Blows ECM-B fuse on start-up after OPS relay mod installed - DTC included below

[95Burbnc]

Troubleshooting Checklist:
Problem: Explain your problem in concise language Cranks, Starts briefly then quits -blows ECM-B fuse (20A)
I'm just finishing up some mods including new GP relay and harness; OPS relay mod & PMDCable ext. kit for bumper mount. If the new OPS relay is unplugged, the fuse doesn't blos and the truck starts normally. Did the OPS relay mod according to instructions. Used +12V from power center post and connected cut gray wire on OPS wiring to relay & LP as instructed. I've wired these relays before).

Demographics: tell us about your truck and your ambient conditions:

- Year: 1995
- Truck model and class (ie: c2500) K2500 Suburban
- Automatic or Standard Auto
- Mileage: (indicate miles or km) 185K Chassis / 14K rebuild engine
- Replaced parts and mods (IP, PMD, chip/PCM, TM, injectors etc) 4" Exh, FTB, Chip, Dual Thermostat, D-Max Fan, Big Oil cooler & repl block fittings & hoses, K&N, PMD relocate, PTI Wiring Solutions Batt & GP cables
- Model: ‘S’ or ‘F’ engine, NA or Turbo, EGR? __F____________
- Ambient outside temp: (indicate *C or *F) __60________
- What fuel are you using? (#1 Diesel, #2 Diesel, Bio, VO, WVO, etc) __#2_______
- What fuel additives are you using? __Lucas_________
- Where are you located? _NC____________

General maintenance: inspect and evaluate the condition of each. When were they last changed?
- Air filter __14K_______
- Fuel filter _new________
- CDR Valve? __14K_______
- Coolant – last flushed/replaced at _14K____________
- Oil – synthetic/regular, changed at _3000K__________
- Batteries – state of charge, (matched set) replaced at __New, Interstate pair___________
- Battery cables and grounds (removed, cleaned and tightened): __PTI Wiring Solutions____________
- Glow Plugs - how many miles on them? _14K________ What type? __Stock___________
- Injectors - last changed at __14K_____________


History: (Describe how the problem first appeared.)
- What were you doing when it happened? __Starting engine after OPS relay mod & new GP relay / GP Harness and PMD relocate____
- How was the truck running fine before this problem appeared? _running fine____________
- Has this problem ever happened before? __No_____________________
- Describe any related factors that might be part of the problem. _Recent install of OPS relay mod & new GP relay / GP Harness & PMD Cable extension.
- Have you checked for DTC (Diagnostic Trouble Codes)? (Y.N) ____ And? _none_______


Diagnostics:

1] Starting:
1a] Does the engine start? _*Yes - if remove new OPS Mod relay________
1b] Does the engine crank over? __Yes_-then fires/starts briefly but immediately shuts down and ECM-B fuse blows 20A_____
1c] Are your batteries fully charged? _Yes________
1d] Have you removed, cleaned, and tightened all battery connections? _Yes_______
1e] Does your Wait to Start light come on for a time? How long? _N/A________
1f] Does your glow plug light come on? _Yes____ For how long? _- 15 sec? (normal)________
1g] Do you see smoke from the exhaust while cranking? What color? __no______
1h] Does the Service Engine Light Glow during Start/bulb check? _not sure_______

2] Fuel Issues (if not running):
2a] Can you hear your lift pump working? ____________
2b] Do you get a steady flow of fuel from the water drain (T-handle) ? _________
2c] Loosen the inlet line from the IP – is fuel coming out (Lift Pump on)? ___________
2d] Loosen an injector line and crank the engine - does fuel come out? _________
2e] Do you hear a vacuum (hiss) when you remove the fuel cap? ___________

3] PMD / IP Issues
3a] Location of PMD (on pump, on intake, remote mounted) _remote__________
3b] Condition of Extension harness (if used) Vendor or Homemade? _PMDCable - brand New________
3c] Condition of grounds / ground wires to IP / PMD. Check Harness. _cleaned________
3d] PMD Make: _Stanadyne__________
3e] PMD Age: __3 yrs___________

_________________________________

 

[bk95td]

I can't see the relay causing the fuse problem. I'd look at the glow plug controller. Are you positive you got the right year controller? 93 and older looks the same but is different.

