Allisonman's Reply to Some of My AT545 Questions

Discussion in 'General RVing' started by chips2002, Jul 18, 2004.

  1. chips2002

    chips2002 New Member

    I recently bought a 1995 Ford F800 with the Cummins 5.9 engine and
    Allison AT545 transmission. The truck has about 40,000 miles and has 210 HP and 485 ft-ib.s @1600 rpm. The total weight of the vehicle plus the load is about 20,000 Ibs.

    The truck gets a lot of local and highway driving. When I drive in
    traffic, the temperature stays within 140 F to 160 F. But the thing that bugs me is when the truck comes to a stop. I don't know if it is the transmission or the brakes, but after downshifting to first (this happens around 15mph), Ifeel this big jerk at around 13 mph. I guess it is some type of stopping mechanism but am not sure what it is. Do you know what this could be? Is this normal?

    It sounds like your modulator is on. You will either have a cable
    mechanical modulator or an Electric modulator and what the modulator
    does is raise and lower shift points from full throttle (high) shift
    points to off throttle (lower) shift points. If you modulator is staying on, all your shift points are at there highest setting and when you downshift - bang- too high of shiftpoint.

    Also, when I drive on the freeway, it barely stays within 60 mph. The
    top speed is around 65 mph on downhills. The engine rpm is at around 2600 rpm. The temperature stays around 150 F. Is this all the AT could handle? Could I push it more? I am afraid it might decrease the transmission life if I drive over 60 mph constantly.

    This transmission does not have lockup which would give you more road
    speed. The only thing you can do is change the differential gearing.
    This truck was probably set up for local deliveries only.

    When shifting from first to second, I feel this big shock when I do not press the throttle too much (40% throttle). It seems like I get better shifts when I push the throttle about 75%. Is it better to kind of floor the truck? How much throttle am I suppose to use when driving? During traffic, is it better to let the transmission change gears and keep the revs low or rather stay in one gear with the revs high?

    You are correct. The best shifts occur at full throttle, but it
    should never be harsh even at part throttle. Most people drive at about 60% throttle.

    I want to change the tranny oil and the filter myself. I have no
    previous service record, but the truck was used by the city. So, I am thinking it has had some maintenance. Looking through the forurm, I see that there is an internal and external (spin on) filter. It seems like their are mixed reviews on whether to change the internal filter or not. Some people say it is not worth the money and labor and is not going to affect the life of the tranny. Others say otherwise. What is the real deal? Should I change the internal filter and
    drop the pan? I also read that there is a new polyster filter for the AT545 that replaces the brass filter. How does this compare to the paper filter? If the internal filter change is necessary, what filter do you recommend? Could you specify the part number and the average cost?

    I recommend the poly filter. YES! Indeed change the internal filter
    every 12 mos - 25,000 or 1000 hrs. service when using Dexron III. You
    also should flush out your cooler at this time and of course change the external filter. The transmission you were reading about is the world transmission which an AT is not one of. You will have to call your local Allison distributor or dealer for a part number and cost.
    Transynde synthetic is great oil but cost more. If you look at website, there is a list of oils that are
    approved. As long as Allison has it on there approval list, it is good
    fluid. We actually use Chevron Dexron III. With an AT545, Delo 400 15-40 engine oil actually makes the shifts better and stays cooler and this oil needs the Poly filter.

    For external spin on filters, I was thinking about using Wix filters
    instead of the original Allison filter. Do you think this filter is safe to use? There was some talk about Transynd synthetic transmission oil on the forum. I see the added benefits are increased service interval and lower transmission temperature. I remember you saying that it is only worth it if the transmission has low miles (~20,000 miles). My truck has about 40,000 miles, do you recommend that I stay with the Dextron or switch to the Transynd? If you recommend the Dextron what oil is reliable? I was thinking
    about using the regular Cheveron Dextron oil they sell at Costco for
    around $13 for 12 quarts. I also see that some people are using C4 motor oil as transmission fluid, what is the difference? Does this have any added benefits?

    This is normal with an AT 545. This model does not have lock-up in
    the converter which means it is all moved hydraulically with the torque convertor. Hydralics cause more friction and heat. You are doing everything correctly by pulling over and reving to 1300 until cooled. The only way to keep it cooler is by adding an additional air to oil cooler in front of your radiator.

    I occassionally tow a Bobcat 863 behind my truck. I see that the
    temperature increases a bit, 180 F when driving and sometimes 200 F when during a stop. When the temperature hits around 200 F, I stop the truck and put the transmission to neutral. Then, I rev the engine to about 1300 rpm. This lowers the transmission temperature. Is there anything else I should do when towing a trailier? How should I drive when towing?

    This transmission is only rated at 30,000 lbs. With the trailer, what
    is the combined weight? When towing a trailer, it would have been better to have a model MT643 but the AT will be ok.

    Thanks for the questions. If you can get into the forum at RVUSA, please could you post that I have been booted out of going in? I think they thought I was promoting my company which as you can tell, I didn't even mention. I was there just to answer peoples questions, but someone must have complained.

    Thank you - AllisonMan - Rick

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