The Blue View - Troubleshooting Engine Tachometers

We interrupt our inland travel once again to bring you another Blue View. Believe it or not, some folks prefer boat talk to travel talk. Travel resumes tomorrow.

In the last Blue View, I talked about troubleshooting problems with engine gauges, specifically temperature, oil pressure, and water temperature gauges. In this BV, I'll discuss diagnosing problems with tachometers.

Despite the fact there are different tachometers intended for outboard two stroke and four stroke engines, inboard gasoline and inboard Diesel engines, they all basically work the same - they all convert the number of pulses per second on the input terminal to engine RPMs.

Different types of engines generate these pulses by various methods. Some engines generate the pulses electronically, and there is one pulse per engine revolution. Many inboard Diesel engines use the alternator output as the tach input, and the number of pulses per revolution is a function of the number of alternator poles and the ratio of the alternator pulley size to the engine pulley size. Other engines use a magnetic pickup mounted next to the flywheel and generate one pulse for each gear tooth - in which case, there are two or three hundred pulses for each engine revolution.

Size, shape and color aside, tachometer manufacturers usually have different models available for different engine types. One option is the full scale range. The old Ford Lehman engine on Nine of Cups has a maximum RPM of 2800, and I don't want a tachometer that indicates a red line at 6000 RPMs and a full scale of 7000 - my idle speed of 800 RPMs would barely deflect the needle. Another option is the "pre-calibration" of the tach. Most manufacturers have models calibrated for the more common engines, thus avoiding the necessity, in most cases, of requiring specialized equipment and an hour or two of time to calibrate each tachometer when it is installed.

Okay, enough theory - let's get on to troubleshooting. There are typically four problems that can occur with the tach: it is totally inoperative and always displays zero; the needle is stuck or permanently pegged; the needle is erratic; or the RPMs are consistently off - either low or high. To test the tachometer, you will need a digital multimeter.

Inoperative

Check the obvious - when the ignition is switched on, is there 12vdc (or 24vdc) between the positive terminal and negative terminals when measured with a voltmeter? If not, use your voltmeter to determine whether the problem is in the wiring on the positive side or the ground side.

If the voltage is correct, the next step is to check the input signal. Remove the signal wire. Set your voltmeter to AC volts and connect it between the signal wire and ground. Start the engine and set the speed to idle. You should get a reading on the multimeter that varies with engine speed. If the tachometer is connected to the alternator, this reading should be a minimum of about 5VAC, otherwise it will be more like 0.3VAC. If you get any indication of a pulse stream, reconnect the signal wire, and if the problem is still evident, the culprit is most likely the tachometer itself.

If you get no indication of a pulse stream, disconnect the tachometer signal wire on the engine and repeat the test there. If you detect a pulse stream at that end, the problem must be in the wiring. Otherwise, the problem is in the signal generator.

Needle is Stuck or Pegged

The needle could be stuck in one place or permanently pegged for a couple of reasons. One is that the tach case is tightened down too much and has distorted due to heat or vibration. Try loosening the clamps holding it in place, then tapping the front face piece gently. If the needle frees itself, retighten the tach clamps just enough to hold it in place.

Another possible cause of a stuck needle is over stressing it electrically. This could happen, for example, if the battery cable was disconnected while the engine was running, the tachometer was subjected to large rf noise from a badly tuned or poorly grounded HF radio or from a lightning strike. It is sometimes possible to correct the problem by placing a magnet on the faceplate over the needle and "pulling" the needle free.

Erratic Reading

An erratic reading is usually due to a poor connection somewhere. Start the engine and wiggle each wire leading into the tachometer to try to isolate the culprit. Have someone watch the tachometer while you wiggle the wires on the engine side.

Another possible cause of an erratic reading is electrical noise. Does the problem only occur when the autopilot is running or when sending emails via your HF radio? The problem can sometimes be corrected by putting a resistor in the signal line. It should be placed on the tachometer end of the wire. You may have to experiment with resistor sizes. A 10k ohm, 1/4 watt resistor will often correct the problem without affecting the calibration. If not, try a 1k ohm, 1/4 watt or a 100 ohm, 1/4 watt resistor.

