question 7745 - Crazy Electrical Issues With New Lamps

Hi Everyone, I am going crazy trying to figure out an issue with new rear turn signal//brake light/hazard lamps on my late 80's Zetor 7745T. I replaced the front turn signal/headlamp/marker lamps as well and replaced all the bulbs and the multi-function signal/horn/beam flash stalk on the dash. The fronts were easy as they are two wires each and coloured to match the existing wires. They each have a single element bulb for turn signal and a single element bulb for the white "headlamps" and everything works correctly. The rears are a bit different as they each contain a one element turn signal and two element brake/marker bulb. There are three wires (a forth wire for license plate on one side and a forth wire for a work lamp on the other side as well) and my schematic indicates green/yellow to the signal and the brown and blue wires on the left side and two black on the right side to the brake light (the wires coming from the new lamp are all black but it is easy to determine the single element wire from the other two and my meter indicated the correct wire for the brake part of the two element bulb so I am pretty sure it is correct). I had some initial issues with dim bulbs and some random craziness but after removing all the fixtures and grinding the metal beneath clean and wiring a dedicated ground to the mounting posts and to the body I am able to get almost everything working with the key inserted but not turned (position "0"). The rear turn signals work correctly but both the turn signal and brake bulb on each side flash (the brake bulb is very weak and only one of the two elements is on) when I put the hazard lights on (this could be normal but I am not sure).

The problem I have is that as soon as I turn the key to the first or second position (position "1" or "2") to activate cab lights and other functions the turn signals stop working and the relay under the dash goes silent (as well as the dash indicator lamp no longer flashing). When I turn the key back to position "0" everything works again as described above. I have tried changing the wiring around on the rear lamps but either the brake light flashes when the signal is turned on or nothing lights up. The only thing I haven't done is run a separate ground wire right from the battery which I will try (I will update if anything changes) but I would expect dim lights or no lights rather than this issue. Could the relay be shorting out but only when the key is turned to on? I don't know how to check that. If it is defective I would think it would just not work. The previous owner definitely did some changes under the dash though.

I am at a loss and am hoping someone can help me. Please and thank you in advance for any help or suggestions.

Hello Richard

First of all - there are various options/circuits for connecting the hazard warning system and also various switches and flasher relays.
It would be helpful if you could show with photos which hazard warning switch you have installed, which hazard warning relay and one of the inner workings and the wiring of the rear lights.

I'm attaching my circuit diagram - this is how I wired my Zetor 5011, it's practically the same for a 7745.
I used the following flasher/hazard warning relay:

The advantage of this compared to the old barrel-shaped design is that it is much easier to connect and you also have separate connections for up to 3 control lamps.

I assume that you know the assignment of the two plugs on the combination lever?


As hazard warning switches, I usually use the 6- or 7-pin hazard warning switches with integrated hazard warning control lamp like the one below. These are available from Hella or as a replica for a little less money.

Connection assignment of a 6 or 7-pin hazard warning switch, valid for original Hella and identical switches.
Connection assignment for 1-circuit flashing/hazard warning switch:
15 = ignition plus (from fuse 54)
30b = 30 switched by the hazard warning switch (not applicable for 6-pin switch! can remain free)
30 = battery plus (from fuse 30)
49 = flashing/hazard warning relay on 49
49a = flashing switch and relay on 49a
L = flashing switch L (left flashing)
R = flashing switch R (right flashing)



I connected the whole thing to my Zetor as follows:


Attention - the colors in my plan do not have to match those on your Zetor because I originally only drew this plan for my tractor. I had redone all of the electrics and in this context drew my own new circuit diagram based on the circuit diagram of the Zetor 5211 to 7745.
I am attaching my circuit diagram here as a ZIP file.
Note - this will (I think) allow you to easily follow how everything is connected and adapt it to your Zetor if necessary.


In general, the indicator system must work in all ignition switch positions, i.e. in 0, 1 and 2:
0 = key inserted, "ignition" on
1 = key inserted, low beam in the bonnet or in the front top of the roof on
2 = key inserted, high beam in the bonnet on


If you still have the old aluminum flasher relay installed, you will probably have a different hazard warning switch and completely different wiring.
It would therefore be interesting to see which components you used and, where possible, how you have connected them up to now.

If more lights are lit or are too dim than they should be, it is often simply a ground problem. In other words, a short circuit to ground or too bad a ground.

You could also check this again.

I would be looking at the ignition switch or check the wires are in the correct place on the rear of the switch.

Hi Joachim and Ixer, thank you for your replies and the info. I really appreciate the help and will send additional details and try the suggestions. Joachim, I have not replaced the relay and have not connected anything under the dash except the new switch with was. direct replacement with two multi-wire plugs that connected straight into the wire harness plugs so I assumed it was wired correctly (possibly unwisely). Other than a new ground wire to the body I have also not modified or replaced any original wiring but am not sure if the previous owner did. It is probably worth replacing the parts and rewiring as you suggest. Thanks again and I will update as I progress.

rlaidmanr said:

but am not sure if the previous owner did.

Click to expand...

That is often the point. You can often not realy trust the former owner or owners, so it's better to check and better as a fire unter the desc.