Switch Hitter - Transmission Digest

Switch Hitter

When the owner was asked whether this was a problem before the truck came in to have the transmission replaced, he said it wasn’t but that the alternator had been replaced recently. Hmmm, could there be a “connection?” (No pun intended)

Switch Hitter

Technically Speaking

Subject: Charging, brake, vacuum and fuel-level lights remain on at all times
Unit: JR403E
Essential Reading: Rebuilder, Diagnostician, R & R
Author: Wayne Colonna, ATSG Transmission Digest Technical Editor

Technically Speaking

  • Subject: Charging, brake, vacuum and fuel-level lights remain on at all times
  • Unit: JR403E
  • Essential Reading: Rebuilder, Diagnostician, R & R
  • Author: Wayne Colonna, ATSG Transmission Digest Technical Editor

A rebuilt JR403E transmission was installed into a 1991 Isuzu NPR. During the initial road test it is discovered that the charging, brake, vacuum and fuel-level lights remain on (see Figure 1).

Besides this, there were no other indications of a problem. The vehicle was running well and the fuel gauge was working correctly. A quick check of all the fuses (see Figure 2) showed that none was blown. Swapping relays around didn’t change anything, either.

When the owner was asked whether this was a problem before the truck came in to have the transmission replaced, he said it wasn’t but that the alternator had been replaced recently. Hmmm, could there be a “connection?” (No pun intended)

A wiring diagram of the dash (see Figure 3) was obtained with the intent of backtracking the ground path of each bulb that was remaining on. After a good look, a notable observation was made: Battery voltage coming from fuse 5 was supplying power for most of the lights in the dash. Nothing odd here, but what becomes unique and very interesting is the ground path of the bulbs remaining lit.

The charging, brake, vacuum and fuel-level light bulbs have their own respective ground path that would normally be used to illuminate the bulb should there be something wrong with that circuit. But they each also have their own splice and diode in their ground circuits that lead them all into a shared circuit going to fuse 19.

Why would this be? Why would voltage be supplied to each of the ground paths through a diode? And isn’t it interesting that it is these four lights that remain on, not the others. It was no surprise that when fuse 5 was pulled out, every light to which this fuse supplies power went out. But when it was put back in and fuse 19 was pulled, only the four lights that were remaining on went out. How odd is that?

This would seem to indicate that somehow fuse 19 was acting as a ground path for all four lights instead of a power supply. After all, if each of the individual ground paths was somehow shorted to ground, they would remain on with fuse 19 removed. So the question was, “Where does fuse 19 get its power?” To answer this question required a different wiring diagram (see Figure 4), and from it we can see that fuse 19 receives power through the charging relay – that is, if everything is working correctly.

After a careful look at the relay circuit in Figure 4 you can see that when all is well, the alternator supplies power from terminal 1 to energize the charging-relay coil, which closes contacts inside the relay and allows power to be supplied to fuse 19. Power from fuse 19 then is sent to each light’s ground circuit. And since each of these circuits does not ground until there is something wrong, these bulbs would remain off. However, if one of the circuits did go to ground to warn the driver of a problem (such as low brake fluid), the diode in the circuit is sufficient to prevent the power supply coming from fuse 19 from interfering with the normal function of turning that light on.

But, if the alternator does not energize this relay coil or the coil in the relay is burned out, the contacts in the relay are normally connected to a ground path. This then would make fuse 19 act as a ground path, lighting up all four lights, which is exactly what has happened with this vehicle. The coil in the relay was damaged and would not energize sufficiently to allow the internal contacts to pull together and supply power to fuse 19 (it may have been damaged by the previous alternator). Instead, it remained grounded, keeping the four lights lit.

So the charging relay is a switch hitter, as it could provide either a ground path or a power supply. Normally its function is to provide power. When it provides the ground path and puts on all four lights, it means that either the alternator is bad, the wire from terminal 1 at the alternator to terminal 4 in the relay is broken, or the relay is malfunctioning. This charging relay will not interchange with the other relays, as it operates slightly differently, and this explains why it is the only blue relay in the panel (refer to Figure 2).

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