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1988 Prevost Transmission issue help needed
Hi all,
Looking for some advice for my neighbor’s ’88 Prevost transmission shifting issues.
I finally drove it the other day and did not like the way it shifted. Takes off ok, then the shift to 2nd is kind of soft, then it shifts to 3rd at around 1700 RPMS and if you drive in the city and stay at around 30-35 mph, the tranny shifts down into 2nd, then back to 3rd, and so on…you can watch the tach and see the RPMS go up and down as the tranny shift back and forth…..if I speed up and goes into 4th it seems fine and then goes into overdrive ok. Seems to hold the overdrive fine over 55 mph with no abnormal down shifting.. I can only see the problem between 2nf and 3rd gears.
The chassis on this 1988 vintage has the 8v92TA DDEC and the HT 755CR ATECtransmission with the the Allison shift padwith 1, 2,3,4 D N R and the Do Not Shift light in the upper right corner.
He says that the the transmission has been shifting this way since he got the bus 10 yrs ago. Complained, had mechanics look at it, no codes flash when the switch under the driver is in diagnostic mode, no solution offered. He does not like the way it shift either so we are now on a quest to find a solution and answer.
The bus has 105k miles. No records on the transmission fluid change. He only put 13k miles in 10yrs on it….so we think it is time to do a fluid change to begin with.
Some folks on forums advocate straight 30 motor oil in the HT 755 but from what we gather in the books it should be Dextron II or Transynd if he wants to go synthetic…Which way to go ? The shift pad does not have the UP and Down arrows to check the fluid level electronically and I don’t know how to check the level on this vintage…looked for a dipstick and did not see one…maybe I missed it.
Do we have any folks that have experience with this transmission, maybe offer some advice what to check for this back and forth 2nd and 3rd gear shifting pattern and what fluid to use, fluid level check and how to change the fluid and filter if there is an easy way.
Any advice appreciated.
Thanks
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7 Replies
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It sounds like the transmission is doing what it is supposed to do, which is to shift. The way you said you know it shifts is by watching the tach sounds like the shift at city speeds is not a harsh event. It might be as simple as the speed cited is at a shift point and if you lift from the gas pedal or step on the gas pedal at that point it is going to shift as it is supposed to do. You might be influenced by your experience with the 6 speed transmission in which the shifts are smooth and hard to perceive and the 5 speed tends to hold shifts until higher RPMs and is more noticeable. The 5 speed can have harsher shifts but it is a very robust transmission and when used in heavy emergency vehicles tends to bang from gear to gear.
Transynd costs an arm and a leg but the synthetic fluid is really good for the Allison transmissions so that is what I would use.
The five speed in the 1988 coach lacks the fluid level sensing so the correct amount is almost impossible to read on the dip stick. In fact it is so clear you tend to want to check it multiple times just to be sure it is at the right level unless you want to use the 1/8″ small pipe threaded plug in the side of the pan.
Overfilling will cause the transmission to over heat so the oil plug is used like the one in the differential. You will have to do some research because I cannot remember if the oil level is checked hot or cold, but it is checked with the transmission in neutral and the engine idling. Maybe someone on here remembers when to check.
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FWIW, many years ago I asked DD about changing to Transynd for my 94 Liberty and they advised against it, told me to stick to Dextron for the 5 speed.
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When I purchased mine, the previous owner had made one transynd change. I completed the changeover process with a second. It does take at least 2 a few thousand miles apart to get a complete change over. That is a $1000 trip.
My temperature dropped a good 50 degrees. It shifts smoothly except on a hard stop the final downshift to first can be hard.
If it has old Dextron now it is likely loaded with adsorbed water and will shift rough. Get that out. It someone put Dextron III in it will have a lot of water.
Check it cold in neutral at an 1/8inch pipe plug on the side of the transmission. fluid should just come out. If overfilled it will foam and overheat. Let it drain out to this level on changeout.
Check it hot in neutral when using the dipstick.
1999 XLV X-6872, liberty #440 -
Thanks Joe. I couldn’t remember if the fluid needed to be hot or cold to use the drain plug method of checking the level. CRS!
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Have the 755CR trans (1991 XL) and went with the Transynd in mine after i bought the coach 9 years ago after changing all the fluids out in coach. . Had 50,000 miles on coach when I bought it. . Has 97,000 on it now. Haven’t noticed any up and down (hunting) between shifting points and shifts firm at every shift.
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Thank you, appreciate the replies.
Does anybody know the part number and/or equivalents for the screw on filter for the 755CR transmission ? Seems to take around 7.5 gallons for a full fluid fillup.
How much does really come out out of the 7.5 gallons when you remove the plug ?
Should he order 8 gallons or actually less ?….some fluid never comes out, right ?
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I ran transynd in my 755 and agree temps were substantially lower. I would definitely go with a full synthetic.
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