While ETC (Electronic Throttle Control or Drive-by-Wire) might sound like a good idea in theory, both the practice and execution often leave everything to be desired, and particularly so on Jeep vehicles- as this article will recount. You may have fought battles with the ETC system on Jeeps before and you may even have won some of them, but in the case of the Jeep vs. the three young and inexperienced mechanics that we featured here a few weeks ago, a Jeep Grand Cherokee and its ASD relay-controlled ETC system were the clear winners- and by a huge margin, too. In this article, we will recount the pertinent points of the case, and explain how it was resolved with the use of an old sealed beam headlight, but let us start by stating-
We need not repeat the history of the three young mechanics here, beyond saying that a) they pooled their resources soon after qualifying as mechanics, and b), that they used their pooled resources to open a small workshop many years before they were ready to accept work from paying customers. We can also say that based on their relatively recent experience with failing to diagnose a simple misfire correctly, they admitted defeat early on in the battle, and chose to call in a consultant: this writer, to be exact. Here are the facts of the case-
The vehicle was a 2010 Jeep Grand Cherokee whose engine cranked and started normally but only ran for two seconds or so before shutting off. It would restart immediately, but only if the ignition was turned off completely before restarting. Attempting to start the engine without first turning off the ignition resulted in a no-crank condition.
According to the three young mechanics, who wrote an extensive report on the symptoms and history of the problem as it was recounted to them by vehicle’s owner, the engine first started cutting out sporadically while diving about 3 months ago. At first, it happened sporadically and unexpectedly under all weather conditions, during most driving conditions, and at almost all engine speeds, engine loads, and ambient temperatures. In short, there was no discernible pattern to the engine cut-outs, but it would always restart if the ignition was first turned off.
After a few weeks, the problem became worse in the sense that the engine would shut down at least once during each commute between the vehicle owner’s home and office. At this time, a dealership replaced the ECU and various sensors, including the crank position sensor, engine coolant sensor, and a part of the engine wiring harness. According to the vehicle’s owner, the dealer could find no trouble codes when they had the vehicle, but despite their assurances (and an eye-watering repair bill) that his Jeep would now be "like new", the opposite happened. The problem got steadily worse until the vehicle had become undrivable a few days before.
To their credit though, the three young mechanics thought of connecting a high-end scan tool, albeit a non-Jeep-specific scan tool, and managed to capture a single trouble code after many attempts to capture live data and freeze frame information during the two seconds they had at their disposal. The code was P2100, which is most commonly defined as “Throttle Actuator "A" Control Motor Circuit/Open”, although it is sometimes defined as “Throttle Actuator Control (TAC) Motor Control Circuit”, depending on the source consulted, which brings us to what the young mechanics did not know -
Nobody knows everything, but sometimes it helps when you know what you don’t know. In this particular case though, the three young mechanics’ collective lack of knowledge was so extensive that they could not reduce the problem to its simplest form, which was the fact that the Jeep did not start because there was a problem with the electronic throttle control system.
However, in the interest of fairness, it must be stated that Jeeps use a rather convoluted start-up logic that is largely unique to Jeeps, but more importantly, that Jeep has never published all the fault-setting criteria for even generic ETC-related trouble codes. Also, while some ETC-related trouble codes will set immediately when the vehicle fails to start, most of these codes are erased automatically when the ignition is turned off and an on within a few seconds. Thus, the practical effect of these combined strategies is that in many cases, it is all but impossible to extract some fault codes and their related freeze frame data if a Jeep’s engine does not run for more than five seconds.
Therefore, in the real world, most experienced technicians diagnose and resolve no-start conditions on Jeeps with the knowledge they have gained over many years. Put differently, this means that if you are faced with a no-start condition on a Jeep, Google is not necessarily your friend, which begs the question of-
Before we get to specifics, it might be worth noting that all vehicles that are made and sold in the USA (including vehicles imported into the USA) are legally required to have a system that automatically shuts down the fuel supply* in some situations, such as accidents or severe impacts to prevent fuel fires.
*Note that while this requirement does not apply to some markets, this feature is typically retained on Jeep vehicles that are manufactured in the USA and then exported to foreign markets.
