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!!

Sorry, forgot to mention, not only wont it print onto the charts, the speedo needle stays on zero alll the time, and kms wont go up on the dial at the bottom. This is the case with the unit I have, and also when i swapped it for a unit I know definitely works.

I have a vague recollection that the earlier Kienzles needed keys to select driver mode. Maybe an issue with the locking of the face and/or selecting the driver position.

Hi Roger,

This unit has a 2 small black nubbings at the top (like turney buttons, but I'll call them nubings!). One for each driver, which you can select 4 options (work, other work, rest, sleep, i think!)...

I'm sure the unit is locking itself properly, as when I close the unit and turn the key, the red warning light goes out. Also, the red light comes on if I have locked the unit without a chart in there and select the work option on the nubbings.

So im pretty sure it's locking itself closed properly. You're absolutely right, if it is not closed properly the speedo needle wont go up or the KM dial wont move. But as the red light goes out on my unit, Im sure it's locked properly.

Next step sounds like troubleshooting the wiring loom from the sender to the tacho head. I am surprised that there is a reduction in voltage - but anything is possible.

Cant Lucas/Kienzle help with wiring diagrams? I'll have a look on my Merc parts DVD and see if they have anything in the digrams that might help - can you post your VIN?

sorry to upset you, But had same problem on my DAF 95 XF. you need to remove the speed sensor and clean it as it will have metal fillings on the head. (I bet you can go at speeds in excess of 60mph).. You must then take it to a Tacho center to be recalibrated and resealed by law. If not you will be issued an instant prohibition by VOSA and an on the spot fine of many.

Oneworld Biker

RTW on a goldwing and prayer.

P.S You also need to clean the contact plate in the gearbox by removing the square-ish plate.

Roger, I will get the VIN tomorrow and pm it to you. Thanks for your help.

Oneworldbiker. When you say the speed sensor, do you mean the unit which bolts into the gearobx? I call that the sender, so just want to check we're talking about the same thing. Will follow advice on cleaning contact in gearbox. I will get the unit calibrated and sealed at a UK place for sure.

Thanks for all your help much appreciated.

Yes, we're talking about the same thing. The contact in the gearbox gets contaminated when using mineral oil. Also check the top of the gearbox housing for small holes from alloy corosion, as this allows water into the gearbox.. sorry about taking so long to get back to you as training new driver how to drive in the forests and to load timber.

No worries Oneworldbiker, sure appreciate the information. I will do this next week and see if it helps. Would like to do it now but currently busy laying ply wood flooring in the body.

Thanks again, Ill let you know if it works. Good luck driving in the forests!

Roger, sory, i clean forgot to get the VIN. I'll get it tomorrow

Completely forgot this thread.

Finally managed to get the tacho working again. I'm not sure which of the following I did that solved the problem but she is operational again:

1. Found that the electrical connections in the plastic screw on cap that go onto the sender were pushed too far into the plastic casing. So when I screwed the cap onto the sender, one of the metal connections wasnt mating properly. Pulled the connecter in question forward a little and crimped it tighter within the casing.

2. Roughed up the end of the sender that screws into the gearbox with sandpaper

3. Found a wire that had been rubbing against a chassis cross member....possibly this was contributing to the 24v vs 12v power problem i talked about earlier.......one of the three wires in the odometer cable was down to about 3 strands of wire so I did a chop and chock job.

One of those 3 problems solved it. Happy days!