How to diagnose terminal connection problems in vehicles

Nov. 1, 2018
The intermittent nature of terminal connection problems make them extremely hard to diagnose since they often occur so fast and also so randomly. One technique is use an oscilloscope to help pick out quick glitches in a circuit that would be hard to locate with other testing methods.

When we think of failures, we often thing in terms of components since most of the time, that is what ends up being replaced especially with ignition problems and crank no-starts. Often times we overlook or never consider something that is becoming more common and that is the terminal contact inside of the connector to a component. The intermittent nature of terminal connection problems make them extremely hard to diagnose since they often occur so fast and also so randomly. One technique is use an oscilloscope to help pick out quick glitches in a circuit that would be hard to locate with other testing methods.

2013 Dodge Durango

The first vehicle is a 2013 Dodge Durango with a 5.7L engine and 52K miles on the odometer. It was towed in with a complaint of a crank-no start. The customer stated the vehicle has been running rough and had low power before it stalled. When I went outside to try to start the Durango, it started right up like most technicians have experienced with a vehicle that was towed in and as most technicians would, I instinctively thought that a fuel pump was going out.  Something else that I noticed was I could not shift out of park, but after repeatedly pressing the brake pedal the shift interlock released so I could move the vehicle.

The 2013 Dodge Durango with a 5.7L engine

When I went to pull the Durango inside it died out while making a turn. I noticed a long crank before it restarted, then after about 20 feet it stalled again and this time it would not restart. After getting some help to push the vehicle in my bay, you guessed it; it started up and idled fine. Ok, let scan the vehicle for codes and take a look at some data before testing the fuel pump. I found a stored code P0627 Fuel Pump Relay Circuit which seemed in tune with the stall and long crank that followed afterwards. Also stored was a P0571 Brake Switch Performance code which explained why I have difficulty shifting out of park.

A good first step is retrieve Diagnostic Trouble Codes (DTCs) that give some direction in deciding where to begin testing.

Shortly after the vehicle was inside the service writer came with some information that the customer has shared with them. The customer informed us that the vehicle had been brought to the dealership earlier in week to have a fuel pump relay recall performed. They only had it back for a day when the stalling started occurring intermittently and then last time it stalled, it would not restart, only crank. We were also informed that prior to being at the dealership for the fuel pump relay recall repair, they did not experience any problems.

The fuel pump relay was originally an integral part of the TIPM, but when the safety recall has been performed, it is visibly mounted on the right front of the engine compartment.

On these vehicles, the fuel pump relay is integral to the Totally Integrated Power Module (TIPM) and is not serviceable separately. It appears that the fuel pump relay is problematic and a recall had been issued for this vehicle. Safety Recall R09/NHTSA 15V-115 addresses this problem, but instead of replacing the entire TIPM assembly, the repair is to rewire some of the TIPM circuits and add an external fuel pump relay.

After getting this information I focused in on the fuel pump relay to find out if anything during the repair was left loose. I printed a copy of the recall repair instructions so I could familiarize myself with the procedure and hopefully narrow down the number of circuits to test. The connectors of the Totally Integrated Power Module are secured by a flip type lock to hold them in place so there was little chance of them not being pushed in fully. Most of the repair consisted of cutting and splicing wires of the fuel pump circuit going to the TIPM into the harness supplied for mounting the fuel pump relay remotely from the module. The repair passed a visual inspection so I attached my oscilloscope to the each of the relay circuits to determine if one of them was causing a loss of voltage to the vehicle’s fuel pump or if I need to look elsewhere.

Since the relay is mounted remotely, it made an easy test point to start looking for a problem related to the DTC and no start concern.

The location of the relay on the fender made an easy point to attach my scope leads to get direction on what is and is not working as designed. Just a heads up, if you open the hood on any of these vehicles that were affected and there is a single relay attached to the right corner of the engine bay held in by a push clip, the recall has already been performed on that particular vehicle. By attaching to the fuel pump relay I could monitor whether the PCM was commanding the relay on and also what the output voltage to the fuel pump was when the vehicle was either cranked or running. It so happened that when cranking the Durango that this time it started and kept idling. As seen on the scope capture, the green and red traces showed a lot of drop outs, which corresponded to the PCM control circuit commanding fuel pump relay on and off and the output from the relay to the fuel pump respectively. The rapid on/off of these to signals continued until the vehicle stalled out.

Reviewing the capture, I found that the Green trace which is the relay command from the PCM was the dropping out first, following closely by the Red trace, which is the voltage output of the relay to the fuel pump.

Monitoring each circuit of the fuel pump relay it didn’t take long to find a problem in the relay control circuit.
Zooming in on the capture we can see that the control circuit from the PCM is losing its signal which in turn is causing the fuel pump relay to open and causing a loss of voltage supplied to the fuel pump.

Reviewing the procedure involved to install the external fuel pump relay, I found that the relay command signal was spliced into a Pink wire with a Green tracer, which goes to Pin 38 of a 40-pin connector at the TIPM. Lifting up the TIPM and inspecting the wiring did not reveal anything loose. The brass splice band and solder joint where the relay wiring attached to the original TIPM wiring also showed good. Inspecting the pin fit in the connector revealed something else. After the technician removed the terminal lock from the 40-pin connector, he must have used an incorrect tool to push the terminal out of its cavity because the terminal was spread and there was no grab felt at all during the pin drag test. As seen in the picture, the terminal flopped around when inserted over the mating male pin. This was the poor connection that was causing the vehicle to stall out when driving or when slight movement of harness existed.

A close up of the spread open terminal connector causing a poor connection at the TIPM.

Since the external fuel pump relay and related wiring alterations were made during a safety recall, we did not perform the repair on the terminal. The customer and dealer were contacted and informed of what we found, and the vehicle was towed back to the dealership for repair. The customer was also advised of the problem with the brake switch input being the cause for having difficulty shifting out of park.

