Location of oil sending unit on 88' V30? Keep blowing ECM B fuse

[Chevy3+3]

I have a new problem blowing the ECM B fuse and it controls the oil sending units and fuel pumps. I here a lot of people saying to check for broken or shorted wires going to the oil sending unit on the back of the block below the intake. I could be wrong but I don't see any sensor or sending unit back there. Running an 454 tbi

I do have what looks like an oil sending unit at the front of the engine below the power steering pump. The sending unit has a 2 wire connector plugged into it and a single wire connector also. I just want to know if anyone knows where the location of my sending unit is so I can start inspecting wires to track down the problem.

Thanks

[chengny]

Yep that's where it should be up front on the left hand side. And, there should be two controls on that christmas tree.

One should have two leads connected. That's the oil pressure switch which is used as part of the fuel control circuit. It's contacts close at about 3-4 and from then on (as long as there is 4 psi oil pressure), it provides a redundant power supply to the fuel pump - in case of an FPR failure.

The other one is the oil pressure sender. It has just one lead connected. The sender creates a variable resistance in the sensing leg of the dash mounted oil pressure indicator. If it's not in the same place as the switch, it might be mounted way back on the block - above the oil filter.

Locations:




Operation & wiring diagram:






FPR diagnostic:

[Chevy3+3]

Thanks for the info! So if I understand what your saying is that if my oil pressure sending unit is faulty it could cause fuel pump related problems since the fuel pump relay gets it power from the oil pressure switch?

Or are you saying that as long as there's 4 psi or greater oil pressure that the oil pressure switch provides power through that circuit in the event that the FPR fails

[chengny]

So if I understand what your saying is that if my oil pressure sending unit is faulty it could cause fuel pump related problems since the fuel pump relay gets it power from the oil pressure switch?

No, the relay and the oil pressure switch are wired in parallel and they are both supplied by the ECMB fuse. The power from the ECMB fuse comes out of the fuse block on two leads (orange). One goes to the pressure switch and the other goes to the FPR. After these leads exit the OPS & FPR (now as tan/white wires), they are spliced together again and head back to the pump as one common lead. But that's not all. In your case (with the 454), there's additional lead spliced to the OPS & FPR outputs. You should have a fuel module under the dash. It supplies power independently of the ECMB fuse - but only for 20 seconds. It is designed to prevent vapor lock in hot start conditions.

Or are you saying that as long as there's 4 psi or greater oil pressure that the oil pressure switch provides power through that circuit in the event that the FPR fails


Yes, the OPS contacts will close at 3-4 psi (that pressure can be reached just by cranking over on the starter) and provide a redundant/parallel power supply to the fuel pump - should the FPR fail to pull in.

[Chevy3+3]

Ok thanks for the all the info. It's probably a bad fuel pump causing the problem. The last few weeks my power has been in & out and not consistent. After the fuse blew and I got towed home I disconnected the connector at the pump and replaced the fuse. When I did this the fuse did not blow again. I plugged the connector back in and the fuel pump primed like it should. I was able to start the truck but after 15 minutes the fuse blew again. I wonder if the fuel pump is getting hot after running for a few minutes or just shorting out and blowing the fuse. Not sure if it means anything but I noticed my fuel gauge after the fuse blew the last time read WAY past the full mark also.

I have ordered 2 new fuel pumps (Delphi) and plan on replacing pumps in both tanks as well as replacing the fuel filter. I hope that fixes the issues I have been having. With the info you provided I should be able to trace wires and make sure nothing is shorting out to ground also.

[nuckollsr]

On my '87 Sierra, ECM=B powers a lot of stuff. It's circuit 440 that runs all over the place. Get the wiring diagrams and check it out.

When I was blowing ECM=B, it happened right after I turned the IGN switch on. It was not related to having the engine run, hence actuation of the oil-prssure switch was not a factor. It took me several days to sift through all the options . . . hand to pull the firewall connector to isolate which side of the firewall. Finally ran it down to a dead-short to ground behind the instrument cluster somewhere. Didn't have the time/inclination to tear out the panel stuff so I ran a new wire around the bad segment and all was once again right with the universe.

I wish I could be more encouraging but circuit 440 is snarl of snakes.

[Chevy3+3]

Originally Posted by nuckollsr

On my '87 Sierra, ECM=B powers a lot of stuff. It's circuit 440 that runs all over the place. Get the wiring diagrams and check it out.

When I was blowing ECM=B, it happened right after I turned the IGN switch on. It was not related to having the engine run, hence actuation of the oil-prssure switch was not a factor. It took me several days to sift through all the options . . . hand to pull the firewall connector to isolate which side of the firewall. Finally ran it down to a dead-short to ground behind the instrument cluster somewhere. Didn't have the time/inclination to tear out the panel stuff so I ran a new wire around the bad segment and all was once again right with the universe.

