Knowledge Equals Power

Diesel World Spring 2006

Instrumentation Basics for your Diesel

Chances are you have an interest in taking care of your diesel engine because unless it’s a small car, the option price was in the thousands pf dollars over and above any standard gas engine. Modern electronics go a long way to preservation, but keeping an eye on that pricey power plant adds both protection and usable information.

Most new diesel trucks have instrumentation that covers the basics: coolant temperature, oil pressure, voltage, and a tach, you might get automatic transmission fluid temperature on a few. In some cases these gauges are properly numbered and quite responsive, in others they appear to operate simply as analog warning lights.” For the owner who wants more, or more precise information, or for the older truck that haunt got the basics, an investment in gauges is worthwhile. Also extremely useful to many diesel pilots are exhaust gas temperature (EGT) and manifold pressure or boost gauges. On a modified truck these should be considered mandatory, yet they serve well on stock trucks as well.

Gauges typically show data in one of three methods: analog, digital or some blend of the two. Analog gauges are the traditional round instruments with markings from 90 to 270 degrees of sweep and a needle indicating the current value. These are often the easiest to see transitions and read quickly because you’ll become familiar with where the needle normally is, and a rising or dropping needle is instantly detectable.

These gauges typically measure 2 1/16-inch, 2 1/4, 2 5/16- inch, or 50- and 75-mm in diameter, before choosing one solely on diameter make sure there is adequate mounting depth as well. Also check the illumination patterns and color to make sure you can read all of it at night and some high tech cool-looking color won’t dazzle your eyes after 15 or 20 minutes: Generally speaking, red or deep amber offers the fastest eye recovery time, and green ls often considered easiest on the eyes over long lengths of time.

Analog gauges are the only type that may offer a choice of mechanical or electrical operation. Mechanical gauges are always on” and usually have a wider needle sweep, but not everyone prefers them and some don’t offer pressure isolators to keep fuel or hot oil or water out of your dashboard in the event of a line or gauge failure. Digital instruments allow much finer detail of data transmittal, down to a single degree or psi for example, but often can’t be read as fast. For example, the numbers 1355, 1555 and 1855 take longer to decipher than a needle at 9 o`clock, 12 o’clock or 3 o`clock, and while 1355 may be a safe EGT in your truck, similar 1555 and 1855 displays probably aren’t. More and more digital displays are including a choice of analog outputs,  essentially an  electronic  picture of a dial/needle arrangement. Some are designed simply as displays, others double as electronic controllers, with even PDAs are getting into the act, and with the fleidbility of electronics displays should continue to offer more and more options.

Whatever you choose, put it in a suitable place. We can’t tell you how many trucks were seen with gauges lined up on the transmission hump or obscured by a tilt wheel. Pillar pods are quite common and put the gauge in clear we; on the down side they often make the pillar even wider and it remains to be seen how theyIl get along with side curtain airbags. Be aware that even modern pickups have lots of sensitive electronics under the dash,and running something hot or a heavy electric load near it may cause problems. Avoid using T-junctions for a temperature sender because temperature readings require fluid flow past the sensor, and a T-junction will not always give an accurate Indication. Your engine (and gearbox or aftermarket diff cover) may well have one or more ports suitable for adding a gauge.

Debate varies on ATF temperature sensing, with many recommending the pan because a sender in a cooler line might weaken the line and restrict the flow through the cooler (aim for 1/8-inch or less intrusion) or cause a pressure drop. Ideally, ATF temperature is measured at the hottest point, but If your rig sends fluid directly from the gearbox to cooler and back to pan, the pan may be the best choice. Many manufacturers publish gauge install tips in Web sites and catalogs. Diesel clubs like the Turbo Diesel Register (Dodge), the Diesel Page (GM), and Power Stroke Registry have model specific mounting systems and tricks.

