Transmission Slip Gauge

[barrys]

My 2007 Holiday Rambler motor home has an Allison MH2500. I have just installed a Banks Economind including the iQ performance monitor system. Among the wide selection of optional gauges is one called TRANS SLIP. The Banks manual defines this gauge as: "Displays the difference in transmission shaft input speed and output speed in revolutions per minute (RPM)"

This seems like important information but I have no idea what parameters would be considered safe or normal.

I've only driven a short 15 mile test to verify the Banks system is working. During that short test I noticed TRANS SLIP mostly 0 but occasionally reading a single digit briefly when accelerating or pulling a moderate hill.

So what amount of slip should I expect to see under more challenging conditions? Is some amount of slip normal? What would be normal or acceptable slip amount in RPM? At what point is my transmission being damaged?

Thank you,
Barry

[BFlinn181]

I think the Allison transmission would let you know if it was in danger of being damaged.

Actually, I'm surprised at such low numbers on the slippage rate. When you're stopped, the engine is turning at 600~800 RPM and the output shaft is stopped. That's quite a bit of slippage I'd think. Same when you apply throttle to move off from a stop, it slips quite a bit and doesn't go into 'lock-up' until the top gear, maybe.

You might contact Banks about the range of numbers you can expect, perhaps Allison could tell you if there's any reason for concern.

[harleyjt]

When stopped there is no slippage in the gear train. According to what I am seeing here there is an input speed and an output speed. The input is 0 when stopped. What I think it's reading is turbine speed which is really the input. When you are stopped in gear all slip is through the fluid coupling - the torque converter.

Output speed is monitored after all transmission gearing. Once running down the road the two sensors will have an rpm difference due to the gearing and the computer knows that and knows what it should see at the output shaft for any input speed. If you have slippage it will show on the screen.


jt
2005 Newmar Kountry Star gas 3742, Workhorse 8.1L w/Allison 5 speed.

Sent from my iPhone using iRV2 - RV Forum

[barrys]

Thanks BFlinn and Harleyjt for your thoughts.

Before installing the Banks system transmission slip was something I never thought about - there is normally no way one would know of small amounts of slippage. Large amount of slip becomes obvious but then it is too late.

I could simply choose another gauge for that window and be happy not knowing, just like before I had this gauge available.

I'm waiting for responses from both Allison and Banks but not holding my breath.

[dbarton291]

Interesting. A transmission slip gauge. It can't be simply comparing turbine shaft speed to output speed because the only time those two speeds are the same is in fourth gear lockup. That's the only time input shaft speed equals output shaft speed.

In ranges one through three, the input speed is faster than the output speed because those gears are reductions.

In ranges five and six, those are overdrives and the input speed will be slower than the output speed.

If it knows the range the transmission is in, the only time you should see slip is during the shifts.

The Allison control system already has range verification codes in it that measure the ratio between the input shaft and the output shaft to make sure the transmission is maintaining the range is is supposed to be in and is not slipping. It will generate a trouble code if the expected range is not attained or maintained for a short amount of time.

[barrys]

Thanks for your thoughts everyone. It seems like the Slip Gauge information may be interesting to look at once in a while but not particularly useful. I received the following from Banks:

Hello Barry,

Interpreting transmission slip by RPMs is difficult since engine RPM and RPM slip must be calculated in real time but that is the PID (Parameter Identifier) the vehicle’s PCM (Powertrain Control Module) will allow the iQ to read. The vehicle’s instrument cluster will display a “Check Engine” light (CEL) whenever the PCM detects transmission slip of 5%. The Banks tuner will enter failsafe mode and shut itself off when the PCM detects a transmission slip of 3%. This is to ensure that the tuner cannot cause harm to the transmission. In this case it is best to let the vehicle’s PCM calculate slip and let the Banks tuner do what it needs to do.

We hope this information proves useful to you.

Regards,

Email Response Team
Technical Support Group
Gale Banks Engineering
546 Duggan Avenue
Azusa, CA 91702
Phone (626)-969-9600
Toll Free (888) 839-2700