M920 electrical issue

[Karl kostman]

Hello Guys and yes I have gone through enough TMs to just about make my eyes bleed on this issue, I now ask for your ideas please! I have a 1980 M920 and when I had it delivered last Wed. it was very cold and the batteries were weak, so of course it would not start, in fact it could just bump the started solenoid and that was about it. When I got into the cab I did notice the speedometer was bouncing between 0 and 20 mph, it would go down to 0 sit there a few seconds then go up to 20 mph and drop back to 0 again? Well I got the truck into my heated shop and got the batteries charged back up to full power and this morning put it all back together and hooked up the 12 Volt wire and I could hear the speedo doing its same thing as before, well I wanted to see if the truck would start so I crawled in the cab and the truck fired up instantly, yet again the whole time the speedo was doing its little dance, it does it whether the engine is running or not. Truck ran great I let the engine temp come up to about 150 degrees, it was running 60 psi for oil pressure at 800 rpm and the air pressure came up to 110 psi. But the speedo is drawing current from the 12 volt side every time it does its thing and I cannot find anything in the TMs that address this in any way, needless to say I am very open to any ideas on what I am missing here, I also unhooked the 12 volt wire when I shut the truck down this morning because it will continue to draw battery power regardless of ignition switch position.
If its any consolation the truck seems perfect and everything else seems to be as it should be! Let me know your ideas on what I am missing in this picture folks?
Thanks much
Karl

 

[R Racing]

The Speedo is not electric . Its driven by a cable from the transmittion so I have no clue why it would move? Only thing that comes to mind is possible issue with the trans engage button. the only power to the spedo is for the tachograph and the lights in it. my guess is a bad tachograph.

 

[R Racing]

Did you try and unplug the electric cable from the tachograph? Also These trucks are known to have a issue with drawing 12V ( I added a 12v rotary switch to my system to kill the 12v when im not using it ) Good luck finding your issue. Also check your relays. I had a burnt 1 that was powering my jake brake when not applied.

 

[Bighurt]

My 12 volt draw is off an on. My 915 killed its batteries a few years back and then it was fine all of 2013, now it's doing it again. I haven't traced it yet but I have a feeling one of these day's I'll convert the whole truck to 12V.

Wish I was closer Karl, then I could be of use. I have a feeling Jim is correct, bad tachograph.

 

[Karl kostman]

Thanks guys do you know of any TMs that would show me how to troubleshoot the tachograph to see if it is in fact the problem? And if it IS the problem is this something that would be fixable or is the just a replacement item? As always any info is more!
than I have right now!
thanks again
KK

 

[R Racing]

Did you try to unplug it ? It's a replacement item. I had to replace mine because it was physically broken inside. And mine made terrible noise going down the road. I still had a battery issue after replacing mine with a new unit. The schematics are in -24 but not sure if it brakes down the tachograph or not?

 

[Artisan]

Are you sure you are not mistaking the RPM needle for the MPH needle?

Maybe "I" am missing something here.

 

[Monkeyboyarmy]

There is a newer replacement tachograph that is all electric. It uses a pulse signal generator on the trans and a wire runs to the tach. I don't know why they do it but I have seen them bounce like that and run the battery down....at least that is what I thought was running the battery down. I've put a couple of battery disconnects on the 915's to combat the problem.

 

[Artisan]

There is a newer replacement tachograph that is all electric. It uses a pulse signal generator on the trans and a wire runs to the tach. I don't know why they do it but I have seen them bounce like that and run the battery down....at least that is what I thought was running the battery down. I've put a couple of battery disconnects on the 915's to combat the problem.

If the speedO and tach are cable driven.............OR CONVERTED as MonkeyboyA just said.
I found THIS w/ a little info and if nothing else a link to buy a key and graphs.

A picture of YOUR TachOgraph "might" give a clue?



The parts TM for M916 says ;

1 711 007 021 SPEEDOMETER TACHOME

8-127-305-185 KEY TACHOGRAPH

Seven Day Disk Pack (7530-01-060-162)

 

[Tow4]

My M915A1 killed the 12 volt tapped batteries. I trouble shot it today.

I pulled the breaker panel and disconnected the leads that supplied 12 volts to breakers 1 and 5,6,and 7. With all the leads disconnected on the supplies and other feeds off, the parasitic amp draw on the battery 12 volt tap was ~50ma. With all leads and breakers hooked up without the tachograph the draw was ~50ma. With the tachograph included it was ~100ma. I haven't looked further, but the 50ma draw is probably from the alternator. That appears to be the only other thing in the circuit after disconnecting wires 10c and 10g at the breaker panel.

I switched the tachograph to CB2 which is switched by the ignition switch. I'm thinking about adding a relay to switch the 12 volt tap on the batteries since there is no parasitic amp draw on the 24 volt side.

The tachograph works correctly, I don't know why it is powered continuously, it doesn't seem necessary. For a vehicle run every couple of weeks a 100ma amp draw is not a problem, but if you have an extended storage period, it will drain the batteries.

 

[Artisan]

The constant draw is for the clock, yes?

 

[Tow4]

Shouldn't draw 50ma. If it does, it's more than I want to see.

 

[Karl kostman]

Ray thanks much for your input on this for right now I have the main power connector (black one) disconnected from the back of the gauge, that took care of the bouncing MPH needle, I have not driven the truck since I did that so I really dont know what is going to work and what is not working with it disconnected? Ray do you know of any ways to TEST these units to see if they are functioning correctly or if the unit itself is bad and needs to be replaced?
Thanks for your input on this subject Ray!
Karl

 

[Tow4]

I didn't find anything useful searching for information. If you google TCO-15-7 tachograph, you will come up with several auction links. Good luck, if you find a tm please post it.