Tacho Problems

[Dave The Hat]

Hi Guys,

Im stumped on this one.

Mercedes 1617, Om366 engine, fitted with a Kenzie analogue tachograph. Took her out for a spin after alot of work we've done on her, and the tacho would not read anything.

It is a Kenzie unit. Has a 6 pin plug connection that goes into a port on the back. One of the plugs is a blank. 2 are earths, one is permanently live (keeps clock going when ignition is turned off I presume), one is live when headlights are on (lights up the unit), and the other is live when ignition comes on.

Both earths are good, and the other 3 are kicking out the correct 24v current. I had the tester on them.

If you disconnect then connect this plug, the clock starts working fine, the lights come on when headlights are turned on, and the red warning light comes on when you open the unit, or if you close it without a chart in there. So Im sure this plug is fine.

Then there is a 4 pin plug, the wiring of which runs to the sender on the gearbox.

1 of these is blank. One is an earth (good), and the other 2 are live.

This is the interesting part. With ignition on the 2 live wires of the 4 pin plug only kick out a 12v current.

However, the Kenzie unit states 24v, and the trucks elec system is 24v. So does the unit have an internal dropper on it? I've not heard of that before. I only thought they came as 12v or 24v?

So I presume I've got a voltage drop problem somewhere.

But if the 6 pin plug is all reading 24v, the current surely passes through the unit to the 4 pin plug, so is the voltage being lost within the unit itself?

Okay, waffling on.

I swapped the sender on the gearbox. That didn't work. I then swapped the Kenzie tacho unit for one from another merc truck (which I know is definitely working), and still nothing!

If I take the plug off the gearbox sender, I get the same 12v reading at this end of the wire as at the 4 pin plug end which is on the back of the tacho unit, so there is no probs with this wiring loom.

Totally stupmed. Any help much appreciated!!