ECM B fuse keeps blowing

[gjr64]

Hi Folks needing some help. Battery got hooked up backwards for a few seconds and now everytime we try to start it the ecmB fuse blows immediately. I tried unhooking sensors to see if I could find one that was shorted but am not getting anywhere. Any ideas? Could it be the ecu itself? In a pinch and need to get this running. It's an 87 1/2 ton, manual with tbi 305 all original.

 

[Cuba]

Is it still carbureted or is it TBI?

 

[gjr64]

it's an all original 305 TBI

 

[Cuba]

Ah ok...I don't remember the exact year they transitioned. The TBI injectors tie into one of the ECM fuses.

Also there's a thread on here that contains all (or most of) the service manual pdfs. Many also include the schematics. That should definitely help track down the issue. I doubt you cooked the ECM, though it's possible... there are many fusible links & fuses that help prevent that.

 

[Cuba]

Here's the link to the wiring schematic thread.

http://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

Also the ignition switch comes to mind as well.... this ties in the same circuit.

 

[gjr64]

ok thanks I've also found some threads talking about the oil pressure switch shorting out as a cause

 

[Cuba]

No problem and good luck. Keep us posted with your findings!

 

[gjr64]

so I've unplugged the fuel pump relay and the oil pressure sending unit and the fuse still blows as soon as I reconnect the battery. I then unplugged the ecu and it does not blow the fuse. I try to connect either one of the ecu connectors and it immediately blows the fuse again. ECU?

 

[Cuba]

I wouldn't jump to that conclusion just yet. You need to "pinout" those wires going to the ECM. For 1, to rule out one of the wires to it... could be melted with another wire.
2nd, you don't want to destroy or burn up another ECM by not verifying the harness & its wires. Go through the schematics.. & do the pinout (measuring the resistance between 1 end of the connector to the other, ex., from ECM to sensor and so forth)

To do a pinout properly, be sure to keep the ECM connectors unplugged and whatever the other end of the connection, so you don't get false readings.

 

[Cuba]

This may be the correct one. 87 305 TBI
Sometimes the colors vary by year but the components should be the same.

A1- grn/wht- this wire is used to energize the fuel pump relay.
A2- not used
A3- not used
A4- gry- to EGR relay.(optional) This is a ground from the ECM to control the EGR relay.
A5- brn/wht- SES(service eng soon) light. This is a ground to turn on the light.
A6- pnk/blk- switched 12v from the ignition relay.
A7- not used
A8- orn- ALDL Serial data. This is your ALDL pin E data for your lap top.
A9- wht/blk- ALDL pin B diag mode. When jumpered to ground will set the ECM to ALDL diagnostic mode.
A10- brn- VSS speed sensor signal to the ECM (optional)
A11- ppl or blk- MAP sensor ground. See ECM ground thread.
A12- blk/wht- System ground. Can be tied with D1. Goes to engine ground.

B1- orn- 12v fused from battery. I use an inline fuse on this. It also ties into C16.
B2- tan/wht- Fuel pump signal from the relay. This one ties into several places, the fuel pump, the relay, and the ecm. When the relay is on it sends 12v thru this wire to turn on the fuel pump, at the same time it tells the ECM the fuel pump is on.
B3- blk/red- EST reference.
B4- not used
B5- ppl/wht- EST high reference
B6- not used
B7- blk- ESC signal This is the knock sensor signal to the ECM. The knock sensor circuit is optional. If not used, tie this wire to C14 the 5v reference.
B8- dk grn- AC signal. This tells the ECM that the AC is turned on. This can be used to set the idle speed higher when you turn the AC on. I use it to increase the idle when using the compressor for on board air. Same thing!! Just tie it to the wire going to the AC clutch.
B9- not used
B10-orn/blk- Park/Neutral sw. This is optional but I have hooked it into the park sw on the tranny. It tells the ECM when you are in Pk or N. It can be used to give a slight adjustment to idle speed when switching from park to drive and back.
B11- not used
B12- not used

