Humvee Transmission Problems

I have a 1985 M998 Humvee with TH350 transmission troubles. All was running fine but one day started it up and the vehicle won't hardly move unless I press the pedal to the floor. It also trys to move forward when placed in reverse. Even placing it in 4WL it still hardly wants to move. The fluid level is at the line. I am thinking torque converter or a transmission overhaul. Any thoughts on what is going on before I tow it to a shop?

Thank you for all thoughts. Kevin

KVislocky said:

I have a 1985 M998 Humvee with TH350 transmission troubles. All was running fine but one day started it up and the vehicle won't hardly move unless I press the pedal to the floor. It also trys to move forward when placed in reverse. Even placing it in 4WL it still hardly wants to move. The fluid level is at the line. I am thinking torque converter or a transmission overhaul. Any thoughts on what is going on before I tow it to a shop?

Thank you for all thoughts. Kevin

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Its a TH400

Before you tear it down try this. Open the hood. On the drivers side by the washer pump there is an electrical connector. Unplug it and see if it will shift then.

Before I ever took it to a shop-I would drop the pan for a visual inspection.

milbri09 said:

Before you tear it down try this. Open the hood. On the drivers side by the washer pump there is an electrical connector. Unplug it and see if it will shift then.

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how would this help anything? the 400 is completely free of any wires or electronics so how would this help?

I'd say you could do more diagnosis but it's sounding like it's going to need some work. the 400 is pretty tough so it could be your torque converter but I doubt it.

do you hear any funny noises? could be your xcase has died but parking it and then trying to move and it doesn't leads me to believe that this is not the case either.

the trans. is a 3L80. The 3L80 and the 400 are the same except for one thing. The 3L80 had a lockup torque converter. However all other parts are the same. They both have an electrical downshift solenoid in the valve body. I do appoligize for my mistake. The solid state relay by the washer bottle is for the fan control. For the trans. you need to remove the wire from the trans. just behind the modulator valve on the driver's side of the trans.

If you can get the vehicle to a transmission shop they can test the pump output pressure to see if thats the problem. The torque converter connects the engine to the trans and then inputs that power to the pump in the trans. The pressure produced by the pump is what makes the whole enchilada work. No pressure, no tranny action. This can be done without tearing down the trans and can help define the problem.

Another thing you should consider is that even if the fluid level is correct if the filter is plugged then the fluid isn't getting to the pump. To check this you will need to drop the pan and inspect it. Before you do this invest in a new filter, fluid, and pan gasket. It's been my experience that they don't reuse well. They're not that expensive.

Kurt

Your problem may be with the engine not making power. It could be as simple as the glass plunger that is in the nipple that is located at the top front of the Injector Pump top cover. The nipple has a glass plunger that is held against returning fuel from the injectors by a spring. If this plunger gets stuck, you loose so much power that you can't even get the vehicle to move. It can stick because of debris from the hoses at the injectors or from internal IP plastic pieces falling apart. You can take the nipple assy off the Ip and try and clean it or do as I have done on other 6.2's, break out the glass plunger. Be shure to get it all out. The spring can stay or just get that out as well. The glass plunger is clear so you can look through the nipple and think it is OK, but you can't blow through it. I have driven vehicles for thousands of miles without the plunger with no problem and just as smooth and idle.
The Humvee three speed transmissions don't have any electric items. They are just the old style Turbo 400 and are called the 3L80. If you have a low fluid condition or foaming fluid, you could be having the same problem. Do you have trans fluid in your air cleaner or vent lines. It could mean that the cooling loop in your Transfer Case is cracked. I think the "Vampire" problem was only on the 242AMG T cases though. Do you have a 218?

Ok I thought that I was loosing my mind, but after further investigation I have discovered this. The 3L80 is the newest version of the THM400. When they went to the new trans. naming system the 400 became the 3L80. Second, it does infact have an electric kickdown solenoid. The switch is mounted on the injection pump and the wire goes directly into the connector on the side of the case of the trans. to the rear of the modulator control. The soleniod is NSN 5945011757318. You can find out the adjustment procedure in TM 9-2320-280-20-2 section 4-43.

