Technically Speaking

• Subject: Intrusion of engine coolant into fuse and relay box
• Unit: JF506E
• Essential Reading: Rebuilder, Diagnostician
• Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor

The Isuzu/GMC Forward Tiltmaster truck (otherwise known as the NPR/W4/4000) with a diesel engine and the JF506E transmission has a setup for recovering engine coolant that has been known to cause hard starting or a no-start condition. And it always seems to become a problem after the transmission needs repair; at least, that is what the customer says until you finally figure out what is the real cause of the problem.

The reserve tank used to capture the expanded engine coolant is placed into a bracket and attached to the frame rail with an overflow hose that drips directly onto a relay box, as you can see in Figure 1. Inside this box are four relays: the exhaust-brake relay, the starter relay, and glow-plug relays 1 and 2 (see Figure 2).

I think the pictures are self explanatory, especially when you learn the function of each of the relays contained in the box. Of course, when the vehicle is new there is a cover on the box to protect the relays and the drip hose is bracketed, directing the dripping away from the box. But these trucks are worked hard, and in time the drip hose gets free from its bracket while the cover either is left off or falls off. As a result, the relays are subject to a sporadic dripping of water that can affect any one or a combination of these relays.

Obviously, if it is the starter relay we could have a no-start complaint. If the exhaust-brake relay fails there is no engine-braking assistance to the transmission, which could be the reason the transmission found its way into the shop for repair in the first place. If it is one or both of the glow-plug relays we could have a hard-start or no-start complaint. The glow-plug relays are controlled by the QOSIII controller to provide quick pre-heating, quick after-glow and after-glow operations. And there is nothing worse than getting both your pre-heat and after-glow cooled off with a little water! So if the customer tells you they never had a hard-start problem until you repaired the transmission, you will know he is all wet.

One other tip: If the glow-plug relays are compromised by a heavy dose of water intrusion, the No. 5 fuse, which is the power supply to the relays, will blow. That fuse is also the power supply to many of the indicator lights in the dash, and if it is blown these lights will not illuminate (see figures 3 and 4).