the engine light, no problems there. I agree, the cost/effort is relatively insignificant and plan on replacing the 2ndary temp sending unit if for no other reason as a standard maintenance measure (other more significant car projects are piling up on other vehicles for now). May boil the old sender and ohm it out of curiosity too. I am curious about one thing. Since the internal water temperature, measured by the sending unit, fully warms up to 195F+ IRREGARDLESS of outside ambient temps being 30F (or 100F or whatever...)... - WHY would ambient outside temperature seem to play a role in this scenario at all? Thanks <SNIP> > If you can find the coolant temperature sensor a quick resistance check with > an ohm meter should give you an indication of what it's thinking the coolant [quoted text clipped - 21 lines] > > Bob > I am curious about one thing. Since the internal water temperature, > measured by the sending unit, fully warms up to 195F+ IRREGARDLESS of > outside ambient temps being 30F (or 100F or whatever...)... - WHY > would ambient outside temperature seem to play a role in this scenario > at all? The temp in the motor and the temp in the rad are two different things. The trans fluid sees the temperature in the radiator. > Thanks. Yes I've counted the blinks on the engine light, no problems > there. I agree, the cost/effort is relatively insignificant and plan [quoted text clipped - 10 lines] > > Thanks The best way to find out would be to hook a diagnostic scanner up while the engine is running and find out what the engine computer thinks the coolant temp is. My thought is the dash gauge may be showing 195, but I'm not sure of the overall accuracy of the gauge. And the engine coolant sensor for the ECM may be thinking the temp is different. All I'm saying is if the coolant sensor is telling the engine computer that 195F is actually 170, you could be on the edge of where the OD lockup kicks out. A cool stream of coolant may cause the ECM temp sensor to read low long before the gauge shows the change, since there is a bit of mechanical damping in those dash gauges/meters. It could be a resistive connection on the top of the sensor where airflow and engine heat is causing expansion and contraction. A lot of electronic problems that I fixed in the past were due to heat/cold issues and mechanical stress. Bob 700r4 is the worst tranny i have ever been around but a bottle of lucas oil trans fix would probably help with that problem Signaturemidgetracing28 http://www.automotiveforums.com >700r4 is the worst tranny i have ever been around but a bottle of lucas >oil trans fix would probably help with that problem Hmm, I hope the one in my S-10 doesn't hear about that since it's got 172,000 miles on it and is still going strong. >>700r4 is the worst tranny i have ever been around but a bottle of lucas >>oil trans fix would probably help with that problem > > Hmm, I hope the one in my S-10 doesn't hear about that since it's got > 172,000 miles on it and is still going strong. ...and I like them too, AC--esp. the later ones, beefed up (first by GM) to handle the power of the TPI engines in the 'vettes, Z28's, & Trans Am's, seemed to perform beyond what I expected. Strong OD's with capabilities to be shifted independently of ECU's. Earlier ones in lighter-duty ap'ns, iirc, also gave good service when used normally. Hopefully, those seeing really bad service maybe got it confused with something along "metric" line. s |
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