Here’s a quick rundown on how to make your diesel gauges work for you

Oil Pressure

The most important factor, especially during startup and low-speed hot operation. Could indicate the wrong grade of oil, low level, high oil temperature (if you’ve no gauge for that), worn oil pump or bearings, or a leak-including a line to or from your turbo(s).

Exhaust Gas Temperature

Behind oil pressure, most important on modified trucks. Your diesel engine has a limit on exhaust gas temperature, above which pistons may be holed or melt to cylinder walls, or some other catastrophic engine failure may result. Limits vary by model and application. An early Dodge diesel, a recent Dodge diesel and a motorhome all with 5.9-liter engines, or Power Stroke ln a pickup versus a VT365 in a delivery van might have different limits because of changes in pistons, valve seats and so on. But most will generally fall in the 1250-1450 (degrees F) range. Pickup Duramax EGT limits are 740 C (about 1365 F) continuous and 780 C (` 1435) for transients less than five minutes. Limits above are quoted by the engine manufacturer at “turbine in” meaning the temperature is measured ln the exhaust manifold, you can measure it after the turbocharger but that max number will be 300-400 degrees F lower than the turbine in value. EGT can also be used as a fuel economy gauge since fuel in a diesel equals heat. If you can cruise at 500-600 F instead of 800-900 F you will likely get better mileage. Smoke out the tailpipe is indicative of unburned fuel and means high EGT. however, many thermocouples will not read and display spikes and a puff of smoke (and fraction of a second of high EGT) is not likely to do permanent damage.

Fuel Pressure

Applies particularly to electronic control Dodge Cummins prior to the 2003 model year common rail. Low pressure from a bad transfer/lift pump will eventually kill the (much more expensive) fuel injection pump on the engine.

Oil temperature

Your oil pressure and grade are all based on nominal operating oil temperature, and while water may warm up quickly it takes a while to heat 3 gallons in that brick of iron to the point that clearances and fluids are all ready for full boost. lt also reflects work load because of both bearing stress and spray coolers on bottom of pistons.

Coolant Temperature

Also useful for knowing how hard the truck is working because it spikes less than EGT but faster than oil temp. May cycle up and down as thermostat opens and closes under low loads. Could be first clue to slipping fan belt, or that the fan clutch may soon engage and you could be down 22-30 hp. For trucks  with  added  lights,  winch, bumper, etc., modified power, or all, you can set power/throttle to balance EGT and coolant temp.


This runs higher in city traffic and towing hills, and lower on level cruise where the torque converter ls locked. Any slippage such’ as idling at traffic lights or low-gear acceleration, added load, or an exhaust brake tends to make it rise. Most stock trucks when operated within design parameters will rarely indicate more than 210-220 F on the factory gauges. Temperature limits depend on manufacturer and fluid used, but on average we found the recommendation (using typical ATE not synthetics) was normal operation in the 175~230 F range and added cooling (or less load/speed) at temperatures above 270-275 FZ.

Voltmeter / Ammeter

Voltmeter measures just that, system voltage. Will be lowest when cranking or when glow plugs/manifold heaters are running. Many owners with multiple batteries on solenoids or isolators use a switch for checking alternate batteries. An ammeter will show current loads to/from battery and be first indication of a broken belt. Because of high loads in diesel pickups, shunted models should be used to avoid accidental firewall arc welding.


Shows pressure of air being shoved into engine, and will drop with loose intercooler or compressor hoses, a blocked or leaking intercooler, and may indicate restricted air or fuel filters with low output.


Useful for trucks that spend a lot of time idling a PTO at a job site or running very low speeds as ln low-range four-wheel drive. If the odometer breaks your records will allow you to calculate average miles/hour (usually 30-35 for mixed use travel) to determine maintenance intervals as



Tires and gears change speed indication but also may affect stability and traction control systems, ABS, cruise control, and automatic transmissions. Always reset for accuracy after a tire size or axle ratio change.


Should be used to avoid lugging, minimize engine speed until lubricating fluids are warmed up, minimize over-speed by downshifting manual too early, and to verify that torque converter is locked up.


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