When driving will act like it has slipped out of gear?

The symptoms you're describing in your 1999 Chevrolet Suburban are indicative of a potential transmission issue. While a faulty transmission speed sensor could contribute to these problems, there are a few other possibilities that could also be causing these symptoms. It's important to consider all potential causes before deciding on a course of action.

Here are a few possibilities:

Transmission Speed Sensor: The transmission speed sensor provides input to the vehicle's computer about the vehicle's speed. If this sensor is malfunctioning, it could cause erratic shifting behavior. However, it's not the only potential culprit.

Transmission Fluid Level and Condition: Low transmission fluid level or dirty/deteriorated fluid can lead to slipping and shifting issues. Check the transmission fluid level and condition. If it's low or looks discolored, you might need a fluid change or a potential leak repair.

Transmission Solenoids: Modern transmissions use solenoids to control the flow of transmission fluid, which affects gear shifting. A malfunctioning solenoid could cause erratic shifting or slipping.

Torque Converter Issues: The torque converter is a component in the transmission that transfers power from the engine to the transmission. If it's not functioning properly, it can cause slipping and other shifting issues.

Transmission Control Module (TCM): The TCM is responsible for controlling various aspects of the transmission's operation. If the TCM is failing or experiencing communication issues, it can lead to shifting problems.

Wiring and Connections: Faulty wiring or poor connections in the transmission system can disrupt the signals between sensors, solenoids, and the TCM, leading to shifting issues.

Thanks for the input, but I do have thermometer, also have rad PSI test pump, my Dad "was " now retired for 23 years " a Mechanic for 47 years, he has seen everything I have gone through, the only thing he had left to add was that the antifreeze brook down and jelled and then hardened, and covers up the back cyclinder walls keeping them from cooling, only came across this once and that was in 68, acid bath the blook, but with todays stuff I didn't think that was my problem, since I had the head off and could see down the back water jacket.
Thanks any way.

What u described and approach sounds(to me is(correct me if I am wrong anytime) - case problems are solve by experienced resources who derived the solution from the past history. Another word. If they never seen that happen b4 then they can't fix a problem. How bout this. Understand what basic principle of the thermodynamics internal combustion engine. Then ones can react and decipher whats valuable information to solves problems even no1 ever encounter b4. Cuz what ever happen out there new or old. It still gotta be hold up under the law of physics. Agree?

Yes I do lose coolant, 3 to 4 liters at a time, when truck is emty and driving the highway, trucks about 1/2 up the Ford gauge(Ford has the word Normal and a bar line on each side of the word, with about a letter space between the word), now for the fisrt 280,000km or so it always stayed at the "N"- truck loaded or not. Then one day for no reason really it went past the last bar to "H" blow the drain plug right of the rad and has been running hot ever since. That was 9yrs ago. NO the gauges do not fall right away, it takes some time, the overflow is also over flowing, when it cools down, the engine sucks all that remains in the overflow until it sucks air. Right now when I pull the trailer I have(17ft), with no thermostat, the gauge will be at the "L"(of the word normal), with 5 to 8 km and stay there. The check engine light at that point will flicker on and off. Speed of travel 90kmp, take your foot out of it and coast down a small hill(300yrds or so) and the gauge will drop between the "M" and "A" of the word on the gauge, step back into it and it will be back to the "L" with in 1Km on the flat. This whole time the clutch fan never kicks in. I still have the one the truck came with, I have tried three different kind of after maket ones(they never kicked in) and the got a forth one right from Ford$$$, and is still on the truck and seem no different than the one that came with the truck, and it also never kicks in, ever. I have even taken some coat hanger wire and wired the pulley bolts to the clutch fan bolts, to hear the sound differents, I know for a fact, that the clutch fan has never kicked in on this truck for over 18yrs. The wires broke, lasted about 70km, to much strain I guess. Sounds like a jet taking off at 90k, lol

Well I will not test the laws of physics, I understand how the system works, and have had payed money to some rad pro shop, they told my 2nd Alum. Rad was no good after only two years( the first one got a rock in it), so they built the 3 core copper rad, with costom tanks and everything. Holding three liters more than stock. Truck was still over heating under load, and 2 months later blew out 27 of the cores and they could not fix it. Thats when I put back in the 2nd Alum. One, for I asked for it back at the time. As for taking it to Ford again and pay 110$ /hr for them to stand there and scatch thier heads, since I already replaced everything, I don't think so.

The clutch fan is the type that has a single 1 1/8" nut that threads to the water pump pulley. The five blade fan bolts to the back side of the fined fluid clutch, with a heat spring on the front side of it, facing the rad. There is some drag to it when the engine is not running, but you can turn it with your hand, the old one that came with the truck is the same way. But these clutch fans never kick in on this truck. It should lock-up for a few seconds when one starts the truck even cold, but never does anything, it does draw air in its slipping stait, but does not lock-up to the engines RPM. Even when the truck engine has been hot, hand I have had to pull over, shut the truck off, over flow tank is over flowing and you can spin that clutch fan by hand, same as when cold.