Milbrio9, The humvee has a mechanical Detent Control cable assy that works off of a rod that is attached to the Throttle braket. It is mechanical and is adjusted by loosening the lock nut and threading it forward or rearwards to reach the desired result. This adjustment is done at the left rear of the engine valley area. I don't think that a miss adjusted Detent would cause "slipping" in forward and reverse though. The switch that is on the right side of the Injector Pump and is actuated by the "Pack Man" looking cam is actually a switch that will open a normally closed circuit to that little electrical box that controls the Fan Clutch solenoid and probably the PCB as well. When the throttle is depressed enough, the circuit is opened and the fan is turned off for a period of time. This is supposedly so the fan can be stopped for water fording even though the vehicle may not have a fording system attached. I guess they had a problem with broken fan blades wheb people were driving into lakes and rivers and had to add this little complexity to the vehicle, Still it should not have anything to do with the Trans problem. The ambulances did have Reverse Lights, but they did not use the switch in the transmission like a GM car would, they had a switch in the shifter assy do the job of powering up the rear back up lights. The Humvee in question could be altered from original condition though. Not trying to be a smartie, just trying not to let someone chase wild geese around.

the wire number is 315 and it is for the kick-down feature. Yes the fan control is on the injection pump too.

Ok, It has something that's called a Kick Down switch. If that item is disconnected, then when driving at a low enough speed in third gear and depressing the throttle pedal enough, it should not shift "kick down" into second gear. Is that correct? Then if the switch is re connected and of course operating correctly, the transmission should "kick down" as normal. I have noticed that the TM's and AM General have interesting names for parts. Take for example the fact that they call a part that is bolted to both the crankshaft and torque converter a Flywheel, but I have been told that this is incorrect and should be called a Flex Plate so don't go crazy about names. The fotos that you posted are too small for me to read and when I enlarge them they get blurry. Was there a wiring diagram there that showed the kick down switch operating something within the transmission? I don't think any of this has anything to do with the original posters "slippage" in both forward and reverse though.

kickdown switch

George,

I've never seen anything in the manual that shows the kickdown solenoid in the trans but it is in the TH400 manual from GM. One of these days I'll get around to creating a .pdf file I can send out.

Kurt

There isen't one inside there Kurt. The Detent cable takes care of that, but I seem to remember one wire, I think it was on the right hand side of the trans, rearward of where the dentent cable enters the trans. I thought it was for the STE ICE system. Maybe a sender for a pressure reading? I pulled the origianl power pack out over five years ago so I could be wrong on that. That STE ICE stuff was pretty much a non interesting item for me so I diden't pay much attention to it.

My agency is getting a 2009 Humvee and they are telling me that It DOES NOT SHIFT INTO HIGHWAY SECOND GEAR MAY BE MODULATOR VALVE?? Anyone have any help into this?

I have had a lot cars with th400 and th350 GM transmissions. It sounds like a problem in the valve body or an internal linkage. If you can put it in 4lo and it still goes forward when you put it in reverse I would guess its in the transmission and not the engine or transfer case. I would disconnect one of the transmission cooling lines and crank the engine to see if fluid comes gushing out if so and it is not burnt and doesn't smell like burnt sugar then I would guess that something in the valve body is stuck or loose. I would probably drop the pan and make sure no linkage parts are sitting in the bottom of it. If not I would disconnect the shifting linkage on the outside of the trans. Then I would move the arm through the different positions watching to see if anything seems stuck and making sure all the valves move in the valve body. BTW you did make sure the engine is spinning the correct direction right???

Traffic 301 said:

My agency is getting a 2009 Humvee and they are telling me that It DOES NOT SHIFT INTO HIGHWAY SECOND GEAR MAY BE MODULATOR VALVE?? Anyone have any help into this?

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A 2009 would be a 4L80E equipped truck, not a TH400, so the trans is most likely in limp mode...that can be for a host of reasons.

It doesnt shift into "highway second gear"? Will it shift into offroad second gear?

I have rebuilt a gas engine and forgot to hook up the modulator valve. It had a vacuum modulator. It would start out in 1st then would not shift. When you let up on the accelerator the trans would just kind of let go.

Hello guys I have an 89 m998 and I only have forward and reverse in HL and L . When in H it feels like it engages but when I give it gas I get noise from the rear end differential. Fluid is check and clean. Any ideas on where to start checking components???? Thanks ALL