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Trouble….24V fed into 12V system by mistake
Hello all,
Looking for some advice. I am looking at a bus for a special purpose ( mobile coffee shop) and I found a 1988 hospitality coach converted by American Coach. The bus is in Washington DC area. It runs and drives fine now but….
A couple of months ago the owner had the chassis batteries changed and the mechanic wired them wrong feeding 24V into the 12v system. He claims that the Allison computer got fried and they changed it. No engine computer so no trouble there…he says.
Since this incident, the Yanmar generator won’t keep running if you take the finger off the start button. Other than that he claims everything in the coach is working.
I want to go over and take a look at it early next week but I’d hate to waste all this time if chances are there are other things that might have been affected and he does not know. I have a 800 mile trip home and the last thing I need is to be on the side of the road with problems.
If anybody has made this mistake in the past ( or has first hand knowledge ) on an older vintage bus please let me know and advise what else could have been affected by this mistake and what to look for from an electrical standpoint.
Thank you. Please chime in.
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6 Replies
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If it has an Allison computer, it has an engine computer. The generator is likely on the house side so the chassis battery issue might be unrelated.
You don’t list the chassis model year, but on early model years there was very little that was not 24V and only the engine and transmission ECM’s and likely the dash radio plus a few other minor things were 12V. Later years lighting became 12V.
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1988 prevost chassis, 8v92ta, American Coach conversion
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That definitely has an engine ECM, likely DDECII unless the chassis model year is 1987. Lights were 24V, headlights 12V, but wired to use 24V. Both ECMs were 12V.
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Thank you Jon, as always you are at the forefront of offering help and insight. Appreciate it.
I confirmed that the VIN shows it as a 1998 chassis and after reviewing the receipt, the DDEC computer had to be programmed and replaced not the Allison computer….that one survived I guess…..
The shop did not report any other issues after the replacement of the ECM.
As the bus is started, all warning lights come on as they should, once air builds up the Low air lights go off and no other warning lights remain on.
Still trying to sort out why the generator only runs as long as the start button is kept pressed and if that has anything to do with the temporary higher voltage in the chassis – but I agree it should not….everything else works as it should on the house side.
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The run solenoid for the generator has 2 windings in it. 1 for start and 1 for run. The run 1 is controlled thru an oil pressure switch and probably a coolant temp switch. It is either one of these or the solenoid itself has failed. The above is the issue 99% of the time
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Thank you.
I’ll have him look at that solenoid and check the sensors.
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