Have not ran the truck with the test pump hooked up, only tested to see if I'm losing psi, I'm not
Exhaust is new, 3" right through, High flow muffler.
Running the engine now with out any thermostat for the past two weeks, will be testing flow(by eye with cap off) tomorrow, and a set in rad gauge.
Yes the engine boils over to the point that the overflow tank can't hold it all and runs all over the ground.
The electric fan is in front of the rad with the fan running in reverse, so it blows, because the big fan on the water pump pulley is still being used. The electric fan has not seemed to have fixed anything, other than maybe gives me a few more mile before I need to let the engine cool down.

New hose's with coil wire in bottom hose, new water pump, had three different new thermostats, (2-194deg-stock setting and even 1-185deg) now running with nothing. New head gasket and NO it's not put in wrong on this engine, can only fit one way. And there was nothing wrong with the old one anyway. Head has been hot bathed, valves and sets resurfaced, and head shaved.007. The engine is malti-port injected, intake has been cleaned, new EGR system parts. Fan is installed correct. Is not burning coolant, engine only uses 1 liter of oil in 6000klm driving emty, and 1liter in three tanks of fuel when pulling 17ft trailer loaded. No coolant in oil. Never has been. Engine has never been rebuilt, this problem started at 286,000km and the truck now has 458,000 on it. And only one owner, me. About the engine running lean, not sure about that, being fuel injected, but I do have another truck, with the same engine, but a year newer, with only 292,000km on it, picked up 100$ after it hit a power pole, thinking of taking the (chip) out of that one and puting it in my truck. But my truck has always been a turn key runner, one crank and it starts. I also have put in new gear to gear timing set, and the timing is set at spec. About the belt slipping, only one belt for all with tensioner, and yes I did replace that some time ago. And ther is NO A/C.
3inch exhaust-front to back, high flow muffler, all good.

The clutch fan is the type that has a single 1 1/8" nut that threads to the water pump pulley. The five blade fan bolts to the back side of the fined fluid clutch, with a heat spring on the front side of it, facing the rad. There is some drag to it when the engine is not running, but you can turn it with your hand, the old one that came with the truck is the same way. But these clutch fans never kick in on this truck. It should lock-up for a few seconds when one starts the truck even cold, but never does anything, it does draw air in its slipping stait, but does not lock-up to the engines RPM. Even when the truck engine has been hot, hand I have had to pull over, shut the truck off, over flow tank is over flowing and you can spin that clutch fan by hand, same as when cold.[/Quote:060836a425]

Try this old school:

"With the truck cold, tie a length of "kite string" to one of the fan blades, and the other end to some point in the engine compartment. Then start
the truck.

The string should be able to hold the fan when the engine is cold, and a good fan will break the string with engine warm."[/Quote:060836a425]

That sounds like fun, I will give that a try, but as I have been told, and have seen this, when cold the clutch fan should lock-up for a few seconds when one first start the engine, and then release and free wheel. Which mine does not do and never has, but my dads does, so does my brothers trucks, and my buddy's trucks, and I have done lots work on his, that what friends are for right. Lol free beer

Well combustion temp is lowered by the EGR system, and this is done also to keep the valves cooler, this is one of the reason why engine last so much longer than the engines of the 60's. But the over heating problom keeps happening. I could maybe try changing the sending unit for the gauge, not sure if it the on board computor to lean out the injectors, it is this would cause high temp combustion. Something try. Thanks

Well there something I never thought of. I do no an engine Tec. Guy that has a tinny little camera on a flex wand. Just like the one a surgin uses to go up one nose and down into one lungs. And yes it even has a tinny little light too. I had the lung check thing done back in 91, I almost died, but still here, lol I will have a chat with him and see what he thinks. Thanks for the idea.

It work and I never expected! Hah!

New hose's with coil wire in bottom hose, new water pump, had three different new thermostats, (2-194deg-stock setting and even 1-185deg) now running with nothing. New head gasket and NO it's not put in wrong on this engine, can only fit one way. And there was nothing wrong with the old one anyway. Head has been hot bathed, valves and sets resurfaced, and head shaved.007. The engine is malti-port injected, intake has been cleaned, new EGR system parts. Fan is installed correct. Is not burning coolant, engine only uses 1 liter of oil in 6000klm driving emty, and 1liter in three tanks of fuel when pulling 17ft trailer loaded. No coolant in oil. Never has been. Engine has never been rebuilt, this problem started at 286,000km and the truck now has 458,000 on it. And only one owner, me. About the engine running lean, not sure about that, being fuel injected, but I do have another truck, with the same engine, but a year newer, with only 292,000km on it, picked up 100$ after it hit a power pole, thinking of taking the (chip) out of that one and puting it in my truck. But my truck has always been a turn key runner, one crank and it starts. I also have put in new gear to gear timing set, and the timing is set at spec. About the belt slipping, only one belt for all with tensioner, and yes I did replace that some time ago. And ther is NO A/C.
3inch exhaust-front to back, high flow muffler, all good.[/Quote:2fc7e0516c]

Wrong signal from O2 sensors at closed loop/partial clogged injectors and low fuel pressure will make it run lean.[/Quote:2fc7e0516c]

O2 sensors? There is three of them, will need to pay for that one to be diagnostic coded, but do have a home hand held one, should give that a try again, injectors have been replaced, new fuel pumps last fall( first time they messed up after 450,000km, not bad don't you think?