Bad ECM?

[H8PVMNT]

So I figured out how to get really great gas milage from my '88 TBI 454... Pull it behind a Dodge.

We recently aquired a 1988 1 ton crew cab dually truck with the TBI 454. Lately it will intermittently blow the ECM fuse. I have looked for bare wires etc and made a few repairs, but the truck still blows the fuse every so often. Usually after fooling with things and replacing the fuse it will fire up again.

Tonight while towing a loaded horse trailor complete with a horse it blew again, this time replacing the fuse didn't work, it just keeps blowing them out. Nothing I can see on the surface should be causing a short. One guy told me he had a similar problem and he had a bad ECM.

-Oh yeah, we have what we believe is a bad fuel pump on the driver's side tank. I allready replace the valve/switch dealy that switches from one line to the other... When we try to use that side it doesn't get any fuel. Could a bad fuel pump blow a fuse?

Does that sound right or could I be missing something else? I'd like a few ideas before I go hunting for an ECM...

Three beers for the guy who gives a correct diagnosis...

[H8PVMNT]

The bed is much lighter than I expected! Replaced the bad fuel pump with a new one. Got fuel coming from both tanks now but still blowing fuses. Searched a bunch of wires but found no suspects yet. I will order up an ECM tommorrow... Those things are actually not too expensive. I'm used to Toyotas where an ECU would be like $1200.

While I'm in there with the bed off for easy access I guess I'll do u-joints and rear shocks as well.

[79k30dually]

The TBI injector wires are twisted so not to pinch them under the air cleaner. Remove the air cleaner and check the wires. In the mean time I'll look at a print for other options, Good luck, Ron

[H8PVMNT]

I just went out and checked the wires to the injectors and they appear to be sitting twisted and out of the way just fine, but you could sure see where they could get pinched... That is a good tip right there! Thanks!

[H8PVMNT]

Replaced the ECM this morning but still blowing the ECM 1 fuse...

Now to the tester and chasing wires?

[79k30dually]

Could you double check the fuse lable, I'm not finding an ECM 1 fuse on my prints. Ron

[H8PVMNT]

Yeah it says ECM 1, right next to the ECM B fuse. After feeling around for burnt and frayed wires in the engine bay it has started once again without blowing the fuse. Sadly the only bare wire I have found was a chaffed alternator lead which didn't appear to be touching anything at the time. Tommorrow I will unplug the coil wires and get them out where I can see them. I suspect that's the problem. I was really manhandleing them before it started. Something there must be making contact.


Pulled the driveshafts and started putting in new u-joints this evening...

[bigblktrk]

The ECM1 slot is a 10 amp fuse and from what I see there is a junction in the harness of 6 wires. GM likes to crimp and solder these then wrap them in fabric tape. An easy way to find it I believe the red wire from injector 1 and the white wire from injector 2 tie into it. If I remember, it is on the passenger side between the head and intake. The other 4 go to the ESC (Electronic Spark Control.. won't apply to you 7.4 does not have this), the ECM (bundle of wires goes through the firewall on passenger side by rear of engine, AIR diverter valve (located by AIR pump), and the EVRV (EGR control) Valve (The vacuum line comes off it to the EGR valve).
The wire is a pink/black wire and is ignition on.
Don't be confused with other pink/black wires other than the above ones.
There is a pink/black wire that comes off of the ignition fuse and feeds a lot of the coils on the relays.
Hope this helps.

[H8PVMNT]

OK so I think I've done it. I found the ECM ground hanging by a thread, a bare spot on the alt cable, the "lean out" 4 gear wires to the trans had a rubbing chaffed spot and the main lead to the coil had a chaffed spot as well. The bundle of wires on the passenger side between the head and intake was a damned mess with the conduit all petrified and crumbling off. I fixed, repaired and rewrapped where necessary and put it all in new plastic conduit, the rewrapped some more. On top of that I took out about 2 and a half aftermarket ghetto trailor wiring harnesses with suspicous constant hot wires that were dangling and chewed all over the place. Talk about an old man wiring disaster... It runs and starts without blowing the fuse and it's about 40 lbs lighter .

[H8PVMNT]

Thanks for the input guys it did help. I'm starting to know my way around this thing now...