 

[THEFERMANATOR]

Recheck your wiring to make sure you got the relay connectors hooked up correctly. The ECM-b fuse powers the fuel pump and an ECM feed.

 

[buddy]

Make sure the relay ground is on 86, and the grey wire from the OPS is on 85, and the wire to the LP is on 30 or 87, and the battery power on the opposite 30 or 87.

Maybe you put the ground on 30 or 87

 

[WarWagon]

X2 on above, you are grounding the fuel pump power with the relay - bad connections or rare, but possible, bad shorted relay.

 

[JMJNet]

Agree, you may criss-cross some cable. The ECM-B can blow even when you jump the wrong pin in the stock relay housing.

 

[95Burbnc]

I found this mod while reading about how to troubleshoot sudden shutdown problems. With your help I found that the LP wasn't running as a result of a dead OPS. So I replaced the OPS and the relay in the power center as well and installed the OPS relay mod to avert future problems.

In implementing this mod, I found relays with the built-in diode and a relay socket for a clean install.
RELAY: R-50ASD http://www.wiringproducts.com/contents/en-us/p564.html
RELAY SOCKET: R-73570 http://www.wiringproducts.com/contents/en-us/p565.html

Well, I found that I had reversed the polarity of the coil wires (85 & 86) so I fixed that easily enough in socket, but didn't think this would cause the problem. I thought the coil polarity didn't matter, but perhaps with the diode that is not the case because diodes are polarity sensitive.
To test the other wiring I jumped +12V from pin 30 to 87 the lead running to the LP, I could hear the pump running. so I re-installed the relay and the fuse blew once again. I had purchased 2 relays - just-in-case - so I tried the other relay and the fuse no longer blows. So, I either trashed the first relay or it was defective.

I am not certain that all is well with my mod, however, because I do not hear the LP continue to run after I shut off the engine, as I understand it should. This '95 model doesn't run the LP on ignition like the '96 and newer ones do but I may get another relay and add Buddy's circuitry to enable this feature.
I did a test drive and didn't experience the sudden shut-off that was occurring but I need to go back and open the overflow valve with the engine running to see if the LP is actually running or not. I'll check-in after I do that test.
Thanks again for your help!

 

[95Burbnc]

I have a working lift pump again with the OPS relay mod installed. Open the valve & the fuel just pours out. Still don't hear the pump after shutdown but at least I know its working.

 

[buddy]

The only time you will hear it after shutdown is when the oil is cold, high viscosity that wont bleed pressure from the OPS so fast. So start it up and shut it down 10 seconds later on first start of a cold morning.

 

[95Burbnc]

Thanks Buddy - if we get a cold snap again here, I'll try it out.

 

[buddy]

The reason the fuse would blow is that a diode allows current to flow only one way. With the coil polarity correct, current can flow through the coil, which is about 80ohms of resistance, so the 12V drops across 80 ohms. Using ohms law, V=IR, then 12/80= 0.15 amps current draw by the coil. Thats why relays are used, low current draw from the trigger source, so the load can pull high current from the power source. This way the ignition switch or OPS doesnt have more current through it.

The diode will prevent current going from positive to negative when polarity is correct. And what it does is allow current to flow from the load to the positive when you shut down the circuit and the magnetic field induces a voltage on the load line up to 1000VDC, even though they are not physically connected. The reason it does that is because the higher voltage will see both the path to ground and positve 12V as lower potential and travel back around the coil, and ultimately through the coil once again to disperse the energy.

When the polarity is reversed, you have a diode allowing current to flow straight from positve to negative, and thus a dead short, pop goes the fuse.

Now to be truly accurate, electrons actually flow from a lower potential to a higher one, so current techincally flows from ground to positive, but back in the day everyone decided it made more sense to draw everything as being from positive to negative, so the world designs all components and draws it this way.