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Old 03-28-2011, 12:56 PM   #1
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tachometer not working '96 peterbilt

We just bought a '96 Peterbilt 378 with a 435hp 3406e and 1.34 million miles. The valve cover says 435hp but we were told that the ecm says 500hp and verifies that many miles, but it doesn't run like 500hp. It seems that our '03 C15's with 425hp can outrun it by just a little bit. We have noticed that the tachometer doesn't work at all and it looks like they just replaced it with a brand new one. I pulled the dash panel out (which i love how easy that is to do on a pete) and wiggled the plug and even unplugged it and plugged it back in all while it was running and the gauge didn't move at all. Any ideas what could be looked at next? The pyro also doesn't work. It looks like there are two wires for that going to the sensor in the exhaust. Same thing, i wiggled the wires and nothing.

Other than that, with that many miles, it seems to run ok. The highest I could get the boost while pulling a load was about 22-23 psi if the boost gauge is reading correct. I'm assuming they have done an inframe with it at some point because it doesn't smoke at all and the head has no paint at all on it and the oil pan is squeaky clean.

Any info as to what to look for and maybe any thing else that is problematic on this year and model of Pete would be greatly appreciated. Thanks everyone!
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Old 03-28-2011, 03:23 PM   #2
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I'd pull the pyrometer sending unit, light a match underneath it and have 'stupid' watch the gauge to report what she sees happening. I'm not a mechanic but this approach seems logical to me.

Tachometer: I'd trouble shoot the sensor coming out the top of transmission and the wires running to it.

Last edited by Rogerstar1; 03-28-2011 at 03:27 PM.
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Old 03-29-2011, 01:06 AM   #3
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tRY THIS http://www.ametekdixson.com/technical/index.cfm
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Old 03-29-2011, 03:15 AM   #4
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Alright, thanks for the link! It gives me somewhere to start anyways. Anyone one else chime in on anything? Thanks!!
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Old 03-29-2011, 03:26 AM   #5
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the above link should cover the tach , the sensor is on the flywheel housing.
the pyrometer can be checked by disconecting the wire s and momentarily connecting a fairly dead 1.5 v battery, the needle should jump , don't contact long enough to peg the needle. if that works check for continuity to the sendor at the turbo if you have wire continuity the sendor is fubar, some where in one of the posts Mike from ISPRO posted the sendor output .
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