I suspect the tachometer in my car (1992 Honda Accord) is not working properly. Kind of hard to explain, but it seems to be a little low at idle and maybe a little high on the highway, otherwise it works fine. It's a small enough difference where I can't tell if the tach is off, or if it's telling the truth and something is going on with my engine. So is there an easy way to verify the RPMS? I guess my only option is to buy an aftermarket tach and hook it up? Keep in mind this car is a 1992, so I can't just plug a scan tool into a diagnostic port to see RPMs.
Easy way to verify RPMs?
- Thread starter exranger06
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You can look up the gear ratios, diff ratio, tire circumference, and then calculate what rpm your car should be running in a gear at a given speed.
If its an automatic, then this may only work well in the top gear with the TC locked up.
If its an automatic, then this may only work well in the top gear with the TC locked up.
If you have a mechanical dizzy you can probe the switched earth at the coil if you have a oscilloscope or buy an aftermarket tach but there's no guarantee it'll be accurate.
Put a timing light on it and cuount how many times you see the mark in a minute or if its obd2 then get the torque app on your phone?
If you're good at math then find your tire circumference, gear ratio, diff ratio etc and time a 1 mile drive at a gps confirmed speed.
Put a timing light on it and cuount how many times you see the mark in a minute or if its obd2 then get the torque app on your phone?
If you're good at math then find your tire circumference, gear ratio, diff ratio etc and time a 1 mile drive at a gps confirmed speed.
Go to a Honda forum and ask/compare results.
Get it out in the sunlight with a camcorder so it can shoot at 1/1000 sec for "freeze frame". Paint a mark on the crank pulley, start it up, hold it at "1800 RPM".
The camcorder will film at 30 frames/sec, or 1800 frames/ min, and the paint mark will appear still if your tach really is 1800 RPM. It'll do the same thing at 900 and 3600 RPM, though the 900 would be "every other frame" yet should still hold still, just flicker.
The camcorder will film at 30 frames/sec, or 1800 frames/ min, and the paint mark will appear still if your tach really is 1800 RPM. It'll do the same thing at 900 and 3600 RPM, though the 900 would be "every other frame" yet should still hold still, just flicker.
I already know it's supposed to be at exactly 3k rpm at 70 mph, as that's how it's always been. Now it's at around 3500 rpm. I can't tell if my TC isn't locking up or if my tach is wrong. I can try to see if the RPMs drop when the TC is supposed to lock up, but I'm usually busy looking at where I'm going, since I'm always either in heavy traffic, or it locks up as I'm trying to merge, etc.
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
Originally Posted By: exranger06
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
You can calculate RPMs using a gear ratio calculator regardless. It does not take any input from your tach.
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
You can calculate RPMs using a gear ratio calculator regardless. It does not take any input from your tach.
Originally Posted By: exranger06
I already know it's supposed to be at exactly 3k rpm at 70 mph, as that's how it's always been. Now it's at around 3500 rpm.
Is it also possible that your speedo is wrong? Have you verified with a GPS?
I already know it's supposed to be at exactly 3k rpm at 70 mph, as that's how it's always been. Now it's at around 3500 rpm.
Is it also possible that your speedo is wrong? Have you verified with a GPS?
Originally Posted By: Quattro Pete
Originally Posted By: exranger06
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
You can calculate RPMs using a gear ratio calculator regardless. It does not take any input from your tach.
I know how to calculate what engine RPM should be by hand with any speed or gear ratios. I've done it before dozens of times. But it's useless when the car is sitting still with the transmission in Park and the engine idling. You can plug in any gear ratio and tire circumference you want, when you get to the part where you multiply by wheel RPM (zero, when the car is sitting still) - anything times zero is....zero.
I'm pretty sure the engine RPM is supposed to be around 700 rpm, not zero.
Originally Posted By: exranger06
Also, the tach seems to be off when idling, so gear ratio calculators don't do me any good there.
You can calculate RPMs using a gear ratio calculator regardless. It does not take any input from your tach.
I know how to calculate what engine RPM should be by hand with any speed or gear ratios. I've done it before dozens of times. But it's useless when the car is sitting still with the transmission in Park and the engine idling. You can plug in any gear ratio and tire circumference you want, when you get to the part where you multiply by wheel RPM (zero, when the car is sitting still) - anything times zero is....zero.
I'm pretty sure the engine RPM is supposed to be around 700 rpm, not zero.
Originally Posted By: Quattro Pete
Originally Posted By: exranger06
I already know it's supposed to be at exactly 3k rpm at 70 mph, as that's how it's always been. Now it's at around 3500 rpm.
Is it also possible that your speedo is wrong? Have you verified with a GPS?
Haven't verified speedo yet. I'll try that too.
Originally Posted By: exranger06
I already know it's supposed to be at exactly 3k rpm at 70 mph, as that's how it's always been. Now it's at around 3500 rpm.
Is it also possible that your speedo is wrong? Have you verified with a GPS?
