Overdrive problem

[cerberusiam]

I wonder then... . if my voltage was low, this may have effected my speed correction box? Are they that sensitive to input voltage?

Yep. Low voltage will cause weirdness to ensue, just as bad grounds will introduce gremlins.

 

[KOG]

Slow launch with manual switch? The 46RH will not allow OD unless it's already in third gear. You can leave the OD switch in the engaged position and it will shift 1,2,OD. Unless you're towing or passing on highway below 60mph there's enough torque to shift 2-OD with no real problem. The solenoid ground lead is an orange with pale green stripe. You can tap into it in the harness bundle at the top of the firewall (engine side) directly ahead of the dash. Then clip that same wire about 12" from the computer so that the PCM own't be trying to engage OD erratically. I traced the OD engagement problem on mine to a bad PCM. Any slight mechanial vibration will cause the PCM to go flakey on the "Wait to Start" an "Water in fuel" lights, which also drops OD. It then shifts into and out of OD rapidly and repeatedly. Not good. And sometimes it just refuses to engage OD at all. With manual switch it works just fine.

I used a Radio Shack 275-617 SPST push on/push off switch ($2. 49) mounted in a 3/8" (. 375) hole in the end of the shift lever knob. There's a fair degree of fabication involved in getting the switch mounted there, but it sure is convenient to use. I'd eventually like to get a good PCM and TPS, but the price of new or rebuilt replacement parts is just a little high for no more than I use the truck.

 

[cerberusiam]

Slow launch with manual switch? The 46RH will not allow OD unless it's already in third gear

If you have the switch engaged and do not shut it off when you come to a stop it will never come out of OD. Unless the VB is modified it will not downshift once OD is engaged when you are using a switch.

 

[KOG]

That's interesting to hear. Apparently no one told my transmission that. It does exactly what the shop manual says. If you're in OD it will kick down to 2 at full throttle and skip third. When you slow down it downshifts just as if OD were turned off. The '92 FSM I have describes this on page 21-59 under shift sequence for four speed overdrive controls. I'm not familiar enough with the 46RH to know what might cause it to hang in OD. I know that on the three speed torqueflites that a sticking governor valve will prevent downshifts to first as you come to a stop. And the 46 is a three speed exactly in the first three gears with a fourth OD tacked on. The OD is the only electrically operated part and the valve to engage it gets no hydraulic pressure until 3rd gear is engaged. I suspect that it was designed that way partly to prevent OD engagement in 1st or 2nd which could feed enough torque into the OD unit to trash it. If yours holds in OD it sure would make for one verrrrry slooooow launch.

 

[dpuckett]

I think the 46RH and A518 are the same critter, are they not? Just different labels.

If yours holds in OD it sure would make for one verrrrry slooooow launch.

That is why you can add more fuel, silly :-laf



DP

 

[cerberusiam]

I think the 46RH and A518 are the same critter, are they not? Just different labels.





That is why you can add more fuel, silly :-laf



DP

Funny guy, huh? Tell ya what, lets try it on your mom's truck and we will tell her it was your idea. Think it would still be funny? :-laf :-laf



Smart guy!!



KOG, verify that we are talking the same thing: mystery switch engaging OD manually, after a full stop your 46RH will drop out of OD and start out in 1 st then shift thru all the gears normally?



Its been a couple years since I switched to the 47RH but that is not what happens with it. Mystery switch on and it stays in OD when you start off. I am remembering the same behaviour with the 46RH, but, it has been a while.

 

[KOG]

Yep, my 46RH shifts normally with "mystery" switch in the "on" position. As I stated it will shift 1,2,OD (engages OD as soon as the 3 shift occurs). Downshifting is OD, 2,1. Downshifts at normal speeds to 2 and 1 at both part throttle and stops. Shop manual indicates that this is the only way a 46RH can shift. Unless the 47 has a different valve body, whcih seems unlikely, it SHOULD shift the same way. I don't have a shop manual covering the 47 so cannot verify that. I was sort of surprised to find that the 46 worked the way it does, but it sure makes using a manual switch less hassle. I don't use OD on cold starts (PCM is programmed to not OD until engine is up to temp, probably a good idea, helps to warm engine faster), and have to manually shift it for heavy loads (towing) or highway passing (no third gear with switch on). Otherwise I can just switch it on and leave it on for routine driving after warm up.

