I'm creating more problems than I'm fixing.

[VileZambonie]

If you have verified all of your connections and performed all of the other checks then swap out the ECM.

[tc197]

Trucks driving me insane. Im not sure what I should be looking for. I have continuity everywhere that seems like I should. Tomorrow im going to run more ground wires around to see if it helps any. I just dont understand how it can start to run and then just die while at the same time my headlights dont work.

[1967KaiserM715]

Having continuity is only half of it. Sometimes you can find a fault with continuity, but if you don't you need to go deeper. Example, on slow cranks sometimes the cables look fine, they have continuity, but corrosion can cause several strands of wire in the cable to become seperated and while this still shows continuity, you can't push the same electrical power through the cable. I have a feeling this is what happened with some of the links behind the engine-some wires got crushed and even though they show continuity, they can't allow enough energy through to power anything


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[tc197]

That makes sense, but what doesnt make sense to me is how it will start if I just crank it for a few seconds but then it will die. Isnt the highest draw on the electrical system while it is cranking? And why does my check engine light come on after it cranks for a few seconds but not when i ignition on? Theres something wrong with ny headlights because even with the ignition off i cant get them to turn on. I think this is my problem but im having trouble tracing it back. My blinkers work, my brake lights work, and my headight switch is new. My headlights dont work, my dimmer switch doesnt work, my high beams dont work, and I dont think my horn works but i have the steering wheel off right now.

[80Chevy4x4]

Time to get a wiring diagram and trace each wire. I would do that instead of fishing....it could be many things.

[tc197]

Finally made some legitimate progress. Found the A1 Chart for no SES light when ignition on here : http://forum.73-87chevytrucks.com/smforum/index.php?topic=25461.msg212295#msg212295

Wasn't sure if I would consider my engine starting or not because it fires for only a second before dieing. Either way I followed both directions in the chart and did the tests.

For engine starts: I disconnected the ecm and probed ckt 419 with the test light at the fuse box. The SES light was NOT on. The solutions to this were centered around an SES light that is no good, however, I know my SES light is good because if I crank it for 3 seconds, the light will eventually come on.

For engine doesn't start: I disconnected the ecm and probed ckts 340 and 439 with test lights at the fuse box. The light was ON for ckt 439 (ecm fuse) and the light was OFF for ckt 340 (ecmb fuse)

Case 2 of "engine doesnt start" seems to be more applicable and has a problem with the ecmb fuse circuit. Looking at the wiring diagram in that same thread it seems that the two items are the fuel pump relay and the oil pressure switch. When I ignition on I hear the fuel pump kick on for 10-20 seconds (buzzing noise inside fuel tank) and then it shuts off. I'll have to look into what the causes of a bad relay are and the same for a bad oil pressure switch.

How this relates to my headlights I have no idea, but at least I feel like I have something to go off of right now.

[80Chevy4x4]

Yea that is a real good start.....the fuel pump runs off the oil switch...if you lose oil pressure the fuel pump will shut off....the fuel pump will always cycle until it reads no oil pressure....so you are on the right track...as far as the headlights.....have you checked the bulbs to make sure they are still good?   even after all the testing?  Bulbs can go at any time...hope this helps.

[tc197]

Havent checked the bulbs recently but along with the headlights I lost my gauge backlight and that hasnt come back yet either. I suppose I could have blown a bunch of things at once but ill have to check.

As for the fuel pump situation, my oil gauge is pegged at max (was before my problems too) so i would assume that would be good for the fuel pump because it will always cycle. Another thing I tried was I found on here that the extra wire sticking out of my pump relay was a test plug. I ran a line from B+ to that connector to see if my pump would cycle and I got nothing (ecm was still disconnected if that matters). Would this mean my pump relay is no good?

Is there any connection between the ecmb circuit and the headlight circuit?

Thanks for the help guys, its been weeks on weeks now trying to fix this one stupid electrical problem. I havent even gotten to enjoy the truck after swapping the trans and its getting cold to keep riding my motorcycle. Hopefully this doesnt take too much longer

[bd]

I've checked the fusible links many times now and I don't see any problems with them. One at the starter, one at the battery, one at the junction box on the firewall, and one at the alternator. As I am doing research on the site I see people saying that the junction box up by the firewall has multiple wires coming out of it and for me this is not the case. I only have 1 wire into the box and 1 wire out of the box with a fusible link on the wire leading in. Is this a problem?

