94 freightliner FLB tach

Replace the gauge if not also the cable that feeds it from the engine speed connector.

You cannot drive a truck properly with out that gauge. Yes Ive heard all about by ear and all that (Been there done that...) but to be at your best you need a working gauge that is accurate.

 

Yes I am familiar with the dip switches on computers but not in big trucks. A minor detail that has escaped me until now.

Thank you for your reminder. That opens up a can o... a barrel of worms now don't it? Might as well be western diamondbacks.

 

Tach is probably bad. 25 year old truck, not a surpise.

 

I've got a 96 FLD with a cummins and it's got it on the passenger side of the flywheel housings. If they're in similar locations you might be able to see it with your hood down, I wouldn't be surprised if its backed out a bit, mines got a jam nut that you have to get super tight or itll just back right out. My tach is bad as well unfortunately for me, the cummins ecm will go into limp mode if the speed sensor is bad so I know it's my tach. Spent a month of trying to figure it out before i came to that conclusion, they go for a pretty penny online and an arm and a leg at my dealer. Talk to your dealer though, see if you can get a build sheet, my build sheet has dip switch positions from factory, it's a decent reference especially if you don't know how it's been played with.

 

I'm unfamiliar with detroits but I would recommend you try measuring the AC or DC voltage present at the plug, it should steadily increase with engine RPM. Because you stated it was functional previously that most likely disqualifies the alternator tach drive idea. That most likely means you have a sensor in the flywheel housing, just a little background research shows that it's inductive and will be highly sensitive to the gap between the flywheel and the pickup. I'm wondering if the 30 pin connector at the ECM is contaminated or not seated well, if you are seeing a voltage that increases with RPM your tach is most likely caput. If it's jumpy and all over the place I would measure the resistance of the wire while someone jiggles the wire to make sure it isn't internally broken.