Astra GTE digi dash rev counter reads too low

[lord13]

Hi all, I recently fitted a GTE digi dash into my mk2 cavalier, everything works as it should, except the rev counter looked to be reading low. I connected up my old dash in tandem to the digi dash so I could cross reference everything and yep, the rev counter reads about 800 - 1000 too low on the digital dash. Does anyone know how to rectify this? Is it a case of swapping out some resistors on the board, or using a different feed? I'm using the original feed that ran the analogue rev counter in my original dash and the dash is the later(?) 'curved' type. Like I said everything else in the dash reads as it should, It's just the revs. I can live with it, I have thought of just blanking out the numbers and adding an small aftermarket digital rev counter that's just a number readout, seen 'em on ebay, cheap tiny rubbish but it only has one job and costs less than a pack o' fags but I'd prefer to get the dash to work properly if i can.

Any help and advice is much appreciated
cheers

[droopsnoot]

Is it consistently 800-1000rpm low, or is it a percentage, so as you rev the engine higher, the gap gets larger?

[Phil H]

Are you sure the old dash wasn't reading 800-1000rpm too high?..

[Rich]

Jan 30, 2020 10:25:26 GMT Phil H said:

Are you sure the old dash wasn't reading 800-1000rpm too high?..

Presumably he would have noticed if his car was idling at 1750 RPM on the tach. I would assume it doesn’t over read.

[Phil H]

I figured it would look odd with a fast idle at 300rpm too...

Does it read 800-1000rpm low at all engine speeds (so an offset issue), or is it 25% low (hence a scaling issue)? My thinking is if you're holding it at (what you think is) 3000rpm, is that 2000rpm on the digi and 3000rpm on the old tach, but tickover is about right - or does it read 100rpm at tickover on the digi dash?

To be honest I'd expect the analogue meter dash to be more wrong that the digital one..

[lord13]

Ok some interesting ideas, I did just assume the old analogue dash was correct, mainly because it reads right at tickover, and seems to be ok at what I'd normally change gear at, basically 950-1000 rpm and 3000-3500 respectively... and when I've got my foot down, changing about 5000-5500. The engine 'sounds right at these revs also.
Where as the digi dash reads about 750 on tickover, about 2200, on normal gearchanges and 4000-5000 when being a bit 'lairy' so to speak.
So I guess it is increasing in increments and not just 800-1000 across the board. So .... any ideas?

[Phil H]

I think you need a third opinion in the form of an aftermarket tacho to see which one is right.

[droopsnoot]

I was convinced that the tacho on my Sportshatch was correct, and that the diff had been swapped somewhere along the line (for a diff of a ratio that Vauxhall never made, but that's another story). Until I did this:





So it wasn't that I had a really low (or high) ratio axle, just a dodgy tachometer.

[lord13]

Nice to see you have one of those rare backwards rev counters, that's an early 7 dial dash isn't it? Been a long time since I messed with a 7 dial
I see what you mean though, I definitely need another opinion on the revs rather than relying on GM equipment to tell me...

[droopsnoot]

No, that's a later 7 dial, with "upside down" tacho and volt meter instead of ammeter. Originally they started at about 7-o'clock, but in the HPF with the small steering wheel that meant the crucial bit from 5000 upwards was covered by the wheel and the drivers right hand, so they moved it round to start at about 1pm. I think they left it like that on all future models, but as the hatch has the same wheel as the Firenza, it's useful there too. In theory, but I don't rev it that high now I've found out how difficult it is to get engine bits.

[darrenh]

lord13 digital dash (in its natural environment) takes the tacho feed direct from the coil, so theres not really any special electronics that would make it consistently under-read

the only scenario i can think of which would make one under read is if you had monza gse/ senator dashboard which look identical but are 6 cylinder....

when you switch the ignition on the dashboard does a power on self test. if it displays 126 (the speedo i'm talking about) its an astra dash, if it displays 125 its a monza/senator