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Old 02-26-2024, 03:13 PM   #1
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Shifting interlock, no engine revving 1990 GMC 6000 SS-31

Hello from a brand new Skoolie! I need help and advice please.

I just bought a 1990 GMC 6000 6.0L 366 CID V8 automatic Ward Volunteer SS-31. Friends of mine drove it to their backyard in 2020 to use as a Covid classroom for their kids and the neighborís kids. It started right up after 4 years of sitting.

The bus started in neutral but the gas pedal didnít rev the engine. I could move the shifter into reverse or forward and the bus would clunk like it wanted to move, but no gas pedal power. The pedal cable moves the cam on the carburetor/injection thing but no revving.

Now, a week later with no tinkering, it wonít go into neutral anymore and is stuck in PB. Starts in PB but shifter stick wonít move.

Conditions:
Plates in carb/injector open manually when engine off but stay shut when opened by hand with engine running. Whatís up with that?!
Air brakes pressures up past 60 and on to 120.
Buzzer is sounding next to driver and back door. Checked continuity. Wire 5(back door and barrel bolt) and wire 57(roof hatches) are continuous. Wire 14(push out windows) is an open circuit.
All fluids are new and full. Bus serviced in 2020.

Questions:
What caused shifter to suddenly lock up?
Is the gas pedal tied to the brakes or something?
Wiring diagrams from a later model seem to show the buzzers are just noisemakers. Not sure. Sign inside bus mentions rear and side door starter interlock but bus starts so that might not be an issue.

Iím getting frustrated. Must be something simple. Help!

See photos.

If you happen to live near Austin, Texas Iíll gladly pay you to come troubleshoot it with me!

Thank you,

Brian in Austin
Attached Thumbnails
IMG_3343.jpg   IMG_3368.jpg   IMG_3093.jpg   IMG_6651.jpg   IMG_3200.jpg  

IMG_3088.jpg   IMG_3214.jpg  

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Old 02-26-2024, 04:01 PM   #2
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That is a throttle body injection unit (TBI).

If you have a buzzer, most likely it is an interlock keeping you from shifting or getting throttle response. Smarter peeps than I will chime in soon with good troubleshooting advice. I know nothing but would think the open circuit on the emergency escape window could be the issue.
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Old 02-26-2024, 06:12 PM   #3
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Isn't a 1990 bus a bit to old to have any electronic interlock devices?

As far as the shifter not moving, is it possible to go underneath the bus, disconnect the shift linkage and try turning the trans shifter by hand to see if it moves.

If it does then maybe it sa shift cable issue?

The bus of course with the engine OFF!

What tranny does you bus have?
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Old 02-26-2024, 06:15 PM   #4
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Thank you Kentucky Dreaming. I’d hate to hunt down each of the four push out window switches. I’ve read about bypassing but can’t wrap my head around a simple DC circuit. I’ve tried jumping #14 to ground with alligator clips and wire but nothing changes.

The plot thickens a bit because there is a starter interlock label that refers to the back door and side door. I never found any side door switch but there is no problem starting it anyway. Not sure which wire # would run through it.
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Old 02-26-2024, 06:23 PM   #5
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Hello Ewo1. I see that you are in Central Texas. Which town? Maybe I can bribe you with BBQ and $ to come to south Austin?!!

I can crawl under there tomorrow afternoon and try to manually shift it. The driver’s shifter must have some kind of locking solenoid. It’s rock solid now whereas it moved easily last week. Don’t know anything about the tranny. Are they typically labeled somewhere?
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Old 02-26-2024, 06:41 PM   #6
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Chock the wheels before you cawl under there!!!!
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Old 02-27-2024, 06:28 AM   #7
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ive not heard of a quadco shifter having an interliock.. GMC typically used those on the bus chassis...


the starter interlock in those years was if the back door was locked the bus wouldnt start.. people often disconnected them as the ywere a PITA.. kids would lock the doors and then the bus wouldnt start and they would laugh at the driver..



im interested to see a pic of your trans shifter... the only interlock i know of is the button you press on the side of the handle that releases the interlock... I suppose its possible something broke in there causing it to lock up.. the shifter contains a neutral safety switch, which is good on yours because you can start the bus..


of course its entirely possible a school system had different things made for them.. this was done all the time.. about the time i think ive seen all the different injection systems, shifters, and body / chassis combos.. someone shows up with something new..



the TBI system has 1 of 2 control mechanisms.. 1 is where there is a little motor that controls the engine speed at idle.. in this configuration I had never see nthe throttle body blades stay closed on a throttle up... the other used a solenoid for high idle.. and on those i seem to remember a linkage of some sort that prevented the throttle blades from opening up more than a small amount till the engine ran a couple minutes. I didnt run into those very often.. these are not drive-by wire systems so the throttle cable is supposed to allow the blades to open once the engine ran a minute or two and established good idle quality.. since it starts easily i dont suspect a bad temp sensor or anything..
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Old 02-27-2024, 08:12 AM   #8
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Quote:
Originally Posted by BigTex View Post
Hello Ewo1. I see that you are in Central Texas. Which town? Maybe I can bribe you with BBQ and $ to come to south Austin?!!

I can crawl under there tomorrow afternoon and try to manually shift it. The driverís shifter must have some kind of locking solenoid. Itís rock solid now whereas it moved easily last week. Donít know anything about the tranny. Are they typically labeled somewhere?
I have a place in Killeen but at this moment I am back in S. florida. Planning to be back in Tx. In about 2 weeks.

If you donít figure it out by then Iíd be happy to swing by.
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Old 02-27-2024, 06:24 PM   #9
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Thank you Cadillac Kid. Iíve let it idle for quite some time so I donít think that triggers anything.

