New factory tach
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Laughing Gas
Joined: Oct 2010
Posts: 997
Likes: 2
From: Rock Valley, IA
New factory tach
I asked in the WHYDTYTT thread but thought this a more appropriate venue.
I put the tach in and kept the aftermarket connected, you know, just in case.
Anyway, I was keeping an eye on both of them and the factory tach was reading about 250 RPM low.
Now, I know the aftermarket was reading correctly. I drive 55mph, have 30.47 tires, and a 4.10 rear end. According to the calc I should be running at 2487 RPM which, according to the aftermarket tach, is spot on.
Now, is there anyway to bring the factory tach up to speed, as it were, or should I just count on adding an additional 250 when looking at it.
I know it's not a big deal, but I just like thing to be "right", ya know?
I put the tach in and kept the aftermarket connected, you know, just in case.
Anyway, I was keeping an eye on both of them and the factory tach was reading about 250 RPM low.
Now, I know the aftermarket was reading correctly. I drive 55mph, have 30.47 tires, and a 4.10 rear end. According to the calc I should be running at 2487 RPM which, according to the aftermarket tach, is spot on.
Now, is there anyway to bring the factory tach up to speed, as it were, or should I just count on adding an additional 250 when looking at it.
I know it's not a big deal, but I just like thing to be "right", ya know?
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
Is the factory tach in your '85 similar to mine, in that it reads pulses off the green wire attached to the coil?
I'd imagine you have a corroded or otherwise resistive connection somewhere between coil and instrument.
I'd imagine you have a corroded or otherwise resistive connection somewhere between coil and instrument.
Thread Starter
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Laughing Gas
Joined: Oct 2010
Posts: 997
Likes: 2
From: Rock Valley, IA
Yup and that's where I have the aftermarket one hooked up as well, I guess that could definitely be causing an issue. I'll disconnect it there and see if I get a different reading.
Cargo Master
Joined: Mar 2007
Posts: 2,935
Likes: 34
From: Arcadia, Fla
On the factory tachs, they have a 6 or 8 cylinder setting on them. For 6cyl leave it ungrounded, for 8 cyl it must be grounded.
This is done through the factory wire harness when it was wired up for your engine type. As some go swap engines from 6 to 8(or 8 to 6), then the tach will be off unless you add/remove that ground
This is done through the factory wire harness when it was wired up for your engine type. As some go swap engines from 6 to 8(or 8 to 6), then the tach will be off unless you add/remove that ground
Cargo Master
Joined: Apr 2003
Posts: 2,408
Likes: 5
From: Parkesburg PA
Um... Does anyone here really expect the factory tack to be anything more than a general guide? Or is it just me?
Car Craft I believe just did a test on 5 tachs under $100. They used a dyno motor and were able to drive the tachs off the same computer at the same time, plus they had the mega-dollar, highly accurate dyno tach to compare to. 50-100 rpm differences were common as well as larger variations.
If it's always low it could be a signal problem. I don't think it's a 6 vs 8 cyl problem as it would read approximately 1.5x higher than real RPM. You'd know it was off...
Car Craft I believe just did a test on 5 tachs under $100. They used a dyno motor and were able to drive the tachs off the same computer at the same time, plus they had the mega-dollar, highly accurate dyno tach to compare to. 50-100 rpm differences were common as well as larger variations.
If it's always low it could be a signal problem. I don't think it's a 6 vs 8 cyl problem as it would read approximately 1.5x higher than real RPM. You'd know it was off...
Tuned
Joined: Aug 2004
Posts: 399
Likes: 1
From: Winnipeg Canada
What's a good brand of reasonably priced aftermarket tachometer based on your experiences? I'm looking for my '80 300 w/manual 4 speed.
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Senior User
Joined: May 2010
Posts: 369
Likes: 2
From: BC Cana'duh
You can try an Autometer/Autogauge.Autogauge is made by Autometer.It's their economy line gauges,but still good quality.I've had two different AutoGauge tachs and never any complaints.Just don't buy any NO NAME or offshore junk.Summit Racing has a good selection.
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Cargo Master
Joined: Apr 2003
Posts: 2,408
Likes: 5
From: Parkesburg PA
It was Hot Rod Mag... Autogauge seemed to be the best for under $100...
Rev Counters - 7 Tachs Under $100 - Hot Rod Magazine
Rev Counters - 7 Tachs Under $100 - Hot Rod Magazine
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
I don't think it is a 6 vs 8 cylinder problem. If it is in 6 mode but the engine is an 8 you'll be showing 1.333 times the actual since you are getting 8 pulses when you expect to get 6 and 8/6 = 1.333. Been there, done that.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
Lavatan - One of these days I'll get to the instruments on Dad's truck and pull the tach out to see what is on the PC board. If there is a pot for adjusting, and there should be as the factory would want to be able to dial them in, maybe we can get something going.
I can put my scope on the lead coming from the coil and see what the waveform looks like. Then I should be able to imitate it via 60 cycle AC, a transformer, and probably a diode. If my math serves, there are 4 pulses per revolution on the engine, so 60 cycles/sec/4 would be 15 rev/sec or 900 RPM. Should be able to turn the pot to read that and be correct at that end of the scale. And, if it is that easy I could make the calibration kit available so you could do your own.
But, I'm not to that point yet. Maybe you can remind me in a few months if we don't find a better way?
I can put my scope on the lead coming from the coil and see what the waveform looks like. Then I should be able to imitate it via 60 cycle AC, a transformer, and probably a diode. If my math serves, there are 4 pulses per revolution on the engine, so 60 cycles/sec/4 would be 15 rev/sec or 900 RPM. Should be able to turn the pot to read that and be correct at that end of the scale. And, if it is that easy I could make the calibration kit available so you could do your own.
But, I'm not to that point yet. Maybe you can remind me in a few months if we don't find a better way?
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
Well, my factory tach doesn't have any means of adjustment. At least no potentiometer to tweak. The only possibility is the wire-wrapped resistor between two posts, but I don't know that's how they did it and it would take some playing to figure it out.
So, I think it is what it is.
So, I think it is what it is.