Tachometer Quit Working

[Glennman]

Hey all, I hope everyone is having a fantastic winter.

I just finished putting my transmission pan back on after having the corner welded where it had a pinhole leak. I'm getting ready to install my new block heater and t-stats that I ordered from US Coach, but I wanted to run the bus for a while for its mid-winter fire up first. It fired up and ran real well. However, the tachometer wouldn't work at all. The last couple of times I drove the bus, the tach would jump around a bit, but it would at least work.

Where in the world (or on the engine) does the measuring of RPM's take place? Is there some kind of device there that I should look for? I'll probably need to pull the tach at the dash, as I suppose it could be a ground problem???

Thanks everyone for all your help. Glennman

[Iceni John]

Guess what?!   My tachometer, a Teleflex from 1990, started showing signs a few months ago of its demise, and the last time I drove the bus it either didn't work at all or it jumped all over the place.   Apparently when tachometers don't return to zero but instead their needle stops on a few hundred RPM, that indicates they're going to go tits up fairly soon;  that's exactly what mine was also doing.

Even though I have a signal generator on the engine which I thought drove the tachometer, it actually is the alternator that drives it.   I'm now not sure what the signal generator is for  -  maybe it's for the DDEC?   I bought a VDO 333-952 Cockpit International 3000RPM tachometer, but I'll have to completely redo all the tach wiring in my gauge panel to connect it.  Then I'll have to adjust it, but at least my ProLink scanner tells me the exact RPM, plus I also have a handheld non-contact tachometer to read the crankshaft pulley.

At this rate I'll soon have replaced all my original gauges!   My new tach is the seventh new gauge  -  only four original Teleflex gauges are still left.

John     

[DoubleEagle]

On my Model 05 Eagle, the sensor is driven by a mechanical tachometer drive on the cam cover at the front of the engine (rear of the bus). I renewed the sensor when I installed an IssPro Tachometer back in 1996. There is a small cap that screws on the cam drive area about 1" in diameter (if a sensor is not mounted there now), and the correct size drive pin is needed to drive the sensor. I had gotten mine at International Bus Parts in Apopka, Florida. Some tachometers are driven off the alternator, or the blower drive on the rear of the engine. The head unit will have to match the drive source.

[Lin]

Is the tach original equipment or aftermarket?  I original, it should be covered in the manual.  If aftermarket, it could be either mechanical or off the alternator.

[Glennman]

The tach on the dash is aftermarket. Do you have a picture of the small cap at the cam area you speak of? Or, is there a wire coming out of the unit that sends the signal to the front? I'll take a look at the alternator area too. I'm not ruling out simply replacing the whole thing. I want to change the dash guages around anyway, and install a new dash altogether. This might be the trigger that justifies it!

Glennman