 

[TomThomas]

Well, I appreciate all info this post has generated. I have a '93 W250 with the same problems. "cleaned" and tested the TPS- seems to be working o. k. OD factory off switch works and disables OD when its engaged. So I'm guessing my problem's the PCM (or faulty wiring). Bottom line is, I think I want the manual OD switch as Zgardulski has installed. But I'm still a little confused on the wiring. Apperently I just need to ground out one wire with a switch, but which one? It kinda sounds like KOG and Cerby are talking about two different "approaches" to the same problem. Many thanks, Tom.

Ps- no, my CC doesn't work, no my AC doesn't work, and yes my CEL is constantly on due to alternator voltage "problems", but the PO already wired in an early model external voltage regulator, so I'm not real excited about chasing wiring problems (right now I'm Laughing Out Loud!)

 

[dpuckett]

Funny guy, huh? Tell ya what, lets try it on your mom's truck and we will tell her it was your idea. Think it would still be funny? :-laf :-laf

Hey, it has problems already and the hillbilly isnt wanting to honor his warranty, so sure, why not? :-laf Maybe he'll warranty it when it's towed to his shop. Though I'm not sure about letting him do it again. He cant install a rear main seal right, but that's another story.



Daniel

 

[TomThomas]

K, well I figured out which wire to ground (orange/lt grn stripe) and OD works when grounded. Thanks again to the knowledge this website transpires. Anyways, why not ground the wire entirely (no switch) and just use the factory OD dash switch to control it for hills, towing, etc? Thanks, Tom.

 

[TomThomas]

K, never mind. The dash switch won't disengage the OD when entirely grounded (probably since it takes the cpu out of the game, and the dash switch probably relays to the cpu).

 

[KOG]

The PCM engages OD by grounding that lead. If you have it permanently grounded you can't switch it out of OD with the PCM.

 

[HRDROKN]

Yep, my 46RH shifts normally with "mystery" switch in the "on" position. As I stated it will shift 1,2,OD (engages OD as soon as the 3 shift occurs). Downshifting is OD, 2,1. Downshifts at normal speeds to 2 and 1 at both part throttle and stops. Shop manual indicates that this is the only way a 46RH can shift. Unless the 47 has a different valve body, whcih seems unlikely, it SHOULD shift the same way. I don't have a shop manual covering the 47 so cannot verify that. I was sort of surprised to find that the 46 worked the way it does, but it sure makes using a manual switch less hassle. I don't use OD on cold starts (PCM is programmed to not OD until engine is up to temp, probably a good idea, helps to warm engine faster), and have to manually shift it for heavy loads (towing) or highway passing (no third gear with switch on). Otherwise I can just switch it on and leave it on for routine driving after warm up.

I tried mine out last night... with the switch on, transmission shifts 1, 2, OD and down shifts back to first, so now I don't need to worry so much when my wife drives the truck :-laf

 

[HRDROKN]

I was just thinking... that was with the OD button off, would it be the same with the OD button on and the switch (engaging OD) on? Looks like another test drive in the making!

 

[KOG]

I just clipped the OD lead from the PCM so that it doesn't matter if the dash button for OD is pushed or not. When you wife drives (unloaded) you can just leave OD on. 3rd is useful when towing or heavily loaded so you actually have to shift it then with the magic button.

 

[MRector]

I just bought a 91. 5 W250 4x4, it says it has the A518 transmission. I am having the same problem with no O/D, the light "lights up" on the dash when pushed for off, but nothing happens. I need overdrive as I travel the interstate A-Lot!!



Where can I get this info on the mystery switch(I've forgotten)? And does the A518 have a lock-up convertor?



I have a 97 with the mystery switch and I love it!!



Mikey

 

[KOG]

Look back earlier in this thread for complete info on the switch. My '92 manual says it's a 46RH, but maybe they changed the name from '91. 5 to '92. In any case it's the same gearbox and the switch works like a charm on my 91. 5.

 

[HRDROKN]

I just bought a 91. 5 W250 4x4, it says it has the A518 transmission. I am having the same problem with no O/D, the light "lights up" on the dash when pushed for off, but nothing happens. I need overdrive as I travel the interstate A-Lot!!



Where can I get this info on the mystery switch(I've forgotten)? And does the A518 have a lock-up convertor?



I have a 97 with the mystery switch and I love it!!



Mikey

No lockout converter... I believe that 1994 model year was the first with lockout with the introduction of the 47re/A618.

 

[MRector]

I fugured out my O/D porblem, I just bought this truck a few days ago..... anyways we were adjusting the idle and turning up the power screw and found that the TPS wasn't even on the truck!!!!! It was in the glove box , the PO had taken it off cause he said it made the truck shift "funny". Now I have O/D and I need to adjust the TPS voltage. All is well, thanks guys.



Mikey