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Tiara Open Cats 3126B RPM & OIL PSI Inop
Hey All,

I have the 3126B engines and I recently changed the oil and I think I broke a wire to the engine monitoring system as it is not reading the RPM's or oil pressure.  I tried looking for obvious lose wires with no luck.  Any advised would be greatly appreciated.


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What type of engine monitors do you have? EMS (tach, quad gauge and alarm annunciator)? MPD (single screen) or something else?

Cat electronically controlled engines typically use either the cat data link or J1939 can bus to communicate to the engine monitor. They are both two wire networks that run up to the dash via the customer connector/wire harness. A broken wire on the op sensor or speed sensor will trip an alarm so I would start by checking and cleaning the customer connector and pins. It is a deutsch 40 pin connector usually in front of the turbo. From there check the connectors on the back of the engine monitors behind the dash. The EMS system has separate connectors that run from the annunciator (which is the brain of the system) to the quad gauge and tach. MPD is a single 24 pin connector.

Your other option is to swap the harness connectors going into the monitors. If the problem stays with the monitor then you have narrowed it down.

This just popped on boatdiesel. Sounds similar to what you describe

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Thanks for the advice. I swapped the wires to the monitors and they are okay, which means its in the wiring. I wish I had a schematic of the wiring harness to identify each wire.
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What wires are you looking for? Assuming you have an MPD display (single screen 9" monitor) engine data travels over J1939 which is a 2 wire + shield canbus network. These connections should be on pins 16,17 and 18 of your customer connector.

If you you switched wiring over and the problem followed the wiring, and you are getting some but not all data on the display (only some instruments) then it is possible you have a problem with the ECM. Did your display show a "J1939 signal loss" error before loosing engine speed and OP?
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    My display dimensions are about 2"x7".  I'm not getting an error code it's only showing "****" and states its not receiving data or something to that effect.
The display was working prior to changing the oil and I think I just broke a wire somewhere but I can't locate it.
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Ok I believe that is a Cat messenger. It uses a j 1939 interface to get engine data from the ECM. The attached link has the customer connector wiring diagram in appendix. It lists the C7 but should be the same for 3126B
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Possible you pulled a wire in the harness or have a bad connection in one of the connectors. The ECM has two plugs/harnesses. One goes to the sensors and injectors and one leads up to the customer connector and the cat diagnostic port. Check the harness that runs up to the customer connector and the harness that runs up to dash from customer connector. They come apart simply with an Allen wrench.

Also possible the j1939 circuitry went bad in the ECM. Apparently this happens from time to time. Hope it's as simple as a wiring problem.
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Cats 3126B RPM & OIL PSI Inop00