Haven't verified speedo yet. I'll try that too.
Originally Posted By: exranger06
I know how to calculate what engine RPM should be by hand with any speed or gear ratios. I've done it before dozens of times. But it's useless when the car is sitting still with the transmission in Park and the engine idling. You can plug in any gear ratio and tire circumference you want, when you get to the part where you multiply by wheel RPM (zero, when the car is sitting still) - anything times zero is....zero.
You do not enter RPM value. You enter the speed value, and the calculator spits out what RPM you should be running at that given speed.
I'm talking about a cruising speed when you're in top gear, not when the car is parked in the driveway. But it sounds like you already know what the RPMs should be.
I know how to calculate what engine RPM should be by hand with any speed or gear ratios. I've done it before dozens of times. But it's useless when the car is sitting still with the transmission in Park and the engine idling. You can plug in any gear ratio and tire circumference you want, when you get to the part where you multiply by wheel RPM (zero, when the car is sitting still) - anything times zero is....zero.
You do not enter RPM value. You enter the speed value, and the calculator spits out what RPM you should be running at that given speed.
I'm talking about a cruising speed when you're in top gear, not when the car is parked in the driveway. But it sounds like you already know what the RPMs should be.
Originally Posted By: Quattro Pete
You do not enter RPM value.
I KNOW that. Where did I say anything about entering engine RPM?
Originally Posted By: Quattro Pete
You enter the speed value, and the calculator spits out what RPM you should be running at that given speed.
Again, I know that. Speed value is wheel RPM, which like I said in my last post, is zero.
Originally Posted By: Quattro Pete
I'm not talking about a cruising speed when you're in top gear, not when the car is parked in the driveway.
I'm trying to figure out what RPM my engine is at when it's sitting still and the transmission in park. A speed/gear ratio calculator is USELESS for that purpose.
You do not enter RPM value.
I KNOW that. Where did I say anything about entering engine RPM?
Originally Posted By: Quattro Pete
You enter the speed value, and the calculator spits out what RPM you should be running at that given speed.
Again, I know that. Speed value is wheel RPM, which like I said in my last post, is zero.
Originally Posted By: Quattro Pete
I'm not talking about a cruising speed when you're in top gear, not when the car is parked in the driveway.
I'm trying to figure out what RPM my engine is at when it's sitting still and the transmission in park. A speed/gear ratio calculator is USELESS for that purpose.
OK, ok. Me and others were suggesting the calculators to verify RPMs in motion, not in park.
Hall effect sensor on the ignition wire, multiply by 2 for non waste spark engine, use as is for waste spark engine.
Easiest way I know of from class is an amp clamp on a spark plug wire with a Multimeter that has RPM function IIRC.
Put a frequency counter on the low voltage end of the ignition system, and count ignition pulses for 10 seconds. Divide by the number of cylinders if it's a lost spark system, divide by half the number of cylinders if it's not. Then times ten for your RPM.
Some of the inductive/digital/vibra tachs are fairly cheap and readily available ($12-$100). Some digital multimeters have an rpm feature. I have an old Sun Pro mini-tach from the 1990's I used to use on my '68 GTX. It only has 200 rpm gradations...maybe 100 rpm accuracy.
"...Get it out in the sunlight with a camcorder so it can shoot at 1/1000 sec for "freeze frame". Paint a mark on the crank pulley, start it up, hold it at "1800 RPM".
The camcorder will film at 30 frames/sec, or 1800 frames/ min, and the paint mark will appear still if your tach really is 1800 RPM. It'll do the same thing at 900 and 3600 RPM, though the 900 would be "every other frame" yet should still hold still, just flicker.."
Your not doing this with an ordinary camcorder. Your camera would have to have the ability to manually set shutter speed and ISO rating. Otherwise the automatic ND filter is kicking in. And don't forget frames per second. The video camera I work with allows 1/1000 shutter speed, only at 24 and 60 FPS, not 30. Since this type of camera is running around $+4k, it's not in everyone's neighborhood. And you still need the mark to show on the camera side, don't you. Otherwise it could always be on the far side of the pulley when the photograph happens?
The playback would then be frame by frame or what? Cool idea, though.
The camcorder will film at 30 frames/sec, or 1800 frames/ min, and the paint mark will appear still if your tach really is 1800 RPM. It'll do the same thing at 900 and 3600 RPM, though the 900 would be "every other frame" yet should still hold still, just flicker.."
Your not doing this with an ordinary camcorder. Your camera would have to have the ability to manually set shutter speed and ISO rating. Otherwise the automatic ND filter is kicking in. And don't forget frames per second. The video camera I work with allows 1/1000 shutter speed, only at 24 and 60 FPS, not 30. Since this type of camera is running around $+4k, it's not in everyone's neighborhood. And you still need the mark to show on the camera side, don't you. Otherwise it could always be on the far side of the pulley when the photograph happens?
The playback would then be frame by frame or what? Cool idea, though.
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