92DuallyCTD

Ok, let me see if I can get this scenario described briefly...yeah, sure I can! ;-) My 92 46RH(or 518 as it may be) is apparently going into overdrive right from the stoplight in low gear. Then it goes ahead and makes the 1-2 and 3-4 shift, but because I believe it has already engaged the o/d clutch in low, I would now be running along in o/d on the highway. Verified with new tach turning 1650rpm @ 60mph in o/d output 0.69:1 ratio(which corresponds with a speed/rpm/tire calculator I found on the web). To compare, it shows it would be 2100rpm @ 60mph for no o/d 3rd gear direct(1:1 ratio). And I do get consistent 20mpg @ 60mph, so I'm pretty sure it IS in o/d at speed. Today I discovered by holding it in manual low that there is a shuddering shift point @ 15mph that drops the rpm from 1500 to 1350. It can then be shifted manually up to 2 and get an upshift, then up to D and get another upshift. No further shifts(I believe due to the o/d clutch already being engaged @ 15mph in low gear). Of course, that means the dash switch O/D OFF does nothing, and never did since I bought it. Now here's what I have already done: New o/d shift solenoid(internally on the valve body, single solenoid as this is not a lockup torque converter trans), checking at that time to see the 3-4 shift spool it controls was moving freely in the valve body(it is), wiring around the PCM with a switchable ground inside the cab for the solenoid, and verifying it with test light, just to make sure the new solenoid was being energized(it is), and finally after discovering today there is an apparent o/d engagement point occuring @ 15mph in manual low, I then completely unplugged the wiring harness to the o/d solenoid in an attempt to make certain the internal solenoid could not be affecting the o/d operation either way(when it is not energized, it is open to vent line pressure away from o/d piston in rear compounder unit), so that should have had the effect of not allowing ANY o/d function at ANY time. Yet it STILL does appear(I say appear because the speed/axle/rpm calculator agrees) o/d is engaging in low gear! The calculator shows 15mph should produce 1433rpm, after the 'shift' in low, my tach reads 1100rpm. Calculator shows same 15mph WITH .69 correction(as though o/d was engaging) as 1029rpm. Same calculator shows 20mph with no overdrive should read 1910rpm, my tach reads 1400rpm. Same calculator reads 20mph WITH .69 correction(o/d engaged) would read 1392rpm. So I do think the shudder/shift in manual low IS the o/d cltch engaging, but with all the built-in safety valves and operations of the valve body, plus the o/d solenoid completely open(disconnected), HOW THE HECK CAN THIS HAPPEN??!! ;-) Or, if this is NOT happening(though all the numbers indicate it is), what IS happening? My main concern is if the o/d is engaging right from the stoplight...well...you can't rely on it to last long pulling a trailer now, can you? On the other hand, if it indicated that it was only engaging after the 2-3 upshift(basically bypassing 3rd gear direct), I'd think you could do a little more with it. Whew! What a question - aren't you glad I was able to keep it brief?? Hope someone can suggest a logical answer, 'cuz I feel like a dummy! Thanks all, and if you're still awake reading all this bs you deserve the 'I survived a 92DuallyCTD post!' refrigerator magnet to proudly display! :-) Here is a link to the web calculators(I used the FIND RPM near the bottom, and entered 30.5in for my tires, 3.54 axle, and 2.45:1 for regular low gear ratio<the CAS> and 1.76:1 for the CAS for the hypothetical o/d engaged low gear ratio): http://www.4lo.com/4LoCalc.htm

wannadiesel

Sounds like the OD valve is stuck.

92DuallyCTD

Hi Dave, thanks for the reply/info. So this valve could cause all/part of these problems, right from a stoplight take-off, by allowing the o/d clutch to engage immediately? Even at that I wonder why the open(unplugged) o/d solenoid still wouldn't prevent that from happening, because reading the manual/service procedures, it seems to indicate that o/d will NOT happen if the solenoid vents the pressure needed(by being open - it's default position when there is no power or grounds to it) to slide the o/d piston into the o/d clutch, causing o/d engagement, correct? And which valve(exactly) are you referring too? I have a factory service manual and don't see any valves/spools named o/d valve(or anything too similar) in the illustrations/parts breakdowns. There are definitions like 3-4 shuttle valve, 3-4 timing valve, 3-4 quick fill valve, and 3-4 shift valve. Would this be a valve within the valve body(most likely eh?), so possible replacement of the entire valve body may cure these symptoms? Basically what I'd like to do is be able to delay a $2000 rebuild just now, if it may be something I could attempt to repair relatively inexpensively first, right? I should just pay 'da price for a new warranty and all, but I get great satisfaction from learning/repairing everything I can(don't we all!!). Do appreciate your input, thanks for your help! :-)

Boatnik

You might, big IF here, get by with installing valve body. The one I had installed literally went through the valve body from one end to the other. We, actually TIMINVA did almost all the work, replaced light springs with heavier ones, drilled holes and replaced a couple spool valves. He even threw in a new heavier arm that holds third engaged because of higher pump pressures. I have had a mess with the solenoid valve myself. The original died and the replacement is grounding inside the pan causing a premature upshift of the o/d. I have a temporary switch in the power supply to make it work.The v/b was $399 plus $60 for an install kit.