I wish I could be more encouraging but circuit 440 is snarl of snakes.

Well being my luck it's something complicated. I hope nothing it shorted out under the dash. I even wondered if it could be in my column itself. The first time I blew the fuse I replaced and and when I turned the key on it blew immediately again. I had to leave it sit for a few hours and then be towed home. When I unplugged the fuel pump connector I replaced the fuse and then everything worked as it should. When I plugged the pump back in it cycled and I started the truck. I just left the truck sit and idle but after approximately 15 minutes it blew again. So I'm thinking the fuel pump is overheating and popping the fuse. I had to take the tank out but that's where I'm going to start. I plan on doing both tanks and using the other tank and pump before I tear into the pump that may be causing my problem. If I'm trouble free running on the other pump then I know the pump in the other tank was most likely the culprit. If the fuse still pops while not running that pump I will know my problem is else where

[nuckollsr]

I feel your pain. Do you have the factory wirebook for your truck? Not one of those one-page, simplified drawings but the multi-page drawings that speak to circuit numbers, wire colors, sizes, and the rat's maze through which all the wires go?

Here's an excerpt for my truck . . .



. . . as an example, I've highlighted circuit 440 (ECM-B Fuse) where it comes through the firewall. The highlight shows color, wire size, and most important, the DRAWING COORDINATE (Q60) where the other end of that wire is found. There are numbers on the top/bottom, letters on the right/left. In this case, Q60 is found on the 2nd sheet of the page.

The problem with fuse blowing is that there can be a fault ANYWHERE on that line . . . or even on another line when the power goes through a relay or switch. When you have multiple branches, it can be exceedingly difficult to locate the fault. In extreme cases, you may have to resort to techniques like cutting into a wire, clipping a lower current fused into the gap (I've used ATC in-line fuse holders with alligator clips on the leads) and seeing if the test fuse pops. If it does, you splice the wire back together and move downstream beyond the next 'branch' to investigate individual branches.

It can get ugly . . . but first make yourself a drawing of ALL the wires, connectors, components downstream of ECM-B fuse. Then develop an investigation strategy for considering ALL the places where a short might be located and go after it. It's like playing the board game Clue. When you find all the places where the short IS NOT, all you will have left is where the short IS.

[Chevy3+3]

Well your scaring me now, lol. I won't be able to look at it for at least a week and no I don't have any diagrams other than what you have posted here. If the pumps don't fix it then I might need you for some more info. Thanks a bunch

[nuckollsr]

What are the details on your truck? 88? what model/engine?

[Chevy3+3]

Originally Posted by nuckollsr

What are the details on your truck? 88? what model/engine?

88' 1 ton 4x4 CC dually 454/th400/np205 originally. Still alive original except I swapped in an NV4500

[nuckollsr]

Okay. Shucks! Just a few days ago I stumbled across somebody's library of GMC wirebooks . . . I discovered that I already had the electrical documents for my truck but he had an extensive library of other models/years.

It may not have been in the Squarbody forum but it's in the family of GMC truck forums. As an alternative, there are MANY commercial purveyors of reproductions for automotive wiring diagrams. Also, you may find original paper books on eBay.

I think I spent $30 for my original wirebook . . . worth several times that amount. Without that data, you're wandering around in a maze with VERY poor lighting.

I wish I had bookmarked that library I cited above. I'll poke around a bit . . .

I'm an electronics engineer retired after 45 years of herding electrons in airplanes. We seldom had problems like this on aircraft . . . it's a design goal to have EACH system be protected by its own breaker/fuse. I'm pondering a design for an inexpensive fault finder that would have made my earlier task MUCH easier . . . yours too.

The idea involves replacing the ECM-B fuse with a resistor . . . or perhaps some light bulbs that excited the shorted line with 5A of current. Then you run around the various branches of the rat maze looking for the wire(s) that are carrying the 5A. There are some really neat integrated circuits that will 'sniff' the magnetic field around a wire.

In the mean time . . . I strongly recommend getting your hands on you truck's road map for electrons.

[nuckollsr]

Originally Posted by Chevy3+3

88' 1 ton 4x4 CC dually 454/th400/np205 originally. Still alive original except I swapped in an NV4500

There are lots of offers for 'wiring diagrams' out there. Some are pretty simplistic . . better than NO diagram but not what you need. Here's a link to the wire book for my truck. You can see the kind of detail that goes to chasing your rats out of the wiring.

http://www.aeroelectric.com/Misc/RV1...NG-TRIMMED.pdf

The book will be 11 x 17 pages . . . probably 25 pages or more . . .