Readings are Consistently Low or High

This is a symptom of the tachometer being out of calibration, which usually doesn't occur unless something is changed. Did you recently replace the tachometer? Alternator? Pulley on the alternator or engine? Some tachometers have multi-position switches or jumpers located on the back or which are accessible through openings on the back. Check that the jumpers are not loose, or if there is a switch, it is possible that the contacts are dirty. Try rotating the switch a few times, then moving it back to its original position.

If necessary, it is usually possible to recalibrate a tachometer. Many older tachometers use switches to set the gross range, and a potentiometer to either fine tune the range or adjust the gain. If the tach is only off by 10%-20% or less, you may be able to recalibrate it using only the potentiometer - otherwise, you will need the manufacturer's documentation to correctly set the switches. Newer tachometers often use a software program for calibration, and you will need a computer, the software and any necessary cabling to calibrate the tachometer.

You will also need a strobe tachometer to determine the engine speed. This is an adjustable strobe light with a digital readout. They used to cost hundreds of dollars, but can now be found online for less than $50. I have also seen apps for smartphones and tablets that simulate a strobe tachometer, and while I can't vouch for the accuracy of these apps, they are probably good enough to calibrate your tachometer to within 5% or so. You can rent, borrow or buy a good one, buy a cheap one or try one of the apps. Then you need to put a mark on the big pulley on the front of the engine. This can be a small piece of tape, a line drawn with a marker, or a dot of paint, just as long as it can be easily seen when you shine a light on it.

Start the engine and increase the engine speed until the engine tachometer displays about half of the engines maximum RPMs. Set the strobe tachometer to the same RPM setting and aim the strobe at the pulley. If the engine tachometer is exactly in calibration, the mark on the pulley will appear to be frozen in place. More likely, the mark will appear to move. If it is slowly drifting around the circumference of the pulley, your tachometer is only slightly out of calibration. If the mark appears randomly all around the circumference of the pulley, your tachometer is very much out of calibration.

Adjust the frequency of the strobe tachometer until the mark on the pulley appears to slow down and eventually stop. The digital display of the strobe tachometer will now be showing the correct engine speed. Adjust the potentiometer on the back of the engine tachometer (or adjust the tachometer using the software) until it reads the same as the strobe tachometer. Check the calibration at 1/4 throttle and 3/4 throttle. In a perfect world, the engine tachometer will now match the strobe tachometer at all three engine speeds. More likely, you will need to adjust the calibration until the error is minimized throughout the engine range.

Now that your engine tachometer problem is figured out, the crew of Nine of Cups is picking up where we left off in the wilds of Swaziland and South Africa. Stay tuned...

The Blue View - Troubleshooting Engine Gauges

We interrupt our inland travel to bring you a Blue View or two, for those of you who prefer boat talk to travel talk. Travel resumes tomorrow.

I recently found it necessary to rebuild the box that houses the cockpit engine gauges on Nine of Cups, which is a whole other blog unto itself for another time. In the process, I had to, of course, disconnect and remove all the instruments... always a dangerous endeavor. The wiring harness is now 29 years old, and although it is still working fine, all that tugging and pulling on wires and connectors was bound to introduce some problems. As it turned out, the wiring made it through the project just fine, but I did have a couple of problems with the gauges themselves. I thought it might be a worthy of a blog or two to discuss how to go about troubleshooting engine gauges. In this blog, I'll discuss temperature, pressure and voltage gauges, and a later blog will be devoted to tachometers.

Voltage Gauges

Voltage gauges are the easiest to troubleshoot. Simply connect your voltmeter to the plus and minus terminals of the gauge and turn on the ignition. The voltmeter should read about the same as the voltage gauge. If the voltage gauge reads zero or is significantly off, it is defective and should be replaced. If both are reading low or zero volts, the problem is in the wiring.

Temperature and Pressure Gauges

Engine temperature and oil pressure gauges both use sensors (or senders) that screw into the engine somewhere. These sensors are nothing more than variable resistors - the resistance changes as either the temperature or pressure changes. The gauge provides a small current that flows through the sensor, then measures the voltage across it. As the resistance changes with increasing temperature or pressure, so does the voltage across it.