This is intended as a safety feature, and while most OEM manufacturers program this function directly into the ECU, Jeep, on the other hand, elected to use an ECU-controlled electro-mechanical 12V relay to both supply power to the fuel pump, injectors, and ignition coils during normal vehicle operation, and to cut power to these systems when the situation demands that power be cut.
In Jeep-speak, this relay is known as the ASD relay (Automatic Shutdown Relay), and this relay is almost invariably the cause of no-crank and/or no-start conditions on all OBD II compliant Jeeps. Also, and to make the start-up logic on Jeeps even more “clunky”, the ASD relay also supplies power to the electronic throttle control actuator. In practice, this means that unlike most other vehicle makes on which the ETC system typically does not share circuits with anything else, issues in one or more circuits that are not directly related to the ETC system can, and often do, disable the ETC system.
We mentioned the chunkiness of the start-up logic of OBD II Jeeps earlier, and while the actual process is too long and convoluted to explain it in fine detail here, we can do the next best thing, which is to provide an overview of the 8 things that must happen (or be present) to prevent the ASD relay from "dropping out", thus cutting power to the fuel and ignition systems-
1) The battery positive terminal on the relay must have a minimum of 12 volts on it at all times
2) The ECU uses the ignition coils' primary circuits as a feedback mechanism to determine the intensity of the ignition sparks. Therefore, if the primary current is weak or missing on the ignition circuits, the ECU will conclude that there are issues in the ignition system, and it will disable the ASD relay as a result.
3) If the crank position sensor is defective, or produces erratic signals, the ECU will disable the ASD relay.
4) If the camshaft position sensor is defective, or produces erratic signals, the ECU will disable the ASD relay.
5) If the ECU detects a miscorrelation between the signals from the crankshaft and camshaft position sensors, it will disable the ASD relay.
6) If any output on the ASD relay falls below a minimum allowable threshold, the ECU will conclude that the relay is defective, and it will disable the relay as a result. Note though that Jeep has never published the value(s) of these minimum allowable thresholds.
7) If any immobiliser-related faults are present or if the ECU detects issues with programming and/or security codes in ignition keys, it will disable the ASD relay.
8) Note that on some late-model Jeeps, the fuel pump is controlled by a dedicated relay in addition to the ADS relay that controls power to the injectors and ignition coils. Thus, the ECU monitors the output of the fuel pump relay, and if this output falls below a minimum allowable threshold, the ECU will disable both the fuel pump relay and the ASD relay.
While the above sounds reasonable enough, the problem is that a) all of the above conditions have to be met during the first two seconds of cranking, and b), no faults must be detected within the first two seconds of cranking for the engine to start and run. Moreover, in some cases, some types of intermittent electrical failures in one or more circuits can cause the ASD relay to “drop out” intermittently or sporadically, causing intermittent, sporadic, and unexpected engine stalls. In many instances, these intermittent failures occur without a discernible pattern, as the owner of the problem Jeep Grand Cherokee had reported, which brings us to-
When this writer questioned the three young mechanics about what diagnostic steps they have performed to find the problem before calling for help, it soon became clear that they had only the vaguest of ideas of what is required for a typical Jeep petrol engine to start and run.
Nonetheless, all experienced mechanics will recognise that it could take several hours to verify that all eight factors that comprise the start-up logic of Jeep engines are within acceptable parameters, but there is an easy way to limit the possible causes of a no-start condition on Jeeps. Long experience with Jeeps has taught that scoping the two circuits that control the movement* of the throttle plate via the stepper motor is usually the quickest way to produce useable diagnostic data and/or clues, so the first step was to connect a digital storage scope to the throttle actuator.
* On Jeeps, the ECU switches the actuator motor’s polarity to open and close the throttle plate, and two dedicated position sensors are used to monitor the actual position of the throttle plate relative to the desired or commanded position.
The image below shows the results of the test, which is a 2-second capture with one lead connected to one live terminal, and another lead connected to the other live terminal in the throttle actuator’s connector. The ground connection was referenced directly to battery negative-
Image source: https://www.searchautoparts.com/sites/default/files/images/FIG4%20copy_0.png
Note that when the ignition was turned on and light pressure was placed on the throttle pedal, the red trace sees 12 volts, but almost immediately, the green trace also becomes active- also at 12 volts. Note the chaotic switching between the two circuits; this is clear evidence of the ECU trying to switch the actuator motor's polarity in an attempt to correct what it interprets as a stuck throttle plate.
