Ural diesel tachometer/ rev counter

[Mark Preston]

Anyone got any experience of how there are wired? Mine's a 1980 model and doesn't agree with the circuit diagrams that I have. The diagrams show an electric tacho and speedo sender system. Mine has a standard cable drive for the speedo and an electric tacho that has a connection to the alternator (suspect that this is for revs pickup) but there is also a mysterious (but definitely factory fit as it's all in the same loom etc.) box with quite a bit of electronics inside. The box connects to the tacho also. This box also connects to a large relay on the firewall of the truck that at some point in the past had pretty heavy gauge wire connected.

Anybody any ideas what's it all about? The only guess I have regarding tacho connecting to control box connecting to big relay would be that the cold start glow plug heater current is automatically cut when the engine speed is above a certain value to prevent some sort of problem, However that is a pure guess.

A correct wiring diagram would be a real life saver.

Any words of wisdom would be gratefully recieved.

Thanks

Mark

[Mark Preston]

No one feeling wise?

I may have worked something out... The box connects to the tacho signal, ignition 'on', ignition 'start' and the starter relay. The only point in that would be to either automatically cut the starter when the engine starts (if someone is daft enough not to release the key), or to prevent someone engaging the starter with the engine already running (if they are daft enough to try that)

naturally I haven't tried either so don't know if my supposition is correct - but I guess it may have been done to stop soldiers trying to break things on purpose?

Any other vehicle got anything similar?

Mark

[paulob1]

hmm my 375d's would be broadly similar in design but obviously with the earlier petrol engine. I don't think i have a cut out if you engage the starter it still turns if the engine is running.

Does the box provide some connection to the fuel pump or is it just to the alternator. Likely if it was a radio box on the back it may have had some function connected with that...ie a cut out relay for dual batteries etc, so it could charge the cab batteries when the truck was moving but not have to run any external generators...


It could be as you say but it would be unusual and I have never heard of one being fitted.

The alternator would be a good place to get the signal for the revs I guess if a bit unreliable subject to belt tension......no idea on that one..

[Mark Preston]

No, nothing to do with the fuel pump... doesn't go anywhere near it.

Connections are:

ignition 'ON'
ignition 'START'
alternator 'speed sense' (goes to tacho too)
start relay coil
earth

So with the two ignition signals being 'inputs' as is the alternator speed sense, then I can't see that the function of the box can be anything other than controlling the starter relay for some reason.

HOWEVER I have found some indication of it in the German version of the 4320 manual (from the download section of this site)

It's not exactly like mine as there is no alternator connection, just a connection to the tacho, and that has a sender of it's own - however it's broadly similar. Trouble is that I don't read German so if there is mention of it in the text that it's going to mean nothing to me!

It's on page 119 and it's the item 39 center bottom.

I'll try and post a copy on here if I can re size it small enough

Cheers

Mark

[paulob1]

Mark isnt there a full manual for the 4320 in the downloads section, have you had a look at that..does the relay not just isolate the alternator during start up or vise versa..

[Mark Preston]

Yeah there is a full manual - but like all these things, it's not that simple. The English manual doesn't tie up with what I physically have. It's not just that the wiring has been modified ('molested' - which it has) but the loom and components are subtely different. I guess, over time, modifications were made to the specification.

The German manual does show this 'mystery box' but I don't speak German.

There is a relay (voltage regulator relay it is called in the English manual) that cuts the alternator excitation voltage but this only operates when the glow heater button is presses along with 'start', this is done presumably so that the alternator is not loading the starter when starting at cold temperatures?

[Mark Preston]

Think I am getting somewhere with this now - wired the box up as per the German drawings and (surprise surprise) it didn't work. I then removed the box and installed a shorting link in the connector so that the truck started OK. I then took the box apart for further investigation. After some (considerable) time I think I have pretty good idea of the circuit schematic and it seems that it is as I thought (+ or - a bit). The general idea seems to be to isolate the starter solenoid once the engine is running and keep it so whilst the alternator is spinning. I need to get a 'scope on the alternator and see what the tacho signal looks like and see how it varies with alternator speed, then I should be able to simulate it on the bench and figure out how it works. The good news is that there is nothing inside that cannot be replaced - the transistors all have readily available western alternatives so there should be nothing stopping getting it fully working again.

I plan to post circuits on here when I have got things 100% nailed.

[Mark Preston]

I have finally figured this out, and this is what I have found:

The box is a starter control box.
The function of it is to prevent abuse of the starter motor.

What is does is only allow the starter motor to be engaged once per 'ignition on', in other words you have to turn the ignition off between attempts to start the engine.

Once the engine is running it will not allow the starter to engage at all.

I think I know sufficient about it and now have a circuit diagram such that I could help repair others if anybody had the same problem. I have western equivalents for the transistors and diodes.

[paulob1]

good one Mark, that sounds really useful, will try it on my ural 375 and see what happens, I am sure we dont have one. But what a sensible thing to do...you can fault the Russians for being imaginative..

[Mark Preston]

I have no idea if these boxes were fitted to the 375. If it were then it might be slightly different, as on the 375 the box would have to take the 'engine running' signal from the distributor/coil wheras on the 4320 it comes from the alternator, so the input to the circuitry might be slightly different.

On the 4320 it is mounted up inside the dash, on a for/aft bracing member by the drivers left knee.

Let me know if you find one! (and if so if it works!)

In my case I had one, it didn't work and had been by-passed.

[Mark Preston]

About the Russians thinking of stuff and being imaginative - it's true, however only the Russians would sent conscript soldiers into situations so desperate that they'd try anything to break the trucks to avoid going!

[paulob1]

cant blame them for that...