Series 2a -3.9 V8 Rev Counter Install Problems

[blz6662]

Hi All,

I'm hoping this will be a simple one with the install of a new Durite 52mm 0-8000 rpm coil fitment rev counter.

The truck has a 3.9 V8 fed via a Holley Truck Avenger Carb, sparks via 1.5ohm Flamethrower coil to a Lucas 35DLM8 electronic distributor and Magnacore leads.

No issues with the running of the truck, I just wanted to replace the dodgy chinese rev counter that read all over the place so I bought a Durite one for coil pick-up.

The problem is, after following the install guide (pretty simple stuff) and checking all the connections - green wire to negative side of coil - the rev counter is not working, not even a blip.

Double checked all connections - rear of unit on selection for 8 cylinders, still nothing.

I did contact Durite direct and no reply.

Should I have an earth lead from the negative on the coil direct to the block as well? Currently negative only has a connection to the electronic amp on the dizzy.

Any Ideas?

Kind regards

Baden  

[FridgeFreezer]

You should not earth the coil to anything - it gets 12v from the igntion and the points/amp switch the ground side to make the sparks.

Did you buy a rev counter that is designed for coil systems because there are plenty around that work from the alternator W terminal for diesels?

[Bigj66]

My rev counter is fired from the Thor ECU. I could only find one ECU type and it wasn’t Durite. I thought the Durite ones were W connection fed?

[FridgeFreezer]

VWP claim theirs is coil-fed:

https://www.vehiclewiringproducts.co.uk/c-125-durite-vehicle-gauges-fittings/p-207-tachometer-12v

 

[blz6662]

Yep, that's the one and I bought it from VWP.

Coil get 12v from ignition source to to amp. Negative at the moment to the amp and that's it, as simple as that. Connected green to negative on coil to the lug where the old rev counter was connected (this always read high so replaced with Durite).

The alternator fitted has a W terminal, but can't connect as this is a sine wave signal, so no good for my coil type.

 

Thanks for the replies so far.

Bigj66 - no ECU carb fed.

Cheers

Baden

[smallfry]

Have you got an earth to the body of the rev counter ?

[FridgeFreezer]

Just to check/confirm:

 

[Bowie69]

The clarify, the black and green aren't joined! 

That's the firewall grommet. 

[FridgeFreezer]

50 minutes ago, Bowie69 said:

That's the firewall grommet. 

[blz6662]

Thanks guys,

Progress last night, I went back through all the connections and tested them and all ok. So I decided to try a more direct earth and 12v supply, result still nothing.

In the kit supplied with the counter is a 1 amp rectifier diode and I put this in a jumper wire between the coil negative and the green wire. This is sometimes installed for points type ignition, but thought I would experiment and see what happens.

Started the truck and got a reading although low, tried revving it but it stayed fairly constant at 750rpm. Maybe I'm onto something.

On the other side of the instructions it talks about  a 20k resistor, 3 amp universal diode and 20k linear potentiometer so maybe I need to try down one of these routes.

Cheers

 

Baden

[FridgeFreezer]

Jeez talk about bodges in a product

I'd be fitting the diode etc. as near the back of the tacho as possible, not in the engine bay as they will really not enjoy life in there.

[smallfry]

That Pertronix Flame Thrower coil has a very low impedance ISTR. I have no idea with electronics, but could this have something to do with it ?

[blz6662]

Hi smallfry,

The 1.5ohms matches the electronic ignition so ok there, although I did wonder that myself, any ideas welcome.

3 ohm is not suitable from the info I have double checked.

Fridge I'll move the diode to the gauge and try again. If I have to fit a different one it will be in the cab. This was only a quick trial to be honest.

Maybe I should think about a counter suitable for the alternator W terminal and stop wasting time. I could try the coil one on my series 3 swb station wagon which is standard on points (at the mo).

[FridgeFreezer]

3 hours ago, blz6662 said:

Fridge I'll move the diode to the gauge and try again. If I have to fit a different one it will be in the cab. This was only a quick trial to be honest.

Moving it won't make any difference to performance, it's just that it's a fragile component and may have a short life just dangling around in a hot engine bay.

