Massive voltage drop when attempting to start; new batteries

[Ryan H]

Hello all,

I just picked up a 1990 7.3 IDI with a ZF5 as a project truck. Previous guy said it was running, bad transmission, etc, but when I go to turn key to try and crank, I'm reading a massive voltage drop (down to less than 2v) and get basically nothing both inside the cab or when trying to start. Both batteries are new, just replaced those as my first stop but wanted to see what the next possible option might be. New Battery Cables? The guy did install a manual switch for the glow plugs but it doesn't seem to be affecting anything as far as I can tell. Thanks

 

[DaveBen]

Starter maybe shot. It is drawing too much power. Is it turning over pretty fast or lugging?

 

[IDIBOBS]

Battery cables, ground, starter. It’s a very very simple system. I’d also replace the starter solenoid just to be safe and keep the old one as a spare.

 

[u2slow]

Figure out which piece of the chain is sustaining the voltage drop. Shouldn't need more than a multimeter, and an assistant to crack the key.

 

[Ryan H]

Starter maybe shot. It is drawing too much power. Is it turning over pretty fast or lugging?

It's not turning over at all. I get basically nothing when I turn the key.

 

[ihc1470]

As was said a voltmeter or test light and a few quick test will get you headed in the correct direction. Lots of could be's right at the moment. Depends on how much money you want to spend and if you want to be a parts changer or actually fix the issue.

 

[u2slow]

A note about solenoids... They can actuate but not conduct. The contacts get burnt. Run into that a few times in the past.

 

[bulletpruf]

As was said a voltmeter or test light and a few quick test will get you headed in the correct direction. Lots of could be's right at the moment. Depends on how much money you want to spend and if you want to be a parts changer or actually fix the issue.

Yep, if you throw enough parts at it, you'll eventually replace the one that's the culprit. That's what's likely to happen if it's brought to a shop - new starter, new starter relay, etc.

 

[bulletpruf]

What you can do without spending any $ - check your battery terminals and your cables. Clean everything. Remove your battery ground cables from the block, clean and reinstall. If your battery cables are suspect, it won't be a waste of $ to replace them. Not cheap if you do the job right, of course, with good quality 2/0 wire.

Make sure the connections on the starter relay/solenoid are good, especially the medium sized red wire from the passenger positive to the relay and the smaller (10 ga?) black signal wire from the relay to the starter.

 

[Black dawg]

Dont replace parts yet. I suspect that a few minutes with a test light before and after the starter sol. will show something.

what DOES work?

 

[Black dawg]

Yep, if you throw enough parts at it, you'll eventually replace the one that's the culprit. That's what's likely to happen if it's brought to a shop - new starter, new starter relay, etc.

It is interesting to me, as I see many shops that charge too much, or do poor work, or take too long, or whatever other problems that are not unique to the industry, but dont know of any that just change parts until the problem goes away. In order to do so, would have to justify (lie) to the customer about why those parts were needed. That kind of business wont be around long.

 

[Clb]

Disconnect the batteries first

Get a wire diagram and familiarize yourself to the routing and color codes.
Grab a multimeter set for continuity (omega/ohms) test the main wires from battery to starter for resistance.
0-1 good the higher the resistance the worse it is. Touch 1 lead on each end and observe the #.
Look for a spike in resistance.
Battery to starter
Battery to relay
Relay to starter
Battery to ign. Key
Key to relay
Keep at it till you find an issue.
Look up testing the starter internally.

Disconnect the batteries first

 

[ihc1470]

Disconnect the batteries first

Get a wire diagram and familiarize yourself to the routing and color codes.
Grab a multimeter set for continuity (omega/ohms) test the main wires from battery to starter for resistance.
0-1 good the higher the resistance the worse it is. Touch 1 lead on each end and observe the #.
Look for a spike in resistance.
Battery to starter
Battery to relay
Relay to starter
Battery to ign. Key
Key to relay
Keep at it till you find an issue.
Look up testing the starter internally.

Disconnect the batteries first

The problem with testing just looking at resistance is most ohm meters we can get our hands on just are not sensitive enough. When you are talking several hundred amps starter draw it does not take much in the way of resistance to create large voltage drops.

I have even tested for large voltage drops as it sounds like the OP has just using my hands. Turn on every electrical load you can for about a minute and then start feeling the connections and cables from the batteries to the starter looking for hot spots. Those hot spots are your high resistance spots.

As to the suggestion of disconnecting the batteries first, in most cases not a bad suggestion. In this case I would not as you are looking for poor connections and the less items you disturb until you locate the issue the better off you are. Find the problem then fix it. If you are disconnecting many things you may make the problem go away but is it really fixed?

 

[divemaster5734]

Hello all,

I just picked up a 1990 7.3 IDI with a ZF5 as a project truck. Previous guy said it was running, bad transmission, etc, but when I go to turn key to try and crank, I'm reading a massive voltage drop (down to less than 2v) and get basically nothing both inside the cab or when trying to start. Both batteries are new, just replaced those as my first stop but wanted to see what the next possible option might be. New Battery Cables? The guy did install a manual switch for the glow plugs but it doesn't seem to be affecting anything as far as I can tell. Thanks

Just saw your post.
I was having problems with one of my 7.3's not turning over fast enough to start also.
Had to trickle charge the newish batteries to 14v before it would eventually start.
After trying everything else first, I finally cut out the battery connectors and crimped new ones with new terminal connectors.
Couldn't believe the difference.
It turns over by battery at almost 1k RPM, and starts immediately.
This one is a '90 U Haul box truck, and was severely abused by harried shop mechanics.

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There were some cheap battery disconnects between the OEM cable and batteries.

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About $8 each side for parts.

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This is all the junk that was between the cables and batteries.
It's rare for the cables to go bad, rather it's the connection that will oxidize and deteriorate.
For less than new 2/0 MTW replacement cables you can get a cheap crimper set, lugs, and heat shrink.
This was an extreme case, but I was amazed at the results.

 

[Brian Wagner]

my money is on the starter but several of these fine gentlemen have given some excellent advice, see divemaster's post. If it turns out to be the starter as I suspect you will still want to clean and tighten all connections, if you have any of those cheap ass cable ends get rid of them and install some quality crimp on's with heat shrink, and inspect the cable's, if you have corrosion running back up the cable; which I highly suspect on a truck thats 35 years old, then you should replace them. I don't advocate buying off the shelf ready made cables that would cost a fortune, myself I always opt for buying the tools to do the job right, then I know what I'm putting on my truck is a quality american made part. When it comes to crimpers, there are many different styles, you want something that forces an anvil into the cable-end. Also keep in mind that they also have solder on connections, simply strip the insulation back slip your heat shrink down the cable stick it in the cable end heat that baby up with a small propane torch and melt solder in it till it's full. easy pezy not chineesy