The tach stoped working on the mack yesterday. Its 84 rd686 with an e-6 in it. Where is the tack pick up at? I didnt look at all yet but ill probly have time tomorro or sat.

 

If it's anything like the old Cummins BigCam motors, it'll be in the bellhousing getting a read off of the flywheel teeth.

 

We don't get much of the old stuff in the shop anymore, but the tach is definately mechanical. I'm pretty sure the cable is driven off of the auxillary shaft on the passenger side of the engine. The aux. shaft is the one bellow the injection pump. It also drives the oil pump and the air compressor.

 

That is pretty close IIRC.Look for a cable housing similar to a speedometer cable.I have changed the broken cable from the top using the old housing,it is not a big job.Use graphite for lubrication.Ron G

 

It should be on the timing cover......look for a trail of oil, because they always leak.

 

I didntr have any time to look at it today but ill try tomorro. Following the oil wont help its an 84 everything seeps oil, but ill try.

 

Ok i did some looking this mourning and dont know if i found much. There is nothing with a wire on the timing cover, there is one wire conected to the back of the injection pump( its a 1inch bolt with 2 nuts on it with a ring terminal between them, seems like it set up to set the bolt at a certain depth). I figure thats the throtle kill for the jake. I looked at the trans a bit but couldnt see anything much on the outside, but looking up at the inspection hole it looks like something is hanging inside there but i didnt have a flash light. Im a little stumped so far.

 

your correct, the wire at the back of the pump is the kill switch for the Jake. The think dangling down inside the clutch housing is probably a grease hose going to the throwout bearing. Your tach isn't electronic, so no wires or sensors. It is mechanical, so it is cable driven, just like your speedometer. Like I said before, I'm not completely sure on where it gets its pickup from, but I think its somewhere near the front of the motor, at the auxillary shaft. Try looking where Logskidder mentioned, it could be at the timing cover, I'm not 100% sure.

 

Follow the tach cable right out the firewall (looks like a speedo cable) it should go over to the right side close to the injector pump, however I have seen some on the left side. Remember your not looking for a pickup your looking for a little gear box.....looks just like a speedo box.

 

our 88 had it on the end of the injector pump

 

Since you are going to have to remove it anyway,pull the panel in the cab that holds the tachometer by removing the phillips screws and lifting it up and away from the dashboard.You will see the mechanical cable housing where it enters the back of the tachometer and the threaded collar that screws onto the connector to hold the cable in place.Maybe the collar has backed off and the cable has separated from the tach and all you have to do is connect it again which I have seen happen.The collar has a lock on it to prevent it from turning and backing off by itself as I remember and you have to disable it to unscrew it,it is just a springloaded lever to engage the serrations on the collar to prevent it from turning but it has been a few years since I worked on one so my memory may not be spot on there.
Once you have the panel lifted plan on replacing the instrument light bulbs that don't work while you have it apart and if nothing else you will know what to look for on the other side of the firewall when tracing the path of it.Ron G

 

Ok i got a better idea of it now. If the monsoon's stop a bit ill head out to the site and pop the dash and trace the cable. Man that would be nice if it just came apart instead of breaking. I spent days rewiring the entire light system on it and new air lines all around, 20+ year old wiring is a pain in the ___ .

 

Found where the tach reads from today. It snapped off at the end of the cable. I grabbed the cable and actually picked it up and off of where it threads in. There is still part of the end fitting on the cable, so i dont know if there is any other part that will still thread out because of the grime on everything. Does anyone know if thats the usual way they break at that end?

 

The part that breaks is usually the triangular shaped end of the cable.Get a new cable,remove the old one and the piece that was broken off in the drive and thread the new cable into the old housing using graphite as a lubricant.You can spin the cable as necessary to get it to seat on the other end.You should be back in business.Ron G

 

When the end snaps off 2 things usually are the problem. The cable has worn through the cable housing or the tach head is getting bad. If the tach needle was jumping prior to breaking the cable has worn through the housing, if you heard noise in the tach head the tach is bad.