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Daniel Wilson
Daniel Wilson,
Category: Medium and Heavy Trucks
Satisfied Customers: 5173
Experience:  Owner at DJ Wilson
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Morning, Im looking for a Mack Tech. We are working on a 2008

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Morning, I'm looking for a Mack Tech. We are working on a 2008 Mack truck with the MP8 engine. The nut backed off the idler gear, which we have replaced. wWe are 100% confident that the gear timing is correct. The engine turns over but will not start. As a test we gave it some starting fluid and it runs fine on that. Its like the ECM is not getting power. There is oil pressure showing. The tach needle is unresponsive, we changed both the cam and flywheel sensors. The only other thing we changed was the starter and it doesnt turn over very long before the thermal over load kicks out(3-4 Sec). Where does the ECM get its power from? Does the starter have anything to do with ignition? We have checked all wiring and have left nothing unhooked. Electrically we only disconnected the harness at the rear of the engine and the starter? Any thoughts would be appreciated. Thanks. Doug.
Hi my name isXXXXX will try to help. Check for an inline ECM fuse between the battery box and ECM and test all your fuses in the fuse panel. Also be sure your fuel system is primed. You are on the correct path. It sounds like your ECM is either not getting power or has failed or you are not getting speed sensor signal from either a harness or sensor problem. Did this problem start immediately after changing the starter? Let me know. Is the check engine light coming on and cycling with the key on as it should? If not the ECM is not communicating. If it is then are there any fault codes?
Customer: replied 3 years ago.
Hey Dan. Check engine light cycles the way it should. Tore the engine apart to change idler gear and installed a new starter instead of the old one. All fuses were checked. I think the key was in the on position when we hooked up the batteries?? Is there anyway to diagnose the ECM?
The easiest way to diagnose an ECM is to test for power to it and verify your ground. If you have power to the ECM which unfortunately I do not have the ECM pinouts for this engine but you should have a constant power at one of the pins and a switchable power key on. If you can remove the ECM connectors and test the pins to find and verify that then the next step would be to find a test ECM to try on your truck if you can.
Customer: replied 3 years ago.
Dan. Two of the pins are grounded, 5 have key on power. No constant power?? We won't have pin locations untill tomorrow. Are you positive about a constant power pin? Thanks again you have been helpful.

There should be a BAT power pin to the ECM as I know Cummins, Cat, and Mack have ECM power from BAT and an inline fuse between the battery box and ECM. Unless of course there are changes to the MP8 which is possible but I doubt it. I would verify this when you have your diagram but I am pretty sure you should have a constant power unless this ECM has an internal battery. Since your check engine light is cycling with the key on it seems your ECM is communicating as it should. I would see if you can locate a test ECM before investing the money for a new one though. Do you have access to another truck with the same engine?

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