can a bad ABS make the allison do funny stuff?
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I beg to differ. Only problems with the rear ABS sensor, also referred to as the VSS, can affect the transmission.
What problem(s) is yours exhibiting?
You really need to pull the code to know what is going on. Front wheel sensor issues often don't set codes, but do cause shift issues on the 2-1 downshift.
Vehicles equipped with the Allison LCT1000 Automatic Transmission may exhibit one or more of the following conditions:
DTCs P0721, C0237, P0500, C0055
Speedometer inoperative, erratic or drops out
Cruise control inoperative
Radio speed control volume (SCV) inoperative
Transmission shifting concerns
ABS light illuminated
The testing should begin at the VSS sensor located in the rear of the transmission on 2WD models and in the transfer case on the 4WD models. An erratic signal caused by a loose or damaged reluctor wheel can cause any of the above concerns.Speedometer inoperative, erratic or drops out
Cruise control inoperative
Radio speed control volume (SCV) inoperative
Transmission shifting concerns
ABS light illuminated
The VSS measures the output shaft speed at the back of the trans on 2WD or the tailshaft of the t-case on 4WD models.
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I think what crafty1 is referring to is that if there is a front wheel speed sensor glitch as you're coming to a strop, the ABS module thinks the wheels are locking up and then the ABS kicks in. When the TCM sees the ABS kick in, it sometimes makes it freak out, go into neutral, hold a gear longer, abort a shift, or prematurely shift......
so if you're coming down to a stop, and a split second before the trans does the 2-1 downshift, the ABS kicks in, the TCM will hold off on doing the 2-1 downshift until maybe half a second or a second after you've completely come to a stop....resulting in a big CLUNK. Which a driver might interpret as a "shifting problem"
Crafty1 correct me if im wrong....
ben
You're correct Ben. That corrosion on the front wheel sensor is a common problem on the GMT800's that often get blamed on the trans. Particulary because it often activates the ABS at low speed causing the 2-1 to abort but sometimes doesn't set a code.
The OP says, "my ABS light has been on for some time, i need to change that i know, i have the new one" which leads to several questions about why the light is on, if there are codes present and what "the new one" they plan to change actually is. Not really enough information to go on to diagnose.
I was initially just trying to answer the general question of, "can that affect the Allison Trans?" but since heymccall begged to differ, I thought that I would add some more info that might help the OP make their decision if it was related.
The OP says, "my ABS light has been on for some time, i need to change that i know, i have the new one" which leads to several questions about why the light is on, if there are codes present and what "the new one" they plan to change actually is. Not really enough information to go on to diagnose.
I was initially just trying to answer the general question of, "can that affect the Allison Trans?" but since heymccall begged to differ, I thought that I would add some more info that might help the OP make their decision if it was related.
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I don't disagree that the ABS, specifically the EBCM and VSS, can cause shifting and other transmission issues.
It's merely that front WSS(s) cannot induce transmission issues. When the ABS light is on FULL time, the front WSS's are OFFLINE.
The problem isn't the front WSS's for the transmission issues. The front WSS may be ONE of the causes for the ABS lamp, but they are moot points 'til the real cause is found.
The ABS can't freak out over front WSS's if the ABS lamp is ON.
It's merely that front WSS(s) cannot induce transmission issues. When the ABS light is on FULL time, the front WSS's are OFFLINE.
The problem isn't the front WSS's for the transmission issues. The front WSS may be ONE of the causes for the ABS lamp, but they are moot points 'til the real cause is found.
The ABS can't freak out over front WSS's if the ABS lamp is ON.
Not to hijack. My ABS light comes on intermittently and will stay on for a while, then go away. Then maybe come back on for a while a few weeks later. Should I see codes if I run a check telling me whats causing it to come on? I have experienced some of the things you guys are talking about i.e. speedometer is out, but I assumed that is the stepper motor, and cruise control not working. Thanks.
try to answer some questions here... My wheel bearing assembly's went bad on both sides, when i changed them the light wouldn't go off... so unplugged the cord on the new one left side and plugged it into the old abs cord.. light went off! Right side it stayed on..the guy at Napa ordered me a new cord,,, i just aint put it on yet,, im going to do that tomorrow.i was getting the going into neutral when it was shifting up into 3rd, it only did that maybe 6-7 times, then one time i was coming to a stop and i was almost stopped and it felt like someone rear ended me...this was all in 2 weeks time,,, i have drove it every day for a few weeks now and nothing, its working fine right now, i did notice i was low on trans fluid and topped that off, but it did do it a few times after that, also i changed the spin on filter. when it would go into neutral id have to stop,, trun it off for a few seconds and then it was fine most times, one time it stayed in 3rd.
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I had this problem. both hubs were bad causing the abs to freak out when comming to stop. The truck would go to shift from 2 to 1 and it would drop into nuetral then slam into first very hard. When i did the lift i replaced the hubs and noticed it was gone so now it all makes sence
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