[blz6662]

Thanks for the comment, I've just bought a 20k linear pot, so I'll update when I get it and give it a go. Or perhaps fit a clock instead😀.

Any other suggestions welcome.

Kind regards

Baden

[smallfry]

Probably a far more useful thing to have.

[FridgeFreezer]

22 hours ago, blz6662 said:

Any other suggestions welcome.

Once you work out the pot value, post up & I'll stick a fixed resistor in the post, bit more robust than a pot floating around behind the dash...

[blz6662]

Hi FridgeFreezer,

thanks for that, I got it in the post yesterday so will give it a blast asap and post up the results.

Kind regards

Baden

[blz6662]

Hi All,

OK, here's the latest.

Removed the diode I previously trial fitted so all connections restored to original.

Wired up the 20k pot. 3 pins facing me, wired the left hand pin with an earth lead, centre pin from negative coil connection (input signal) and right hand pin (output) to rev counter.

Result - Nothing!! - no reading, not a blip. Tried various dial positions throughout the range and still nothing.

Last attempt, swopped the input / output wiring over - (which would be full ON anti-clockwise) - and yes you've guessed it - nothing.

So what am I doing wrong.

I think I've given up with this unit and I need to carry on finishing the outer wiring jobs on the truck and put it back in circulation. Got to get the Sankey on the back and get more logs in ready for 'that' time of the year.

Any answers gratefully received.

Baden

[FridgeFreezer]

Depending on the rating of the pot it might not appreciate getting tacho spikes across it to ground, I'd have expected them to just wire it in-line (2-pin modes below) to just act as a variable resistance in the tach line, not with a ground connection.

 

[blz6662]

Hi Fridge,

I did first of all, try the last 2 options. I'll try the others and report back.

[FridgeFreezer]

No need to try all of them, the 2-pin modes are basically the same for your purposes.

[blz6662]

Hi All,

OK, this was it, my last go at solving this one. Tried the 2 pin modes above and still the needle hovers around 750 rpm and no movement when revving the engine.

So, back tracked and rewired in on all new direct connections my old 'el cheapo' unit previously fitted and when fired up the needle hovers around 3500RPM on idle and spins up to max when lightly revved.

This is what it always did, first put in the 1 amp diode - result smoother needle same readings as above. Took that out and put the 20k pot in 2 pin mode and no change at any point on the scale so no good.

Right then, one more go, disconnected the Flame Thrower coil and fitted a good DLB198 Lucas 1.5ohm coil as fitted to my 92 disco. Connected up with new Durite tacho standard wiring set-up, no gizmo's, fired up the engine and the tacho did the same again and stuck at 750RPM with no movement on revving. So went though each method again and same result - GIVE UP.

The truck has no starting or running issues with either coil and is as sweet as' so time to stop.

I don't like being beaten but this has, so this might be a reference guide to anyone with the same set-up as mine.

Thanks Fridge for all your help.

Cheers

Baden 

[FridgeFreezer]

I'm going to tag @TSD to see if he's got any thoughts on this since he's got form in this area with the megasquirts... as you say, it's frustrating to be beaten with something like this.

[TSD]

Sorry for the radio silence, been away for a few days...

I haven't read the full thread yet, but first thought is that I think you may have the pot the wrong way around. One end to ground, the other end to the coil, and the wiper (middle pin) to the tacho. Also watch out for 'suppressor' caps that have been added to the ignition wiring - you can just disconnect them briefly for testing if you find them. On a clockwork ignition, I'd be wondering if the dwell angle was right, but I assume it's not adjustable on the electronic version of the dizzy?

Also, in my experience coils like the FlameThrower cause more issues than they cure, especially on an engine like the RV8 with its absurd firing order. Even with Magnecors, HT varnish and careful lead positioning, they can generate repeated misfires that are hard to find. @FridgeFreezer had one (in pre MS days) that arced out from the coil tower, down the insulator boot, through the dielectric grease or whatever varnish we'd used, and grounded on the mounting bracket, and it was almost impossible to stop. At dusk, his engine bay was like blackpool seafront on fireworks night.