Note, however, that the chaotic switching between polarities occurs at a substantially lower voltage, which could mean one of three things. The first is that an abnormally high resistance between the ASD relay (which supplies the ETC with power) is reducing the current that reaches the ETC; the second is that the ASD relay itself might be defective in a way that affects its output to the ETC, and the third is that there is a problem with the ECU or with the circuit between the ECU and the ASD relay. As a practical matter, remember that the ECU monitors the ADS relays’ output, which in this case, was substantially below 12 volts when the engine was cranking.
Therefore, since the relay’s output fell below the specified 12-volt value, the ECU disabled the relay’s power feed circuit after 2 seconds, and set code P2100, which implicated the throttle control actuator's control circuit(s). However, code P2100 is one among many codes that are erased automatically when the ignition is turned off, so if you cannot manage to capture it in the two seconds you have available, you won’t know it was ever set.
To put the chaotic switching between polarities into perspective, consider the known-good trace of the same function on a similar vehicle below-
Image source: https://www.searchautoparts.com/sites/default/files/images/FIG10%20copy.png
In this capture, the green trace represents the inactive, but stable circuit on the throttle actuator, while the yellow trace shows clearly defined steps in the current that the ECU is using to close the throttle plate. If the throttle were being opened, the image would be inverted; the yellow trace would be stable, while the green trace would show even, clearly defined steps.
So, given that Jeep does not publish some fault setting parameters, how do you know if you are dealing with a simple wiring issue, or with component failures, but more to the point, how do you test components to rule out or confirm component failure(s)? Well, at this point it helps if you can think creatively, or just happen to know that ASD relay failure is the most common cause of no-start conditions on Jeeps. Here are some quick tests you can run to test the relay f you don’t own a specialised relay testing tool-
This is where sealed beam headlights shine as diagnostic aids because where ASD relays are involved, there is no point in testing either the ASD relays’ power feed circuit, or the outputs leading to the throttle actuator with a digital multimeter. While a multimeter might show a reading of 12 volts, a multimeter does not load the circuit, so you won't know whether the circuits under test break down under their rated loads. With a headlight, on the other hand, you load the circuit properly.
So here is a simple trick: connect one terminal of an old sealed beam to the ASD relays’ power feed circuit at the relay, and connect the other terminal on the headlight to a good grounding point. If the circuit is good and the ECU delivers 12 volts to the circuit, the headlight will glow brightly. If it does not, or does not light up at all, there are three possibilities to consider, these being-
If the power feed circuit is good, reconnect the ASD relay, and repeat the test on all of the ASD relay's outputs using the headlight to load the circuits you are testing. In the case of the problem Jeep, one of the relays’ outputs leading to the throttle actuator only managed to make the headlight glow dimly, thus identifying the ASD relay as the problem*, which was confirmed by the chaotic switching between the circuits.
*Note that a common problem with these relays involves burnt internal contacts. Even though worn or pitted contacts may severely limit current flow through the relay, this will not necessarily cause the relay to heat up noticeably, which explains why the three young mechanics’ thermal imager did not reveal the problem when they looked for hot spots caused by short circuits and/or abnormally high resistances.
In this case, the fix was simple: an OEM-equivalent relay was obtained and installed, and the Jeep started right up on the first attempt. A subsequent test drive with a scan tool connected showed no new issues, and no new codes were stored, which ruled out lingering or incipient problems in other parts/components of the vehicles’ starting circuits and electronic throttle control system, which leaves us with this-
Although no-start conditions on Jeeps can sometimes be tricky and time-consuming to diagnose and resolve because specific repair and service information is difficult to find, it is not impossible to fix these kinds of issues with some basic tools and a good measure of creative thinking.
The trick to solving these kinds of issues is to remember that the ASD relay on a Jeep lies at the heart of the start-up logic, and as such, many problems stem directly from this relay if it does not work as intended or designed. In short, if you fix the relay, you usually fix the no-start condition- it’s as simple as that.