[chengny]

Below are the entire combined 440 ORN & 120 TAN/WHT circuits, mapped out from the fuse block to one of the fuel pumps. The 120 circuit is included because, when the OPS or FPR contacts are closed, it becomes an extension of the 440 circuit. Consequently, in addition to the 440, a short circuit in the 120 will also cause the 10A ECMB fuse to blow.

440 ORN circuit starts at the fuse block (10A ECMB), then goes into the engine compartment and feeds both of the fuel system controls (OPS/FPR) used with L19 applications. It also goes to both banks of the ECM - as the fused power supply:




The power coming out of the controls is now carried on the 120 TAN/WHT lead. The 120 passes through the firewall and back into the cab. Once inside the cab, the power from the hot fuel handling module is also spliced into the 120 TAN/WHT circuit.

With dual tank applications, that tan/wht wire doesn't go all the way to the fuel pump. It ends at the tank select switch. The power out of the switch will be carried on either a tan or gray wire - depending on switch position. Both the tan and gray leads then pass through the firewall and continue on back to the pumps/transfer valve.

Which ever one of those conductors that is connected to the tan/wht- through the switch contacts - is the one that will provide power to the fuel pump. It will be switched so that power from the 120 will go to the positive side of the corresponding pump windings. The other wire will be connected to the negative side of the pump windings - and the other end will be connected to the main ground bus (via the select tank switch).

The same hot lead used to power the pump will also provide a polarized power supply to the transfer valve's operating coil. When the coil is energized, the internal plumbing will be shifted to the active tank (i.e. the one with the online pump).

Finally, in addition to energizing the appropriate pump and shifting the valve over to the active tank, when the position of the select switch is changed, one other thing happens . The sensing leg for the fuel tank gauge is also shifted to the active tank. There is no power involved in this process, the contacts for the tank level indication are directly coupled to the transfer valve internals. So, when the valve rotates and changes the fuel supply from one tank to the other, the sensing gauge's leg moves along with it.

The single sheet wiring diagram for the 440 and 120 circuits:




The wiring diagram for RPO NL2 (dual tanks):

[nuckollsr]

Very useful posting! Looks quite similar to if not identical to my truck. I'm going to capture your posting for my library. The 440/120 web is probably the most complicated system in the truck. Looking for a short there is not easy to begin with . . . make it an intermittent short and you've got a real test of your diagnostic skills!

[Chevy3+3]

Originally Posted by nuckollsr

Very useful posting! Looks quite similar to if not identical to my truck. I'm going to capture your posting for my library. The 440/120 web is probably the most complicated system in the truck. Looking for a short there is not easy to begin with . . . make it an intermittent short and you've got a real test of your diagnostic skills!

I started looking into it today but only briefly as I am already a little stumped, lol. I plugged the fuel pump connector back in and the truck started. I wanted to find out which color wire to my driver side pump was positive. Through years of fading the colors appeared to be a pale orange & red (maybe pinkish red). The red colored wire was the one that supplied power to the drivers side pump.

I shut the truck off, turned my ignition back on and switched my dash selector switch to the passenger side tank which hasn't worked since I bout the truck 2 years ago. Even though the pump did not cycle I assumed the pump would be bad but what I found was that there was no power even going to the pump.

So now my new problem is why can't I get power to my passenger side fuel pump. I also found that even when my selector switch was in the passenger side tank position that I still had power at the reddish colored wire going to my drivers side pump connector. I will look into it more tomorrow if I have time. Whats more irritating than anything is the problem blowing the ECM-B fuse is intermittent. Its hard to diagnose an intermittent electrical problem.

[nuckollsr]

With dual tanks, there is an electric solenoid valve and switch assembly that determines which pump and sender combination is plumbed to the engine.

The wiring for that system looks generally like this.

http://www.aeroelectric.com/Misc/Dua...k%20Wiring.jpg

The valve itself looks like this

http://www.aeroelectric.com/Misc/Dua...tch%20assy.jpg

If one of your tanks is not working, the first thing you need to check is the valve. If you cannot hear it translate when the tank selector switch is operated, then it's probably bad. I replaced mine a few years back for about $45 with local car-parts retailer.

If you study the action of the fuel selector switch, you will find that it simply reverses the connections that run to the valve. Applying 12v to one side while grounding the other moves the valve to one extreme, reversing those connections moves it to the other extreme. The fuel pumps get power by simply connecting to opposite sides of the valve signal . . . since only one side is 'hot' at a time, only one pump is powered at a time.

This simplified diagram illustrates ECMB (440) power routing to the oil pressure switch and fuel pump relay. The fuel pump relay has to close during engine cranking to bring the pump on or you could not crank the engine. Once the engine is running, the oil pressure switch takes over duties to power the pump.

