Auto Service World
Feature   December 1, 2003   by Jim Anderton

Making the Right Connection

Installing replacement electronics requires care and attention. Bad connections can give trouble that mimics a bad unit.

The scan tool isolated the circuit, and a little component testing confirmed it: the TPS is bad. Or the MAP, MAF, CTS sensors. Or maybe the ECU itself. In each case, the unit has to come out, and a replacement installed. Screws and bolts are a no-brainer, but what about the electrical connections? While they look like “plug and play” in fact there’s a lot going on between male and female contacts, and a lot of it keeps the current from flowing properly. How do you avoid problems? A good strategy is to start the thinking process before you remove the unit in question. How does the harness and connector look? As part of the test procedure, you likely removed a multi-pin connector and probed the pins with a ‘scope or multimeter. Before searching for power or signals, however, are the contacts corroded or dirty? The most common problems are caused by borderline cases. The problem originates in the materials used for the connector pins and sockets. Electronic sensors and actuators normally have pins (male end) molded into the unit housing, with a female connector attached by a locking tab, wire bail, or by friction alone. The pins are commonly aluminum, mainly because it’s cheap and conducts electricity well. It also has the handy property of protecting itself against corrosion by forming an incredibly thin oxide layer (think millionths of an inch) instantly when exposed to air. The oxide layer is essentially a ceramic, and like a ceramic, electrical conductivity isn’t a feature. The system works by scratching the oxide layer with a snug-fitting connector. Unfortunately, you usually can’t see or feel how close the interference fit is, especially if the connector uses an o-ring seal. And breaking into the circuit to check resistance across the pins is unreliable and time consuming. A good solution? Using a spark plug magnifier, the kind with an integral flashlight, look into both the female and male ends of the connection and check for a good contact. What you’re looking for is white, powdery corrosion in aluminum pins and sockets, and telltale scratches showing tight fit. Magnification helps here because modern low-current sensing circuits use small connector designs. It’s important not to confuse residue from white dielectric compound for the dry, powdery corrosion products. Is it bad? Consider replacing the connector along with the reman part it plugs into. This may be the only reliable option if a multi-pin female is badly corroded, as it’s usually difficult-to-impossible to get them clean enough for a good contact. If you’re old enough to remember replacing the field/sense circuit two-wire connector on reman Delco Si-series alternators, you’ll remember how it drastically reduced comebacks, and was often mandatory for a warranty claim. Modern fuel injection and emission control circuitry uses more connections, smaller contacts and lower current, so it can be cheap insurance if replacements are readily available.

The connector is good (or new) and the reman unit is in place. Plug in and go? Maybe. Silicone dielectric compound i.e. spark plug grease, is a good practice anywhere where moisture is present, but it’s important to remember what the ‘di’ in dielectric means: Insulation. Use too much and there is a risk of insulating the very connection you’re trying to protect. Use a thin film at the mating surfaces of the plastic shell and a little to lubricate the o-ring, if any. Don’t forget to read the label if you’re not sure about the product. Ford for example, has an excellent dielectric compound (part number F8AZ-19G208-AA) that’s great for many applications, but can’t be used on high-voltage connections like coil or spark plug wires.

Is there any way to enhance the electrical contact before assembling the connector? In a word, yes. Compounds that help conduct electricity (as opposed to the dielectric grease) are available, and do help maintain conductivity in tough applications. They are dispersions of finely powdered conductive metals like copper in a petroleum base. The opposite warnings to the dielectric grease apply: get it on the pin or socket, but keep it out of anything else! The stuff is extremely conductive and careless use can create a mystery short or signal dropout that can be a bear to track down.

If these procedures seem elementary, you’re right. But they’re ignored in a surprising number of shops, especially where high workloads and slim margins mean turning vehicles at ever-faster rates. The time needed to really inspect, and even replace, a three-dollar connector can pay off big-time when it’s attached to a three hundred dollar sensor or actuator. Sometimes comebacks are unavoidable, but when they happen, confidence in your connections makes troubleshooting faster, if not easier.


GM MAP/pressure sensors:

The “General” has several iterations of this part, but three are closely related. GM #2503679 (Standard # AS 4) is a common sensor good from engine vacuum to atmospheric used in a wide variety of GM vehicles. GM # 16009886 (Standard # AS 5) is used on turbo applications like the 91-93 GMC Syclone & Typhoon 4.3 turbo, 87-90 Pontiac Sunbird Turbo 2.0 and 84-86 Sunbird Turbo 1.8. They share common pinouts, with a ground, sensor output and +5 volt connections, with different connector keying. Don’t assume that the manufacturer has installed the wrong connector and alter (remove) the keying! You might get away with a turbo unit in a normally aspirated car, but it definitely won’t work the other way around.

Injector Oddities

Reman injector components are now so cost effective that replacement is often more efficient and cost effective than bench repair or component cleaning. Be careful however, to note the little things when disassembling fuel injection systems. Take 1987 4.3L GM light duty truck engines, for example. This TBI unit uses injectors with different flow rates for each throttle bore! Units with this peculiarity are model numbers 17087090 and 17087162. The throttle-lever-side bore uses an orange-green injector with an OEM part number or 17111928, while the TPS-side-bore carries a pink-brown injector with an OEM part number of 17111929. It may not seem to make sense, but orientation is important in this application.

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