You can change it as many times as you want. And if the filter is new, see how it looks, and be smart about it.

 

If the fluid looks good there's no need to service the tranny again so soon. Wait 10k-20K and change it again if you need too. Synthetic ATF is good stuff but its not miracle juice and its pricey. If the unit shifts good and the fluid looks good I'd leave it be a while.

 

Tranny flushers hook into the cooler lines, old fluid comes out one line and new fluid goes in the other. Where does the junk come from that these flushers are pushing into places it doesn't belong. The old fluid is being picked up through the filter so the junk should get trapped in there. I believe in flushing but NOT using the chemical. If the chemicals are so good why doesn't Ford sell it, after all they want to make money, too. Our shop does about 50-60 flushes a month (no chemical), and the only tranny failure I have seen is when the Express Lubie somehow got water mixed with ATF. Also our shop's flushers use the tranny's pump to do the exchange so maybe that is the key. I've fixed many a torque convertor shudder with a flusher.

 

My 98 Expy has a factory drain plug in the pan. At 43,000 I drained the pan and converter and refilled with Mercon V. At 93,000 I drained the pan, converter and changed the filter and refilled with Mercon V. Fluid is always clean and I haven't had any tranny problems at all.

 

I basically do exactly what Mudman does, same mileage and same result. People usually don't change it period and that is a problem. If you drain as much as possible, fill with new, it'll mix a little but will be about 90-95% new.

 

Remember, there is a difference between a power flush machine and fluid exchange machine. Many power flushes stir junk up that should be left alone because of the pressure involved and back flushing. A fluid exchange uses the vehicles transmission pump to replace fluid. It does not put stress on or overpressurize the unit. Years ago I had the fluid exchanged on a Ranger, it took 45 minutes for the transmission to push all its old fluid and pull in the new. This is safe method, back flushing with chemicals is not.

 

Deleted.

 

How would you backflush when ATF fluid only flows one way when the tranny is running? Shouldn't the filter catch all the junk that is stirred up? I think it is the chemical additives that is the problem.

 

Good point. The only way one could backflush a transmission is to remove the pump. The pump will act like a check valve, preventing the flow from going backwards.

 

I hate to beat a dead horse, but I'm still a little confussed.
My truck uses the MERCON III, will it harm it if I drain the pan and converter and put in the synthetic?
I know I won't be able to get all the old fluid out of the lines and cooler.
So I guess I want to know if it will mix. Thanks everyone for your input

 

Yes you can mix it. If you drain the pan and converter, you'll only have about a pint left in the cooler lines & cooler anyway.