Wont start, but has power

[garzas23]

1985 gmc 1500, 350, th350c auto. Ok recently got my truck on the road. It sat for atleast a year. But now, it cranked and drove just fine for about 4 days. Then yesterday morning I went to crank it and and nothing happened at all. I have lights, brake lights, radio and I hear the clock working. But it doesn't even act like its going to turn over. I used a test light, turned the key, and it lights up when I touch the starter solenoids so its getting power but I don't know how much because my multimeter was dead. Since I showed power at the starter solenoid I assumed it meant a bad starter, I took it to oriellys and it passed multiple tests. I had power and the fusible link on the fire wall also, and check every single fuse. But it seems like if its getting any power at the starter it would atleast try to turn over.

Would either the ignition switch, or the neutral safety switch cause it to have power at the starter and not turn over?

 

[garzas23]

Also before I got it running again, it did have the esc computer under the dash above the glove box. The wires went thru the firewall to the dist. and grounded to the passenger head, then the passenger head was grounded to the firewall. I removed the esc and all the wires thru the firewall, and the ground. Was that ground from the passenger head to firewall for the esc only? Or does it still need to be there?

 

[rich weyand]

The ground from the head to the firewall needs to be there. The truck really has three electrically isolated parts: the engine, the frame, and the cab. They are electrically isolated because the engine and the cab are both mounted to the frame with rubber mounts. So that ground has to be there or the cab electricals are not grounded to the battery.

Another likely suspect is the battery cables. They deteriorate internally with no outward sign they are shot. Any cables over 10 years old are suspect. To check, connect the voltmeter to the two ends of the negative cable, at the negative battery post and the alternator bracket. Have someone try to turn it over while reading the voltage. If it's more than half a volt, change the cable. Same with the positive cable.

 

[garzas23]

I will definetly check the cables, but the pos. and neg. are new. I will put the ground strap back on. I have not had the ground strap on there for the few days it been driving and starting great, could I have damaged something running it without the ground strap to the firewall?

 

[350runner]

i always replace 30 year old fuse links usually they are too worn out and won't allow the amperage needed for the starter yup pass through.

Sent from the dust in front of you!

 

[bucket]

Eh, if a fusible link works, then it works well. All mine are original links still and they all start even when it's 15* below in the morning. That's just my 2 cents though.

 

[chengny]

While it true that 2 out of the 3 fusible links are connected to the "B" terminal on the starter solenoid, they are not part of the primary starter circuit. Consequently, their condition has no effect on starter/solenoid operation.

They are inserted into the wiring that supplies the two main branch lines. Their sole purpose is to protect any sections of the truck's electrical harness that provide power to the fuse block (line side) and are thus are not yet fuse protected. Additionally, the links provide excess current protection for those few circuits that do not pass through the fuse block at all (e.g. alternator, headlights, control side of the horn relay, etc.)

The fact that they are connected to the starter solenoid is simply due to convenience, current capacity and cost concerns.

The solenoid "B" terminal is fed by a straight run of heavy gauge cable. So, as far as current capacity, making a connection there is an excellent choice. It is basically as good as if it were made at the battery.

Also, using the B terminal (rather than the battery positive), allows for the use of shorter primary leads. Shorter leads equals less copper - and less copper equals reduced costs. From a performance standpoint, shorter runs of wire also reduces the degree of line losses incurred.

 

[chengny]

I used a test light, turned the key, and it lights up when I touch the starter solenoids so its getting power

Which terminal (on the solenoid) did you connect the test light to? There are two:

A big one - labeled "B". This has two leads connected to it. A big one that comes in from the positive battery post and a medium one that quickly splits into two separate branch lies. Provided your positive cable is intact, this terminal (and any wires connected to it) should show full battery voltage at all times. Since it is connected directly to the battery positive, it will always be hot - this is true whether the key is on or off.

The only other wire is a small purple one. This is the lead that controls starter operation. It is only energized when the key/ignition switch is in the START position.

Think of the big black wire that comes from the battery as an explosive charge and imagine that the small purple wire is the detonator. By itself (although it has a huge amount of power) the black wire can't do anything. It needs a small outside signal to operate. This small signal is referred to as the control circuit. When the control circuit is activated, the power available in the big black wire is unleashed and this is what causes the starter to engage and crank the engine. Without the impulse provided by the purple wire, the black wire just stands by and waits to run the starter motor.

Enough with the analogies.

If you did not have your test lamp connected to the purple wire, do the test again- this time checking that the purple wire becomes energized when the ignition switch is moved to START.

If it does not light up, there are basically three possible causes:

1. The ignition switch is not closing the start contacts

2. The clutch safety switch is not closing (manual transmission only).

By the way; your suggestion that it might be the neutral safety switch was a good diagnostic guess - if your truck was a 1984 (or older) model year.

By the time your truck was built in 1985, the NSS was obsolete. Instead of an electrical switch, starting the engine (with the gear shift in anything other than P or N) was prevented by a mechanical interlock within the steering column.

The switch is still installed at the base of the steering column, but only the contacts for the back-up lights are used. The starter control wires - that previously ran through the NSS contacts (yellow in and purple out) - were replaced with a single purple lead. It now runs past the NSS switch and directly to the starter . See the wiring diagram below.

3. A loss of continuity (i.e. broken wire) in the control circuit. This could be anywhere in the purple wire - from the ignition switch to the "S" terminal on the starter solenoid).

Do the test for power to the S terminal with key in START first.

If it tests out okay, and you have power at the B terminal - but it still won't crank - next thing is to look closely at your main ground cable. As you probably already know, it runs from the battery negative to the alternator bracket (usually). Check for tight/clean connections on both ends and broken conductors at the crimps. It might be worth it to just by a new one and eliminate that as a possible cause. They are fairly cheap.


Wiring diagrams showing the purple wire (I am color blind so the lines might not be purple - I took a guess):

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Same dwgs- unmarked:

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Given the information you provided on what is still working (the radio for example) I think we can rule out the fusible link that feeds that side of the system. How about your windshield wipers - are they working?

 

[garzas23]

Thanks for all the great advice everybody. I went and bought a ground strap on the way home and placed it on the passenger side head to the firewall, and it fired right up! Woke up fired it up again to go to work this morning and fired right up again! problem solved.

 

[chengny]

Good luck and I hope that really was the cause. But I have a gut feeling it was not. If I were a gambling man, I'd have to bet that it will pop up again at some point.

I think that about half of these trucks - the ones that are still on the road - are driven daily without that ground strap. Usually it is either broken or completely gone.

Whether or not it helps the cab mounted components is a subject for debate, but one thing for sure - it has nothing to do with starter operation.

The only way I can imagine it helping the starter is; if your main ground cable is shot and replacing the firewall strap created a secondary ground path.

Instead of the usual ground path (from the starter casing, through the engine, alternator bracket and on the main cable over to the battery negative.

Your starter now is grounded by going from the casing, up through the engine, across on the braided strap and into the firewall, down the length of the fender, into the radiator support frame and finally back to the negative post via the front lighting ground leads.

Again, I hope you got it fixed.