Should the engine die for any reason, oil pressure goes away and the pumps are un-powered. Keep in mind that this switch is different from and in addition to any oil pressure sender.

[Chevy3+3]

Thanks. I do believe the selector valve in the frame may not be switching when it's asked to. However I am confused as to why the truck will die if I flip the switch to the pump that isn't working but still have power to my drivers side pump. If the drivers side pump still has 12v to it even after I switch to the passenger side pump then I would think the truck would stay running because the pump is still powered.

[nuckollsr]

Hmmm . . . if your truck is wired like my truck, power to operate the pumps come directly though the fuel tank selector switch. For right tank, the right pump gets power and the left pump is grounded. For the left tank, the left pump gets power and the right pump is grounded.

If you have a pump running in both switch positions, the pump is mis-wired or the switch is bad. If you got the truck with one side unusable, it may well be that the previous owner rewired the working side to run all the time . . . if wiring in that area has been 'fiddled with' . . . it may be the source of your fuse blowing incident as well.

You didn't mention whether or not you tamed the fuse blowing dragon.

[Chevy3+3]

The dragon has not been tamed. Hopefully I will be able to rule some things out tomorrow.

So if the tank selector switch sends power directly to the pumps then what are the 5 or 6 wires going to the tank selector valve? I assumed the switch sent power to the pumps through the valve somehow. I apologize that I haven't had a lot of time to tear into it or even look closely at the wiring diagrams posted.

I will add another problem I have been having for a couple of months but hasn't caused a fuse to blow. I see my tach needle bounces uncontrollably when holding steady at various rpms. I wondered if it could be a direct short to ground somewhere in my ignition system. Replaced dizzy, wires, plugs, etc. Didn't do coil yet. Kinda wondering if all this could be related but maybe it's gotten bad enough of a short that it's blowing the fuse now

[nuckollsr]

Originally Posted by Chevy3+3

The dragon has not been tamed. Hopefully I will be able to rule some things out tomorrow.

So if the tank selector switch sends power directly to the pumps then what are the 5 or 6 wires going to the tank selector valve?

Two of the wires operate the valve motor. It's a permanent magnet motor that reverses direction when polarity of voltage is reverse. This is what your tank selector switch does, flip two wires between power and ground.

Three of the wires go to a switch inside that transfers your fuel gage to the sender in the tank selected by position of the valve. The sixth wire location in the connector is not used.

The pump motors take advantage of the fact that only one of the wires to the valve motor is hot at any given time. Hence, the two pump motors can be connected to valve motor wires and only one pump will run at a time depending on position of the tank selector switch.

[nuckollsr]

Originally Posted by Chevy3+3

I see my tach needle bounces uncontrollably when holding steady at various rpms. I wondered if it could be a direct short to ground somewhere in my ignition system. Replaced dizzy, wires, plugs, etc. Didn't do coil yet. Kinda wondering if all this could be related but maybe it's gotten bad enough of a short that it's blowing the fuse now

I kinda doubt it. I think the tachometer gets a signal from the ignition module. The only wiring I can find in my library shows a single wire from the ignition module "to tachometer". This would have to be a simple signal that is interpreted by electronics in the gage. This suggests the gage is powered and has three or more wires to it, +12v, signal and ground.

I don't see anything in that area that could fail and put a short on ECM-B fuse.

[hatzie]

A picture is worth a thousand words...
Where IGN power comes from depends on the year.
If the colors match actual wire colors in your harness it's purely accidental.

1. Fuel pump relay. This relay is commanded by the ECM. It turns the fuel pump on for 10-30 seconds when the ignition is switched ON and continually when the ignition is switched to START. It primes the fuel system and prevents the oil pressure switch contacts from burning out from the fuel pump start current.
2. Fuel pump contacts of the Oil pressure switch (runs the fuel pump most of the time). Contacts are closed when engine oil pressure is above @3-8PSI. This switch will shut off the fuel pump as soon as there's no oil pressure for safety in an accident.
3. Hot fuel handling relay. Runs the fuel pump for @ 10-30 seconds after engine shutdown to help with vapor lock.

As shown the RH fuel sender and pump are selected. The LH fuel pump will have two grounds and no hot with the dash switch in the RH position.

Selector switch RH detail TBI .pdf

[JW76]

Check the diamond shaped plastic block on the firewall (to the left of the brake booster) that that power wires connect to. There is a metal tie bar on the back side. On my truck the back of the block cracked (couldn't see it from the front) and when the cab flexed it would contact the firewall blowing the ECM B fuse.

[finewayne]

^^
agreed, the cab flexed would contact the firewall