Stop and regroup!  I concur with hatzie and VileZambonie.  Either a fusible link did its job or it is disconnected or severed.  Chances are that you either created a dead short that still exists or a transitory short that doesn't.  Whichever scenario occurred, it resulted in a zapped (or open) fusible link.  In a new unit of time re-inventory the fusible links.  Don't make ANY assumptions no matter how many times you have already looked at it.  Observe as though it's the first time through, using the 1987 Wiring Manual for reference.  But be advised, the 1987 Wiring Manual requires patience to interpret correctly and understand.

Fusible links are generally about 6" long located at the "hot" end of the wiring they protect.  The circuit number of a fusible link may differ from the circuit number it protects.  Unless the wiring in your truck has been modified, there are two fusible links connecting to the starter 3/8" solenoid stud (Ckts 2E and 2I).  One of the links (Ckt 2I) routes through a red wire directly to the firewall bulkhead connector and into the cab (Ckt 2H); the other fusible link (Ckt 2E) connects through a red wire to the firewall junction block (Ckt 2G).  A redundant fusible link (Ckt 2F) exits the firewall junction block and runs through red wire, again to the firewall bulkhead connector and into the cab (Ckt 2C).  No fusible links connect to the battery or alternator. 

Unfortunately, the 87 diagram is vague regarding the transition of Ckts 2H and 2C through the bulkhead connector.  I believe Ckt 2H becomes Ckt 2E and powers the ignition switch, which in turn powers the ECM-B fuse.  I also believe Ckt 2C splits into Ckts 2A and 2B; Ckt 2A powers the hazard flasher and small appliances requiring constant B+ and Ckt 2B powers the headlamps through the headlamp switch.  Perhaps hatzie or VileZambonie can help clarify circuit 2 transitions through the bulkhead.  Otherwise, you can verify the circuits yourself by observing how and where they connect.

Describe how you have been checking the fusible links.  Headlamps are a high-current circuit.  A fusible link can be partially melted and limit the current flow through a high-current circuit, but still measure continuity if checked with an ohmmeter or checked with a voltmeter when the circuit isn't loaded.  Have you been using a voltmeter, an ohmmeter, a test light, or __________ to check the fusible links?

[tc197]

I appreciate the extensive response bd. My truck is definitely wired wrong. I won't be able to get to it until this weekend but here is some info for now that I am 100% on.

What I have:
- 2 wires coming off b+, thick one to starter and thinner one to alternator. The thinner one has a fusible link immediately after the b+ terminal before the alternator.

- after the alternator I have a second fusible link on a red wire.

- on the same starter stud as the thick b+ cable from the battery, I have a third fusible link that is on a smaller red wire running up behind the motor.

- finally my firewall junction block has only 1 wire in and 1 wire out. I don't know which side is in and which side is out, but one side has a fourth fusible link on it.

For testing fusible links, Ive pulled on them, checked continuity with a dvm, and checked for 12ish volts with a dvm. The fourth fusible link (firewall junction) I replaced as the plastic housing was cracked. I did not load test them or use a test light on them.

ive replaced the headlight switch, ignition switch, ignition coil, plug wires, egr valve (before problems), and cleaned (sanded to shiny metal with a dremel) the grounds at the b- to frame, b- to radiator support, altenator bracket, drivers headlight to sheetmetal, passenger headlight to radiator support, ecm to back of cylinder head, and inside the dash the 6 prong connector which has 1 ground cable connected.i also checked continuity between various ground locations to the b- terminal.

The fan blower, turn signals, brake lights, and wipers all work.

The headlights, dash lights, high beams, radio (disconnected now), and I think horn (steering wheel is off) dont work.

When i turn the ignition on, my voltage gauge reads 12ish, my oil pressure is maxed, no dash lights come on, the fuel pump runs for like 15 seconds, and it will crank. If I crank it for about 3 seconds, the check engine light will come on, and shortly after the truck will start for a split second and die out. If throttle is applied in the split second its running it will run slightly longer.

[bd]

...For testing fusible links, Ive pulled on them, checked continuity with a dvm, and checked for 12ish volts with a dvm.  ...I did not load test them or use a test light on them.

With respect to any future diagnoses of power delivery to various components, unless it is specified in a diagnostic manual an ohmmeter is the wrong tool to use.  An ohmmeter can be used successfully to compare the measured resistance of a component to its engineered resistance when the engineered resistance is a published constant, but using an ohmmeter for general diagnosis of power problems can be very misleading - more so when high current flow is involved.  For instance, if a starter draws 150 amps at 12 volts, the starter motor has an effective resistance of only 0.08 Ohm.  A resistance in a battery cable of merely 1 Ohm (which is 10x the resistance of the starter motor) could effectively prevent the starter from cranking.  Only a laboratory grade meter can accurately measure resistance that low.  So the typical DVM would show excellent continuity on the cable, yet the engine wouldn't crank.

