Fixing 91 U300 Allison MTB 648 Do Not Shift Light

October 01, 2023, 03:50:42 pm

Texas summer has been brutal for the last few months. My 91 GV has been in storage for those months. Recently the weather finally cools down a little bit in the morning. I decide to check the coach out.

6V92 engine starts right up, battery stay charged. Everything is good until I see the "Do Not Shift " Light blinking and of course, I can't put the transmission in gears. This happened before about two years ago. , so I am not too worried about it.

I know it is about the voltage to the TCU (Transmission Control Unit). Last time I called Mr. Large to fixed it. But at $400 plus for each field visit, it is pretty steep to justify for a 30 year old RV. So this time I decide to find the root cause of the issue.

If you want to skip the diagnostic process, here is the solution worked for me:

the 10AMP fuse to the ATEC computer is corroded. It can still maintains connectivity, but it will drop the voltage to 9.2 volt. After clean the contact area, the "Do Not Shift" light went away. Hope this helps some body

Now the long diagnostic process.

First thing I want to rule out is the ground issue. Last time Mr. Large cleaned the battery terminal and the TCU start working again. So I start by cleaning all the battery terminal and they all looks pretty clean.

Then check the ground lug on the side wall of the battery apartment

There are quite some corrosion. So clean up using CRC QD connector and sand paper. All looks shiny. Then applied dielectric grease to prevent from future corrosion. All looks good. Also cleaned few other connectors in the panel area.

I was having high hope to fix the transmission issue. But when I turn the key, the "DNS" (Do Not Shift) light is still flashing. very annoying

Then I went to the front firewall, the TCU is mounted right behind the front grill. There is a wire suppose to connect to the bus chassis. Take that apart, and it is corroded. So take it apart, used sand paper to clean up the connector and applied dielectric grease to prevent further corrosion. Contact looks good. Now I have high hope again. But the DNS light is still flashing.

Little bit disappointed, but I decide to get to the bottom of this issue. If it is voltage issue, it has to be caused by some corrosion. On the TCU, there are two connector. Facing the TCU, I took the connector on the left side. There are two red wire labeled 202A and 223 A. They are the left two pin in the middle row of the connector. turn the power on, measure the voltage and no surprise they are only 9.2 volt. According to allision document, if the voltage is below 10Volt, the DNS will flashing and it will not generate and code since the TCU does not have enough voltage to work.

checking the diagram

202 and 223 are connected together to 77 Red to relay #4

Take off relay and measure the voltage at the terminal, they all show at 9.2 volt so the issue is the voltage supplied to the relay is not high enough.

Checking again the diagram, I notice the only wire that supply the voltage are the thin wire goes to the ATEC fuse. Took the fuse out, it does has a little bit corrosion. Tried a new fuse, but it is fitting well in position. So used the sand paper to clean the old fuse connector and applied some dielectric grease. Installed it back, this time, the voltage is measured at 12.44V. I know this is the cause of it. Feels great.

Put everything back in place, start the engine and just as expected, the Do Not Shift light is gone. I can shift into gears. some tears almost come out.

Well, not that dramatic, but it was pretty exciting. The whole process was little stressful. Some foul language was used along the way.

Looking back to the whole process, it really echo to what @stump said back when I just got the coach: "Relax, don't panic". Most issues for old coaches are caused by corrosion. As long as you find the right place, clean the terminal should fixed it.

Thanks for reading my story till this far. I know less and less 91 GV on the forum now. I did search in the forum, most "Do Not Shift" light flashing posts do not include a solution to it. I hope the mistake I made could help somebody in the future.

Re: Fixing 91 U300 Allison MTB 648 Do Not Shift Light

Reply #1 – October 01, 2023, 04:04:24 pm

While cleaning the contacts on the side wall in the battery appartment, I break the fuse to the water pump. Can any one tell me what part no. is that? Where can I buy a replacement? Thanks

Re: Fixing 91 U300 Allison MTB 648 Do Not Shift Light

Reply #2 – October 01, 2023, 04:48:28 pm

I have never had to replace any but if you google "Circuit Breaker 15A AC 12 V DC Automatic Reset Chassis Mount" you see various suppliers.
Here one on Amazon but it has a plastic housing: Amazon.com: Fastronix 15A 12V Type 1 Automatic Reset Circuit Breaker with...
I think the plastic housing would be fine.

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