2005 Chevrolet Aveo

The next example is a 2005 Chevy Aveo with a 1.6L (L91) 4-cylinder engine and 89K miles. I was asked by the tech to look at this vehicle before he proceeded with any further diagnostics. The vehicle was brought to us for an intermittent misfire that was especially noticeable on acceleration. The original tech performed a scan for codes and a P0302 Misfire Cylinder 2 and P0303 Misfire Cylinder 3 were in history. While loading the engine on a test drive the misfire was evident. As part of his visual inspection he removed the spark plugs and noticed that they were worn out and in desperate need of replacement, so new ones were installed as a starting point however the misfire still persisted while driving. So with the scan tool attached during the test drive watching misfires, he noticed that both cylinders 2 and 3 were still the ones showing a misfire. Since coils are a very common ignition system failure component, especially under load it was the next logical assumption. Normally he would have swapped coils if this was a COP (Coil on Plug) system, but since this vehicle utilizes a 1-piece dual coil pack for all 4 cylinders, this technique would not help on this vehicle. This is a waste spark system that uses 1 coil for a pair of cylinders and fires both the event cylinder, that is on its compression stroke as well as its companion cylinder also known as the waste cylinder, which is on its exhaust stroke. Most of the voltage in this series circuit is used by the cylinder firing the plug on the compression stroke due to the higher pressure in that cylinder. The waste spark cylinder has a less resistance due to the lower cylinder pressure of the exhaust stroke.

Due to the seat of the pants feel we develop as techs, an ignition misfire has a characteristic feel that hints to us it is spark related. Going by his gut instinct, the spark plugs wires were the only option left that could cause this type of misfire. A new set of spark plug wires were ordered and installed. Confident that this would repair the concern the vehicle was test driven once again but much to his disappointment, the same symptoms remained.

So at this point is where I get asked to assist with the diagnosis. A visual under the hood shows everything was installed properly and the spark plug cables were routed correctly. I attached the scan tool to monitor misfires and go for a test drive, sure enough, under a moderate to hard acceleration the misfire occurs only on cylinders 2 and 3 along with a large jerk from worn out engine and transmission mounts.

As I pull back into the shop I was immediately asked by the tech if I thought it was ignition related? It did feel like an ignition misfire but I really needed to dig a little deeper before I could be absolutely sure. I pulled up a wiring diagram of the ignition system and then started to get my oscilloscope out. I think that most good diagnostic techs have a set routine they perform to isolate problems when diagnosing vehicles and by sticking to that regiment they cannot only prove what is not working correctly but also, and just as important, what is working correctly.

I like to try to find some commonality when I see multiple trouble codes to see if a single problem can be causing all of them. Looking at the wiring diagram for the ignition system I find that both cylinders that are misfiring also happen to be on the same coil of the dual coil assembly. The ignition coil module has a 3-wire connector with Pin B receiving voltage in Run and Start through the 15 amp DIS/Injector Fuse.  Pin A is Ignition Coil 2 and 3 Control while Pin C is Ignition Coil 1 and 4 Control. The Powertrain Control Module (PCM) controls the dwell of both Ignition Control circuits. Even though the ignition coil assembly is new it is the one item that both misfiring cylinders have in common. So with the scope attached to the ignition coil I go for a test drive and record my findings.

The blue trace shows the ignition control circuit for the non-misfiring cylinders (1 and 4) while the red trace shows the ignition control circuit for misfiring cylinders (2 and 3).
A close up look at the drop outs in the ignition control circuit for the misfiring cylinders.

Without even needing to pull out of the parking lot I get enough misfires to determine if I am headed in the right direction. Sure enough I notice something occurring on the voltage trace of Pin A of the Ignition Control Module signal which is the Ignition Control for cylinders 2 and 3. The pattern has a lot of drop outs and while the voltage signal for Pin C which controls the ignition timing for cylinders 1 and 4 looks normal. Now I can narrow it down to something in the Ignition Control circuit that is causing the signal to drop out. The downward saw tooth pattern noticed during the signal drop is caused by an intermittent open in the ignition control signal.

When attempting to perform a pin drag test at the connector for the ignition module, the terminal pushed out of the back side of the connector.

With the vehicle in the bay I performed a wiggle test on the harness to try and duplicate the loaded driving conditions and found that when moving the section near the connector of the ignition coil module the engine misfired heavily. Inspection of the harness revealed no breaks in the wiring but when removing the connector of the ignition coil module and attempting to perform a pin drag test on the Pin A for cylinders 2 and 3, the wire pushed out of the connector.

This is how loosely the terminal fit onto the mating male pin at the TIPM. A correctly fitting terminal will not wiggle and exhibit a drag when sliding on mating pin.

The locking tab of the terminal was bent as to allow it to slip out of the connector. I’m not sure if someone tried to remove the terminal at one time or as noted earlier, the powertrain mounts were broken and allowing the engine to jump when loaded which may have caused damage to the locking tab. When the engine moved and the harness flexed, it allowed the terminal to have a poor connection with the mating male terminal and lose signal causing misfires for the corresponding cylinders. When the harness was relaxed during idle and cruise conditions, the connection was good enough that no misfires occurred.

The locking tang of the terminal was bent and flattened allowing it to back out of the connector causing an intermittent circuit problem.

Repairing the locking tab of terminal fixed the misfiring concern, but the customer declined replacing the powertrain mounts even knowing what that it could lead to repeat failures of the harness and its connectors.

Finding either of these failures without the use of a scope would have been a much more lengthy and involved process, especially with the number of circuits on a modern vehicle. By narrowing down a fault to a particular circuit, the diagnosis can be performed more efficiently without unnecessarily replacing components.

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