Well, fusible links are just high-current, slow-blow fuses.  Any damage to a link can impact its ability to transfer current effectively.  A damaged link that hasn't actually severed can check fine for continuity using an ohmmeter and yet not function when placed into service under load.

The best methods for checking fusible links are:  1) inspect for blistering and/or rupture of the link insulation and for evidence of heat caused stress, 2) give a tug to see if the link stretches like a rubber band or breaks, 3) measure voltage drop across the link while the link is in an operating circuit and under load, and 4) use an incandescent test light for a quick test while the link is in an operating circuit and under load.  In a nutshell, you should recheck all of the links.

Post pics of the fuse links connecting to the alternator and to the starter, as well as pics of the bulkhead connector from the engine compartment.  How many 12-gauge red wires enter the bulkhead connector from the engine compartment?  There should be two and both should measure B+.  Don't be confused by all the little things that don't function correctly at this time.  Some may resolve by reestablishing power into the cab through the bulkhead; others are unrelated and can be resolved later.  Did you ever check for power into the ignition switch and headlamp switch?  FYI - the headlamp highbeams/lowbeams are powered separately from the running lights and instrument lights.  There is a procedure posted in the Technical Pages of the forum regarding diagnosing dash lights that don't work.

More later after you post the pics.

[tc197]

Thanks for all the help!

I won't have a chance to get pictures of the links until this weekend so instead what I've done is ordered 4' of fusible link and some terminal connectors. I'll make up new fusible links and just replace all of them to eliminate the errors I'm making in testing the old ones. Unfortunately it's looking like I'll only have friday night to get some work done on the truck this week so I'll replace all the links and take pictures, check on the red wires entering the bulkhead, and make sure those red wires have B+.

I think i was misunderstanding how the wiring was setup, I was assuming that since the motor cranked and fired that the fusible links with continuity would be a valid check. They had continuity, and they could support the draw necessary to crank the motor. What I am now understanding is that the fusible link coming off the starter and at the bulk head don't care about the cranking current, but rather come into play once the motor fires and power needs to reach the rest of the trucks components. This would make sense why the truck starts but immediately dies because the weak wires can't support the current load for more than just a quick start. Sorry that it took like 4 different people pointing this out for it to finally click with me.

If replacing these links does in fact work I guess the next question is what happened to cause them to fail to being with. I did a lot of work and did move the motor around when swapping the trans so it is possible they were just old links that I stretched/stressed and that was enough to blow them. I guess I'll find out when I go to start the truck up and if the new links blow then I know for sure I have an issue somewhere else.

It's a frustrating process but without all these set backs I wouldn't know nearly as much about my truck as I do now. Thanks again for all the help guys.

[bd]

Gauge choice of fusible wire is strictly dependent on the gauge size of the wiring being protected.  See How to make a Fusible Link for a description of the relationship.  The only fusible links with which you should be concerned at the moment are the three (two at the starter and one at the firewall junction block) that feed power into the cab through the firewall bulkhead connector.

[tc197]

Sounds good BD, thanks for the link. Wasn't able to get to the truck this weekend. Fusible link I ordered was delivered late so it's looking like later this week I'll have a chance to get some work done.

[hatzie]

You can use the much easier to read and interperet 1988 RV manual as a loose reference crutch till you get up to speed on what GM did in the 1987 manual.  It's worth noting that the  1987 & 1988 manuals are "full line" manuals so the T400 CK models are in there along with G & P vans and ST trucks too.  You'll need to page to the RV series sections.

There are differences between 87 & 88 but the fusible links and primary circuit 2 wiring should be close if not the same.

Print the 1987 RV wiring manual pages you need and mark em up with hi-liter or colored pencil.  The original page size is 11x17.
If you have a local Fedex Office or access to a printer that will handle the original 11x17 Tabloid size pages, or even better these PDF files were scanned at a high enough DPI to print on 12x18 Arch B by scaling in the print dialog of Adobe Reader... 

The wiring manuals list the wire gauge of the fusible links and each individual wire so there's no need to guess.  They also include jacket color and information on wiring jacket composition ex SXL, TXL, GXL...


In 1988...
The "thin" 1.0mm fusible link at the starter goes to the bulkhead plug and on to the ignition switch and fuse panel.  1.0mm = 16AWG
The "heavy" 2.0mm fusible link at the starter feeds the junction stud on the firewall.  2.0mm = 14AWG

 look at the 1987 power distribution pages to verify GM didn't change something.