Let me attempt to upload pictures of the Felsted shifter and label.

https://1drv.ms/i/s!AsXWoDHhsynqhcINeB8gQMGcdZL9fg

https://1drv.ms/i/s!AsXWoDHhsynqhcIQP8LlL85OyNH5xw

https://1drv.ms/i/s!AsXWoDHhsynqhcISz2CoXCHgEipgag
Looking at shifter from below.

Maybe you can make sense of this type of system?
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Old 02-27-2024, 06:37 PM   #10
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Quote:
Originally Posted by ewo1 View Post
I have a place in Killeen but at this moment I am back in S. florida. Planning to be back in Tx. In about 2 weeks.

If you donít figure it out by then Iíd be happy to swing by.
ewo1,
I very much appreciate the offer. While I hope I’m no longer struggling with it, knowing you might be available when you return gives me hope!

I’ve been handing out flyers at local school district bus barns. So far, no luck. Those mechanics are probably tired of seeing buses by the end of the day.
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Old 02-27-2024, 07:47 PM   #11
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this almost act like it points to an interlock like the bus door being open and / or it thinking the school lights are activated...


I had totally forgotten about the felsted shifters... ive never persoanlly worked on one.. only seen them.. I domnt remember seeing a lock on them though.. but its possible an interlock was mounted as a custom..
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Old 02-27-2024, 08:39 PM   #12
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Quote:
Originally Posted by BigTex View Post
Thank you Cadillac Kid. Iíve let it idle for quite some time so I donít think that triggers anything.

Let me attempt to upload pictures of the Felsted shifter and label.

https://1drv.ms/i/s!AsXWoDHhsynqhcINeB8gQMGcdZL9fg

https://1drv.ms/i/s!AsXWoDHhsynqhcIQP8LlL85OyNH5xw

https://1drv.ms/i/s!AsXWoDHhsynqhcISz2CoXCHgEipgag
Looking at shifter from below.

Maybe you can make sense of this type of system?
In the first picture can you confirm something please.
Behind the shifter I see what looks like a 3 position terminal strip. It has a diode with blue connectors.

Take a look, is that circuit involved with the shifter?
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Old 02-27-2024, 09:01 PM   #13
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felsted shifters

Here is a Felsted Shifter catalog. On page 6 it explains a little on how it operates. You should know this information since your tranny may not have a park pawl.
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Old 02-28-2024, 05:55 PM   #14
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Felsted catalog

Quote:
Originally Posted by ewo1 View Post
Here is a Felsted Shifter catalog. On page 6 it explains a little on how it operates. You should know this information since your tranny may not have a park pawl.
Thanks for the catalog link. Page 6 inches me closer to understanding that aspect of my troubleshooting although I canít figure out what purpose the wires serve. Electrical brake actuation? What keeps it frozen in PB?

https://1drv.ms/i/s!AsXWoDHhsynqhcINeB8gQMGcdZL9fg
Repeat photo of shifter

https://1drv.ms/i/s!AsXWoDHhsynqhcI5apgg_Vm_oLaJSw
Hereís a bottom up view

https://1drv.ms/i/s!AsXWoDHhsynqhcI6g4ofNfZjOWYXHg
Side view

https://1drv.ms/i/s!AsXWoDHhsynqhcI7ZaMjx4pv9-NGkQ
Other side view with terminal strip
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Old 02-28-2024, 06:07 PM   #15
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Originally Posted by ewo1 View Post
In the first picture can you confirm something please.
Behind the shifter I see what looks like a 3 position terminal strip. It has a diode with blue connectors.

Take a look, is that circuit involved with the shifter?
Here are the many angles of the shifter and those wires going to the terminal strip. It appears that the blue and black go to the shifter. I need to pay closer attention when I go back over to the bus now and trace those two exactly to confirm.


(Iím figuring out how to upload photos rather than link.)
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IMG_3397.jpg   IMG_3398.jpg   IMG_3412.jpg   IMG_3362.jpg  
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Old 02-29-2024, 06:24 AM   #16
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the stuck shifter is interesting because in all the doicumentation and talking t oa few people in the enthusiast groups im in, have said that the wiring to the shifter is a switch and not a lock.. its micro switch that triggers the parking brake to be actuated.



the parking brake itself is a solenoid.. they are thinking the locked shifter is related to the release collar not releasing the shifter handle. or the shift cable failed and froze up (less likely)


what does it look like at the transmisison end? are there any indications where the shifter lever goes into the transmission that there is an electrically actuated pin or such? I know someome mentioned manually shifting the transmission. (pulling the cottrer pin and disconnecting the shift cable and m,aking sure you can move the shift lever at the trans back and forth (engine off of course).. just to rule out an issue there..
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Old 03-03-2024, 07:15 AM   #17
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Quote:
Originally Posted by cadillackid View Post
the stuck shifter is interesting because in all the doicumentation and talking t oa few people in the enthusiast groups im in, have said that the wiring to the shifter is a switch and not a lock.. its micro switch that triggers the parking brake to be actuated.



the parking brake itself is a solenoid.. they are thinking the locked shifter is related to the release collar not releasing the shifter handle. or the shift cable failed and froze up (less likely)


what does it look like at the transmisison end? are there any indications where the shifter lever goes into the transmission that there is an electrically actuated pin or such? I know someome mentioned manually shifting the transmission. (pulling the cottrer pin and disconnecting the shift cable and m,aking sure you can move the shift lever at the trans back and forth (engine off of course).. just to rule out an issue there..
cadillackid, the bus is still parked in the backyard of a friend who rents his house out as an AirBnB. They’ve had a full paying customer lately and I haven’t been able to crawl under the bus to try the transmission linkage. I think that is the best logical next test. Like you said, the Felsted shifter movement doesn’t seem to be limited electrically.

Thank you for sticking with me while I await an open day. I appreciate it!
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