Series 1 Tacho - wiring

Phil Robson

Well-Known Member
#1
Please let me first state that my electrical skills are sorely lacking & are my 'Achilles Heel' when it comes to P6s & in fact cars in general :(.

I've had an intermittent speedo fault on Hazel (Series 1 V8) which looks to have been just the angle drive working a bit loose at the back of the instrument panel, meaning that it sometimes located with the speedo & sometimes didn't. I've replaced it with a spare & fully screwed it up. It worked fine on a test drive.

However, before I put everything back together, I've had a tacho/clock to fit for years, so this is a good opportunity. I can just about work out what wires go into the clock, but haven't a clue about the tacho itself. I can't find it mentioned anywhere on series 1 circuit diagrams either, which doesn't help.

Here is the back of the unit:

IMG_5969.JPG

I think the green, black & white/red are for the power, earth & lamp respectively (like the clock) so please can someone tell me where the others go?

Many thanks.
 

Demetris

Well-Known Member
#2
You are correct in you assumptions about the first wires. Just take note of the polarity, in case the tacho positive earth (although i doubt it). Regarding the white/yellow and white/black wires, one of them goes to the coil negative and the other to the distributor. I cannot find something relevant in a wiring diagram, so i cannot tell you exactly what goes where, but it doesn't matter. The idea is that the tachometer is connected inline to the cable between the distributor and the coil negative (for negative earth systems). Ideally, you should find the respective connectors somewhere behind the speedo/clock. If not, you can pull your own cables in order to connect it in the way that i have described above. Obviously, the current line between the distributor and the coil negative should be deleted.
 
#4
That looks nice, can you come and do mine as well!

Seriously were the wires behind the speedo as Demetris said and easy to identify? I've been shown how to get in there but haven't taken the plunge as yet!
 

Phil Robson

Well-Known Member
#5
Seriously were the wires behind the speedo as Demetris said and easy to identify? I've been shown how to get in there but haven't taken the plunge as yet!
I couldn't see any, so I routed a pair from the dashboard to near the coil & wired the tacho in-line. I bound the wires together with insulation tape so that it resembled a part of the 'loom'.
 
#7
In use, the rev counter appears to read a little low, which is probably impossible if I understand the principle correctly, given that I've got standard profile tyres on. I was going to check with a multimeter but I have then read the very useful RPM conversion charts on this forum RPM Conversion charts for Rover P6

I've been told in the past that the 3500 autos have the same gearing in top as the 3500S. From the charts it looks like the revs on a 3500S are around 2,500 at 60mph (approx) whereas I thought it should have been nearer 3,000 revs at this speed. 'Hazel' is showing around 2,500, so is probably correct. :cool: I think I've always had the 4 cylinder figures in mind (basically double the revs for your speed - give or take a few decimal places) but this is with a different final drive ratio to the V8s.

The other thing that I find interesting about the charts is just how little difference the LT77 'box makes to the revs (about 20%). I have such a box & bell-housing to go in 'BOP' (a V8S), but I'm thinking I'm now in less of a hurry to do this than I was a few minutes ago! Certainly a project if the current box breaks, but not too essential up to that point unless I was doing a huge mileage.
 
#8
Both manual and auto P6s are 1:1 in top with the same final drive. On standard tyres that's 24.7mph/1000rpm. The auto will slip of course as it doesn't lock.

The LT 77 with 0.77:1 gives 32 mph/1000rpm. That's a HUGE/MASSIVE/COLOSSAL difference regards to cruising speed and where the torque peak is.

In truth the final drive of the V8 is too high in an attempt to bridge where 4th and a non-existant 5th is. You want around 22/22.5mph/1000 in 4th and then 29/30 in 5th.

Like a lot of cars at the time, the motorway age wasn't kind to them.
 
#9
The figures do look good on paper, but if most of my modest mileage is below 60mph then at the worst my engine is revving 2,430 instead of 1,875 times per minute. That’s a difference of 555 at the legal maximum in most of the UK. Usually it will be well below this.

I agree an LT77 is an improvement otherwise it wouldn’t have been designed, and I myself have one ready to install. The question is; in normal driving is it worth the bother, unless I need to replace the original box?
 
#11
Well, all I can say is I think the ZF conversion I did is transformative. I can get 32 mpg on a run and it is much more relaxed. Admittedly not many people drive from Zürich, Switzerland to Faringdon in a 50 year old car. Add into that the much smoother shifting, lack of clonking etc.
 
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