C1- not used
C2- not used
C3- IAC- Idle air control
C4- IAC
C5- IAC
C6- IAC -The ECM controls the IACmotor. It adjust the amount of air that is bypassed around the butterflies to adjust the idle speed.
C7- not used
C8- not used
C9- ppl/wht- Starter crank signal. Goes to the small terminal of the starter. The stock scout wire is white that goes there. Tells the ECM you are trying to start the engine.
C10- yel- Temp sender signal
C11- lt grn- Map sensor signal
C12- not used
C13- dk blu- TPS sensor signal
C14- gry- 5v reference signal to map and TPS.
C15- grn- inj B ground signal. Most systems do not use this. It was used on some of the bigger engines to share the load of the injector signal.
C16- orn- fused 12v tied to B1

D1- blk/wht- System ground, ties to A12 and engine. See ECM ground thread.
D2- blk- Sensor ground. To eng.
D3- not used
D4- wht- EST control
D5- tan/blk- EST bypass. This is the wire that has a plug near the distr that you disconnect to set your timing.
D6- Tan- O2 ground to engine.
D7- ppl- O2 sensor signal
D8- D13 not used
D14- lt grn- Injector B ground signal
D15- blu- optional injector A ground signal
D16- blu- Injector A ground signal

 

[chengny]

Hi Folks needing some help. Battery got hooked up backwards for a few seconds and now everytime we try to start it the ecmB fuse blows immediately. I tried unhooking sensors to see if I could find one that was shorted but am not getting anywhere. Any ideas? Could it be the ecu itself? In a pinch and need to get this running. It's an 87 1/2 ton, manual with tbi 305 all original.

The ECM B fuse feeds the common 440 ORN circuit. There are a limited number of components serviced by that 440 ORN feed. It runs from the ECM B fuse in the fuse block straight to the main bulkhead connector where it passes through the firewall. Once in the engine compartment it is split off into 4 branch leads as follows:

1. The line side of the oil pressure switch (i.e. the one used for backup power to the fuel pump in case the relay doesn't pull in - not the one used for a dash warning light). It is usually located directly above the oil filter.

2. The line side of the fuel pump relay contacts

3 & 4. These two branch leads transit back through the firewall (over on the righthand side - inboard of the HVAC blower) along with the rest of the ECM connected wiring. These are the "keep alive" power supplies for the ECM - one feeds the A/B banks (@ B1) and the other feeds the C/D banks (@ C16).

That's the entire circuit. The only other wiring/components that could blow that fuse would be on the load side of the oil pressure switch or the pump relay. When you crank the engine, the ECM pulls the pump relay in - and also (if 4 psi of oil pressure is developed), the oil pressure switch contacts close. When those two things happen, an additional branch circuit is added onto the 440 ORN - the 120 TAN/WHT. That circuit primarily feeds the fuel pump and a signal to the ECM (X2). If you had a fuel module (you don't) it would also go there. The fuel pump feed from the pressure switch is unfused (except for the ECM B). The feed from the pump relay is fused but that fuse is rate at 20 amps, so the 10 amp ECM B will blow first.

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[chengny]

so I've unplugged the fuel pump relay and the oil pressure sending unit and the fuse still blows as soon as I reconnect the battery. I then unplugged the ecu and it does not blow the fuse. I try to connect either one of the ecu connectors and it immediately blows the fuse again. ECU?

That doesn't sound good. If you turn the key to on, do you get a SES light on the dash? Probably not.

Check for a flash code 55. Bend a paperclip into a U shape and stick the ends in the A & B sockets of the ALDL under the dash. Turn the key on and count the flashes. They will come in two groups. If you get 5 flashes, a pause and then 5 more flashes...the ECM is NFG.

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[Cuba]

Ah yes! Could call on the SES with key on feature. The simple things that can verify ECM function.

 

[Honky Kong jr]

I'm gonna throw this out there I have the ECM from my 87 350 V10 that's collecting dust.....pay for the ride it's yours if yours is junk. I'm never gonna use it. I tear that **** out. It's to ass backwards to me. But there are quite a few on here that know this stuff like it's their job. And those are the ones that are helping you.

 

[gjr64]

I appreciate the help folks and especially the ECM offer. I have a specialty junkyard near me that does square bodies and he had an ECM. Plugged it in and TADA! Back on the road again. Thanks again everyone.