M1088a1 no oil press, no 12v, no water temp, no fuel.

[Rikjames92]

I'm tasked with finding why these 4 gauges aren't working. It's been a nightmare. I have the schematics. I've found continuity from the pdm2 to the MMDC. From the MMDC to the gauge plugs. And no continuity from any power to grounds. I've swapped the gauges individually, I've swapped the MMDC, I've swapped the PDM2 and PDM4 and still no change. The annunciator lights are working but don't show "low oil pressure" or "no charge" I have continuity from annunciator wiring to the other ends. I'm at a loss. If you have any idea what I should do next please tell me. I've verified voltages at the gauges and the MMDC also. Please help.

 

[Ronmar]

I'm tasked with finding why these 4 gauges aren't working. It's been a nightmare. I have the schematics. I've found continuity from the pdm2 to the MMDC. From the MMDC to the gauge plugs. And no continuity from any power to grounds. I've swapped the gauges individually, I've swapped the MMDC, I've swapped the PDM2 and PDM4 and still no change. The annunciator lights are working but don't show "low oil pressure" or "no charge" I have continuity from annunciator wiring to the other ends. I'm at a loss. If you have any idea what I should do next please tell me. I've verified voltages at the gauges and the MMDC also. Please help.

like any other data system, garbage in = garbage out…

The fuel gauge is driven by it’s own sender connected to the MMDC. It should be an isspro 33-240 ohm sender. Disconnect it at the tank and measure it with an ohm meter. You should see a resistance in that range equating to a relative fuel level. iE 1/2 tank should = ~135 ohms, then reconnect it and repeate the test at the MMDC connector PX7-19 to cab ground. You should see a similar resistance.

I assume 12 V is sensed either from the 12v battery or 12v ignition lines feeding the MMDC. Are both those present? PX 38 pin E & F Respectively. These come from PDM2 cb 38 and 76 respectively. Pin D on that connector is also your main tie to cab/chassis ground I would confirm it does indeed have a good circuit to cab ground, and it would be the best place to test the fuel level sensor loop resistance. PX7-19 - PX38-D…

Oil Pressure, Temperature, RPM, Speed, Voltage(24V), Check trans, Trans temp, and Trailer ABS all arrive at the MMDC via the J 1587 data bus from the ECU, TCU and ABS units. Now it could be that something on the bus is causing a conflict and suppressing the oil pressure and temp data, but I would think that less likley than you having an issue with the oil and temp senders feeding the ECU. It could also perhaps be an ECU issue…

Do you know someone with a scanner to see if you can read that data off the ECU directly from 1587 bus?

 

[Rikjames92]

like any other data system, garbage in = garbage out…

The fuel gauge is driven by it’s own sender connected to the MMDC. It should be an isspro 33-240 ohm sender. Disconnect it at the tank and measure it with an ohm meter. You should see a resistance in that range equating to a relative fuel level. iE 1/2 tank should = ~135 ohms, then reconnect it and repeate the test at the MMDC connector PX7-19 to cab ground. You should see a similar resistance.

I assume 12 V is sensed either from the 12v battery or 12v ignition lines feeding the MMDC. Are both those present? PX 38 pin E & F Respectively. These come from PDM2 cb 38 and 76 respectively. Pin D on that connector is also your main tie to cab/chassis ground I would confirm it does indeed have a good circuit to cab ground, and it would be the best place to test the fuel level sensor loop resistance. PX7-19 - PX38-D…

Oil Pressure, Temperature, RPM, Speed, Voltage(24V), Check trans, Trans temp, and Trailer ABS all arrive at the MMDC via the J 1587 data bus from the ECU, TCU and ABS units. Now it could be that something on the bus is causing a conflict and suppressing the oil pressure and temp data, but I would think that less likley than you having an issue with the oil and temp senders feeding the ECU. It could also perhaps be an ECU issue…

Do you know someone with a scanner to see if you can read that data off the ECU directly from 1587 bus?

Thank you for the direction. Yes I have a snap on scanner and cat-et. I'm sure one of them will work they have loads of adapters. It runs really good other than these 4 gauges not working.

 

[Rikjames92]

Update*** I had multiple problems. The MMDC and gauges must have been fried. After swapping both they started working. I think it had a voltage spike on the 12v circuit. After I get it put together I'll check to see if anything else is fried. Anything off the top of your head that's 12v I should check?

 

[Ronmar]

No, if you have 12v batt and ign supply to the connection points I mentioned above That should be it got 12v. So you wound up swapping the MMDC and gauges twice? Ouch!

Not a fan of glass cockpits and integrated gauges, I like my failures in small bite-sized pieces

 

[GIJake777]

Update*** I had multiple problems. The MMDC and gauges must have been fried. After swapping both they started working. I think it had a voltage spike on the 12v circuit. After I get it put together I'll check to see if anything else is fried. Anything off the top of your head that's 12v I should check?

You definitely want to check your voltage regulator! We had the same issue on several of our trucks, and swapping for a good regulator fixed the issue.

 

[Third From Texas]

You definitely want to check your voltage regulator! We had the same issue on several of our trucks, and swapping for a good regulator fixed the issue.

Oddly enough, my A1R has this issue when the CTIS controller is plugged in. I suspect the ground on the CTIS controller in my case. All was fine until I removed the controller to paint the heater panel. Unplugged, all functions normally. Plug in the CTIS and a mile or two down the road, the same gauges all die. Unplug and they all instantly work again.