There are a few different standard sensors used by different manufacturers and in different parts of the world, but all have resistances that vary from zero ohms to a few hundred ohms as the temperature or pressure varies through the normal engine operating range.

These gauges have three inputs: V+, ground, and the sensor input. V+ may be labeled "+" or "I", ground will be labeled "Gnd" or "-", and the sensor or sender input will be marked "S" or "In". There may also be another one or two connections for the lamp.

The process for testing the gauge is as follows:

1. Check power to the gauge by connecting your voltmeter to the V+ and ground terminals of the gauge and switching the ignition on. You should see 12 volts (or 24 volts). If not, check the power and ground connections, wiring and fuses. If the alarm horn and other gauges are working, the problem is probably between the ignition switch and the gauge or in the ground connection.

2. Disconnect the sensor input from the "S" or " In" terminal and switch the ignition on. On most gauges, the needle will stay all the way to the left side of the gauge face. Next, short the input terminal ("S" or "In") to the ground terminal. If you have a jumper, great, but a screwdriver will do the trick (as long as your arms are long enough to hold the screwdriver in place while looking at the front of the gauge without shorting the ground and power terminals). On most gauges, the needle will deflect to full scale. Some gauges, depending on the manufacturer or model, will work the reverse of this. Just be sure the gauge goes to both ends of the scale when the sensor input is switched from open circuit to grounded. If the gauge does not pass this test, it is most likely defective and should be replaced. If it does pass this test, continue on to the next step.

3. With the sensor wire disconnected from the gauge, connect an ohmmeter between it and ground. You should see a resistance of a few hundred ohms. If not, move down to the engine room, remove the wire from the sensor and repeat the resistance check there. If there is a problem, you can now determine whether it is in the wiring or the sensor.

4. If everything checks okay, things start getting mysterious. It could be a mismatched gauge and sensor - did you recently change one or the other? It could be an intermittent connection - try starting the engine and wiggling all the wires leading to the gauge to see whether it starts working again and ,if so, which wire or connector is causing the problem.

If you do replace either the gauge or the sensor, make sure it matches the old one. If you aren't sure and can't figure it out, replace both the gauge and the sensor.

We'll tackle the tachometer gauge soon.

The Blue View - Broken Rudder Arm...again

On a passage from Sarasota, Florida to Colon, Panama in 2009, the rudder arm on Nine of Cups broke. The arm connects the hydraulic steering system to the rudder, and without it we can't steer with either the wheel or the autopilot. The original rudder arm was a massive hunk of bronze, and I was amazed that the rudder built up enough force to break it. This occurred about 25 nm out of Colon, and with the heavy ship traffic into and out of the canal, it wasn't a good place to be without maneuverability. We dug out the emergency tiller and hove-to until I was able to jury rig something to get us into port. A few days later, I found a machinist, and I had him make a another rudder arm for Nine of Cups. The new one was considerably beefier than the old one. It was made from a 1” (25mm) thick steel plate, and was held in place on the rudder stock with two 3/8” (10mm) bolts. I thought this would certainly be up to the job, and it did fine for five years.

It broke again on the last leg of our Indian Ocean passage from Mauritius to Durban. We had following seas at the time, and apparently the force was sufficient to shear one of the 3/8” bolts in half. I suspect that one of the bolts loosened up, causing a little play between the arm and the rudder. As the waves overtook Cups and hit the rudder, this play would have increased the stress on the bolt.

If it had to happen at all, we were fortunate that it occurred when it did. The weather was reasonably benign at the time and there were no ships bearing down on us. We were also fortunate in that there was enough remaining of the old bolt to grip it with a pair of pliers and back it out. Once again, we retrieved the emergency tiller and hove-to while I worked on it. It took about half an hour to remove the arm, back the broken bolt out, and secure the arm back in place with a new bolt.

Here in Durban, I increased the bolt size to 1/2” (12.7mm) bolts. The yacht club boatyard next door was kind enough to allow me the use of their drill press, and I was able to drill and the tap the  holes myself. The new bolts have an extra dose of Locktite along with lock washers to keep them tight. I am optimistic this will do the job.

It's really good I didn't pick civil engineering as my career choice. If the first two iterations of the bridges I designed all fell down while I worked out the problems, it probably would have been a very short career.