A customer truck had been plagued with intermittent no start problems. For instance, once it was in for a service and it started on the lot to pull it in for the work, again after the oil change, then again to pull it out and park it.
When the customer came to pick it up the next day however, it wouldn't start and was throwing codes for low IDM volts.
I dragged the thing into the shop, where it naturally started right up.
Testing the IDM circuit
The standard test for the IDM circuit is to plug a sealed beam headlight across pins 30 and 87 of the IDM relay socket. I did so and the headlight lit brightly and remained lit for over a minute, so the circuit passed the test.
The relay was also a possibility, so I swapped it with the horn relay and blew the horn several times, including a long blast causing a negative reaction from my shop mates.
Even though the circuit seemed good I had to check the battery box. The Weatherpack connectors to the IDM and ECM are prone to problems in the 4300 model. However, on this truck I had already cut the connectors out of the harness and soldered the wires together in an earlier version of this same issue. The battery cable ends looked OK, but I've been doing this long enough to know that just looking isn't good enough, particularly when you know there is a problem. I took all the ends off the batteries and cleaned them all up, although they appeared good when taken apart.
Back again
Despite the fact that I didn't find a problem, the customer needed the truck. A few days later the truck was again at the shop. One of the other guys had gone and jump started it after it again failed to start and the driver had cranked the batteries dead. Now it wouldn't start for me, either. Before pulling it into the shop I wanted to try to see what was up while the no start condition was active.
The scanner pulled codes for the IDM and batt volts, but those were inactive and probably the result of the batteries being cranked down.
I was really getting tired of this truck and this problem and the customer was less than pleased also. I really needed to nail this thing somehow. Sometimes a fresh pair of eyes is needed for something like this, so we had J. take a look. He is the resident shop genius at electrical diagnosis and the go-to guy for really tough problems in general.
He couldn't find anything, either. However, a couple of times during all this the thing did fail to start, but without attendant codes.
4300 fusebox corrosion
Above the driver steer tire is the fusebox containing the module relays and fuses. I took it apart to check the connections on the back of the relay socket. On 4300 models the box is prone to corroded contacts often causing no starts. This one was clean and all voltages were getting through. Now it was time to really look close at the rest of the IDM circuit.
The wires that feed the IDM and ECM relays are wrapped in fabric tape and run along with the battery cables. However on this truck somebody had repaired the circuit to the IDM relay by splicing in a piece of wire. The splices were covered with tape, which is always a bad sign. Under the tape were shrink type butt connectors. They were good quality, glue-lined connectors, but they had been crimped with a crimper for uninsulated connectors, which pierced the insulation and let road salt into the splice.
When I cut these connectors out to redo the work I noticed the wire had also been pierced with a test light not once but in 3 separate places, and those spots were also corroded. I replaced the splices with new wire and uninsulated butt connectors, which were then soldered with rosin core solder. The ends of the wire could have just been soldered together but I find it easier and neater to first connect the ends with a butt connector, then add solder to the joint. The connection is then covered with glue lined heat shrink tube for a weatherproof seal.
After this repair the truck started every time and also the next day. It has now been in possession of the owner for over a month with no issues.
Proper circuit repair is critical
This no start experience highlights several things:
- Never pierce a wire's insulation to check a circuit. You may get away with it inside a cab or another place protected from the weather, but certainly not under a truck and not on a critical, high amperage circuit.
- If you use shrinkable connectors for a repair, use the proper crimper so the insulation isn't broken.
- Don't even bother using electrical tape to protect a connector from the weather. It just doesn't work. At least use a shrinkable, glue lined connector. For any circuit to a computer or a circuit critical to vehicle operation, use a soldered connection covered with glue-lined shrink tube.
- I was always told that the headlight test for the IDM circuit was definitive, but apparently that isn't so. In this case it looks like there was enough circuit capacity to light the headlight, but not enough to power the IDM.
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