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Please HELP....6.5 non turbo diesel 4L80E in a chevy p30 school bus will not shift into overdrive.

S

Stefdawson

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We have had transmission rebuilt. Still will not shift into overdrive. Also speedometer reads high 70 when only going about 40. We replaced the drac but nothing. We have not had it read for codes because none of our mechanics have the hookups. We are dumbfounded!!!
 
Welcome. You found the right place.
You have a control module that gets a couple signals from engine and sends info to transmission, called tcm or ecm (same device but different names in different books). Silver device in the cab under the dash somewhere about 8” wide and 12” tall.

You do not have correct signal coming from transmission to the dracc then to the tcm.
are you sure it is for your axle ratio and tire size? It sounds like someone maybe put in a wrong one. Doing the dip switch modification or replacement with one already done might be an easier solution. I am 95% that your problem is the drac being the wrong one.

I can’t say for sure where the error is- maybe during rebuild wrong part in there, maybe one of the two speed sensors on the side of the transmission, maybe tps (throttle position sensor) on the injection pump is not adjusted correctly. Not likely the issue because we know your speed is wrong, but it helps control this also. I wouldn’t touch it until the speedometer is accurate.

You need to find out is yours obd1 or obd2. Look at, or post picture here of the connection port. Many p30 have obd2 port but operate on obd1 tcm.

Check back regularly, folks will have answers for ya soon.
 
Will L. said:
Welcome. You found the right place.
You have a control module that gets a couple signals from engine and sends info to transmission, called tcm or ecm (same device but different names in different books). Silver device in the cab under the dash somewhere about 8” wide and 12” tall.

You do not have correct signal coming from transmission to the dracc then to the tcm.
are you sure it is for your axle ratio and tire size? It sounds like someone maybe put in a wrong one. Doing the dip switch modification or replacement with one already done might be an easier solution. I am 95% that your problem is the drac being the wrong one.

I can’t say for sure where the error is- maybe during rebuild wrong part in there, maybe one of the two speed sensors on the side of the transmission, maybe tps (throttle position sensor) on the injection pump is not adjusted correctly. Not likely the issue because we know your speed is wrong, but it helps control this also. I wouldn’t touch it until the speedometer is accurate.

You need to find out is yours obd1 or obd2. Look at, or post picture here of the connection port. Many p30 have obd2 port but operate on obd1 tcm.

Check back regularly, folks will have answers for ya soon.
Click to expand...
Thank you for all of this. You are so helpful. We just put in a new drac that fit our gear ratio and tire size. Kept the old one just in case and that did not solve the problem. I am not sure about the obd1 or 2 you mentioned. We just took it to a mechanic today to start troubleshooting as the garage who just rebuilt our transmission was out of ideas. Any info you can help us with is winderful. This bus will soon be our home on wheels and we are just so frustrated right now.
 
ak diesel driver said:
Are you sure it's not going into overdrive or is the TCC not locking up?
Click to expand...
AK makes a good point. With a properly working 4L80E you should feel a 1-2, 2-3, 3-4 then the TCC clutch lock up at a cruising rpm above, iirc, 55 mph/1800rpm. The person who needs to jump in on this thread is our resident transmission guru and expert, @THEFERMANATOR . To me, it sounds like the speedometer being almost 2x's off of real speed suggests something amiss either with the transmission rebuild and the impulse count going to the Vehicle Speedometer Sender Module is way high due to misinstallation of the reluctor or the wrong one was installed, or the Module has gone bad. It could also be something wrong with the rear speed sensor on the transmission. And what ever it is is telling the PCM that the rpms are all wrong for the speed input it's receiving and it's not allowing the transmission to shift up or lock. I still think this is related to the transmission rebuild as the cause, just my three cents worth - inflation you know.

Allen Schreiber
 
How long have you had this van? When did the problem start? Why was the trans rebuilt. A 99 should use a vssb, not a drac(similar boxes, but the vssb has more outputs). Has any wiring work/repairs been made to this vehicle?
 
@Stefdawson , THEFERM has weighed in, you need to have a conversation with him on here, he is an EXPERT on the 4L80/85E automatic transmission, as well as several other automatics by GM and other manufacturers. He'll get you to the bottom of this mess PDQ.

@THEFERMANATOR , remember we're dealing with a P-30 Step Van chassis here, they usually lag a couple of model years behind the GMT-400 P/U and Suburban platforms.
 
@THEFERMANATOR @Stefdawson

Just FYI, so for diag - I would verify components.

I have seen 1999 p30 / topkicks/ kodiaks that were db2 obd1, ds2 obd1, ds4 obd2. I haven’t seen but would not be amazed to find ones like my hummer that is db2 obd2 plug with obd1 system.
Medium duty class rigs were not subject to the 1996 obd2 law. They had a 5 year waiver nation wide.
 
When we bought the bus it did not shift into overdrive. Speedometer ran high. Of course this was not told to us at purchase. We thought getting transmission rebuilt would solve the problem. It has done nothing. The mechanic who rebuilt the transmission says I do not know what else to do. We currently took it to another mechanic who is going over everything and definitely is more knowledgable. He called and stated the temperature sensor circuit was stuck "open". So he was going to start with that. Any help you can give me is appreciated. This bus is our home so I need this thing to run. We have not been able to take it anywhere as it cannot be run like this. Thank you all for your help!
 
The temperature sensor (the one next to the thermostat housing is for the computer and it does control things like the TCC locking up and shifting into Overdrive, which it will NOT do if it's telling the computer that the engine is too cold because the sensor is bad. The temperature sensor in the Driver side head by the #1 fuel injector is for the dashboard temp gauge only).
 
And I'm still of the opinion (but @THEFERMANATOR is the REAL expert) that the speedometer is off because the wrong reluctor was used in a rebuild, and then was used over again in the last "rebuild", or that the rear output shaft sensor is going/gone bad and giving you false speed readings. If it's a mechanical, not electric, speedometer in the van chassis, it could also have the wrong speedometer drive gear in the transmission, too. Definitely need more info like the option codes for the build, etc. FERM?
 
If the dash gauge is seeing the wrong speed, then the computer telling the transmission it is ok to shift is seeing the wrong speed. Think the transmission engineers would want to allow shifting at the speed you are reading on the gauge?

Don’t waste your time with temperature sensors or other nonsense.

Fix the speedometer issue and 99.9% sure the transmission will work properly in all gears.
Heck- it is working properly because it shouldn’t do anything different according to the speed of the dash gauge.

Get it to a GM dealership if you can’t find a top notch 4l80e transmission shop.
When they hook it to a computer they will see the issue and probably tell you transmission has to come out and be redone with correct parts.
 
I was to the thinking that the coolant temp has to get to a certain temp before the TCC will lock up and for OD to engage. If a shop has a scanner and it is not seeing proper coolant temps then that too needs to get fixed any way. I could he rong though. 🤷‍♂️
 
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