Bad RPM reading in tachometer
08-11-2006, 11:04 AM
#1
5th Gear
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Bad RPM reading in tachometer
Hi all, the other day I found a way to read the motor RMPs from the HVAC and found that when it reads about 600 RMPs the tachometer reads about 1300 RPMs, and so in any RMP range (for 3000 RPM in the tachometer I read 1500 in the HVAC). I check this readings with the ECM850 software and found that the HVAC reading was right. My question is, is there any way to adjust or calibrate the tachometer?
08-11-2006, 12:14 PM
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C7 of the Year - Unmodified Finalist 2021
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yeah, sounds tach filter-related
you need to fill out your profile with year and transmission
you need to fill out your profile with year and transmission
08-11-2006, 04:26 PM
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That's were I would look first, tach filter.
A while back I was told how to check them on the vehicle but I cannot remember right now. Once you cut it out of the wire harness assembly and have it on the bench it's much easier to test with an ohm meter.
I'll look when I get home to see if I have that info.
Did a little searching... the person who truly knows this is member named Tom Piper this was from one of his posts:
The purpose of the tach filter is to filter out the high-frequency component (in other words, the dampened oscillation or "ringing" from the ignition coil) and pass the low-frequency component (the square wave that the dampened oscillation rides on). The tach filter is simply an RC (resistance-capacitance) filter tuned to attenuate the high-frequency component while passing the low-frequency component.
He is the one who helped me so if we could ever find him... or I find his comments to me!
A while back I was told how to check them on the vehicle but I cannot remember right now. Once you cut it out of the wire harness assembly and have it on the bench it's much easier to test with an ohm meter.
I'll look when I get home to see if I have that info.
Did a little searching... the person who truly knows this is member named Tom Piper this was from one of his posts:
The purpose of the tach filter is to filter out the high-frequency component (in other words, the dampened oscillation or "ringing" from the ignition coil) and pass the low-frequency component (the square wave that the dampened oscillation rides on). The tach filter is simply an RC (resistance-capacitance) filter tuned to attenuate the high-frequency component while passing the low-frequency component.
He is the one who helped me so if we could ever find him... or I find his comments to me!
Last edited by Jet-Jock; 08-11-2006 at 04:45 PM.
08-11-2006, 04:49 PM
#6
Le Mans Master
Originally Posted by Jet-Jock
He is the one who helped me so if we could ever find him... or I find his comments to me!
08-11-2006, 06:57 PM
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Well, Jet-Jock, I'll appreciate if you can post that info again because I tried to email Tom Piper, but he disable the send email in his profile... =( thank you anyway RRT Vette! Here then arise to me a new question... Even that the tachometer reading is not acurate, seems very stable to me (no flickering). Even so, could it be the tach filter?
08-11-2006, 07:55 PM
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I've looked and can't seem to find it.
I think that you can remove the ground connection to isolate the filter out. It has two white wires and one black one. The black one is the ground and goes to the engine ground just below the head on the passenger side. The white wire is actually a pig tail where one end goes into the CCM and the other into the tach itself. I believe after you remove the ground it's raw signal going to the tach.
As described above, it's purpose is to isolate the square wave signal for the tach. If it were bad I would expect a wildly jumping tach, not a smooth but not accurate indication. Giving a little more thought to this I'd be more suspicious of the instrument cluster than the tach filter.
When mine went the tach just went nuts. I draw your attention to the cluster because I'm sort of getting a similar situation now except with the oil pressure. Recently mine has gone spastic. With feather application of the accelerator the needle pegs out ease off it goes back to a normal indication, hard acceleration it behaves perfect. The schematic goes from the sensor to the gauge. I've replaced the sensor and no change. So I believe the gauge in the cluster is just not interpreting the signal correctly. Which I tend to liken to your situation.
When you said you compared it how did it compare to higher RPMs is it still off?
I think that you can remove the ground connection to isolate the filter out. It has two white wires and one black one. The black one is the ground and goes to the engine ground just below the head on the passenger side. The white wire is actually a pig tail where one end goes into the CCM and the other into the tach itself. I believe after you remove the ground it's raw signal going to the tach.
As described above, it's purpose is to isolate the square wave signal for the tach. If it were bad I would expect a wildly jumping tach, not a smooth but not accurate indication. Giving a little more thought to this I'd be more suspicious of the instrument cluster than the tach filter.
When mine went the tach just went nuts. I draw your attention to the cluster because I'm sort of getting a similar situation now except with the oil pressure. Recently mine has gone spastic. With feather application of the accelerator the needle pegs out ease off it goes back to a normal indication, hard acceleration it behaves perfect. The schematic goes from the sensor to the gauge. I've replaced the sensor and no change. So I believe the gauge in the cluster is just not interpreting the signal correctly. Which I tend to liken to your situation.
When you said you compared it how did it compare to higher RPMs is it still off?
08-11-2006, 09:39 PM
#10
Burning Brakes
I have a 1990 Auto and my Tachometer is off by 300%, ie. at 700 RPM's my tach reads 2100 or so. It is my understanding that the connector at the bulkhead can get moisture in it and cause bad readings. I was told to remove the bottom connector from the firewall (bulkhead) and dry it out and reconnect it and to check that the drain tube was not draining into the connector. I tried it, but I made only a half-a** attempt. I plan on trying it again tomorrow with a little more try thid time. I'll probably use a hair dryer. I'll post my results.
Here is what Rick Lambert told me to do:
The connector is under the brake booster-big bundle of wires in a connection attached to the firewall (bulkhead) tach pin on botom row. Make sure your cowl drip tube(rubber drain tube) is not draining directly on it, if it is re-direct it. When you open the connection if you see a whiteish/greenish look-that's water, blow it out, don't remove the greasy stuff, that's an anti corrosive, just buckle it back up and use something like silicon to seal both the top and sides.
Here is what Rick Lambert told me to do:
The connector is under the brake booster-big bundle of wires in a connection attached to the firewall (bulkhead) tach pin on botom row. Make sure your cowl drip tube(rubber drain tube) is not draining directly on it, if it is re-direct it. When you open the connection if you see a whiteish/greenish look-that's water, blow it out, don't remove the greasy stuff, that's an anti corrosive, just buckle it back up and use something like silicon to seal both the top and sides.
08-11-2006, 10:51 PM
#12
Burning Brakes
Exactly. Those directions came from Gordon Kilebrew. I called him myself and he told me the same thing. He told me that this is due to a design flaw that is especially evident to the 1990 Vettes.
