M38 tub grounding strap

Kenk · Post by **Kenk** » Tue Feb 10, 2009 6:56 pm

Does anyone know if a grounding strap is required for the tub? I have the engine grounded to the frame. I'm trying to diagnose a oil and fuel sender gauge problem and I am wondering if this might be the problem since I have heard that the gauge cluster has to have a good ground to the body. When I removed the tub from the frame I seem to remember one being there. I think it was one the right side behind the passenger seat. Could this strap have been required for a radio installation?

Post by **wesk** » Tue Feb 10, 2009 10:53 pm

The M38 didn't get any extra straps for radio set-ups. Using a simple ohm meter to check for any reistance between the gage cluster and the tub and between the tub and the frame and between the gas tank and the tub is all you need to do. If you have zero resistance you don't need a strap.

The problems with the restorations is we take too much care to paint everything too well. This results in poor continuity between these parts listed above. If you have any resistance in any of your readings you should add a 1.2" wide or wider flat braided strap between those two items.

TomM · Post by **TomM** » Wed Feb 11, 2009 6:12 am

This is an excellent question. Think throught the circuit and find the weak points.
Battery ground needs to be to a very clean spot on the battery bracket. You can polish that spot with a dremel before connecting the cable.
The engine to frame ground is well known but dig in and use internal/external lock washers and be sure you have cleaned all paint away from where the connections are made.
Fenders have a fighting chance of bonding if you are sure the threads of the frame are completely cleaned where the two vertical bolts support the channel.
Use the original fender-body bolts that have been cleaned completely. Check the big flat-spot washers and be sure the teeth on the internal stars are bright and sharp. They will be part of the circuit diging through the fender paint and connecting to the body. The nut side of those connections is hard to clean.
Tub to frame - none of the hardware was too electrical friendly so you have to be sure to clean up a couple and extra take care that its bonded to the frame. The two bolts that go through the body gussets to the frame can give you an excellent body bond. Clean the frame threads and use lock washers that can dig through the paint or clean the paint off the tab, bolt to frame, paint over.
Instrument cluster to body. As far as I can tell this connectin is via the zeus (not sure if thats the correct term) to the dash. When you refinish your cluster panel leave the paint out from under the screws. But check the zeus side thats crimped to the dash. These can be cleaned up with a dremel or they can also be removed for blasting and replaced with new bright pieces. Reinstall after blasting the body while there is not yet paint.
Tank. You can easily hide a new short wire from the sender or pickup to the seat bolt..
As Wes mentioned, check with a good ohmmeter. Using a DVM you should see typically see about 1 ohm (depends on how far you have the leads) through your connections.

Kenk · Post by **Kenk** » Wed Feb 11, 2009 4:35 pm

Wes,

I tryed what you recommended on the tank sendor. I put my meter in the 2K range and put one lead to the top of the sendor and the other to one of the the tub cover plate bolts(unpainted). Started out with a reading unconnected of 1. and ended up with a reading of close to zero(.001. ) So based on your recommendation, the tank should be grounded to the frame. I will check the instument cluster to see if I have the same result. Please confirm I should have the cluster installed in the dash. That makes sense to me.
If the tank gauge still does not work, do you have any recommendation?
I had installed a new level sendor from Midwest.
As for the oil pressure, the sendor is a NOS 1-120 from Debella. The gauge is original 0=120 and seems to stick or read low. It is not the external resistor type .I still have to check actual pressure off the main oil gallery with a mechanical gauge. I dont what to pull the sendor out yet so will go to the first port toward the from of the engine from it. If all looks good there then I would have to think I have gauge issues.

Does this make sense?

Ken

Post by **wesk** » Thu Feb 12, 2009 12:26 am

I may be wrong here but I assumed you started the gage troubleshooting with the M series gage troubleshooting manual. Whenever you suspect a ground (continuity between parts) issue you can immediately connect a alligator clipped jumper between them to help you continue with the troubleshooting.

The cluster should be installed with it's four 1/4 turn fasteners. I never trusted these to provide an adequate ground. I always added a ground jumper between a bolt/nut on the cluster to one of the circuit breaker mount screws on the side of the cowl battery box. You can install a quick insulated spade or knife connector to this ground strap for ease of cluster removal.

dv3468 · Post by **dv3468** » Sun Feb 15, 2009 10:00 pm

Wes: I have a question on the same lines as this post regarding grounding the gas tank. My M-38 was converted to a 12 volt system very early in its life and when I restored it, I left it that way. I did a frame off restoration and installed a new tank, sending unit and pick-up. The gas gauge is a civilian 12 volt. I noticed from day one, that I have to tap on the fuel gauge to get it to move. When it did, it went right to full and it appears to stay there. After reading theses posts I realized that the gas tank was not grounded to the body,so I ran a #10 ground wire from a screw on the gas pick up plate to the side of the tub. I have several ground straps from the tub to the chassis. I checked the positive wire on the top of the gas pick up unit and I get a 3.1 volt reading. Shouldn't I be getting a 12 volt reading? I checked the gas gauge on the dash cluster and I get a .001 reading on the ohm meter. So I assuming the gauge ground is OK. I'm wondering if I have a six volt sending unit instead of a twelve volt. Maybe that explains why the needle is always pinned on full. What do you think?

Regards Dennis

Post by **wesk** » Mon Feb 16, 2009 12:25 am

First off the voltage reading you are taking is a transient reading since sender is a variable resistor the voltage at it's input screw will vary depending on the load applied by the position of the sender's float.

Next the voltage reading you get at the sender screw will depend on the ground continuity of the tank and tub.

Third the original sender would have been a 24 volt unit not 6 volts. Unless the tank you had was not an M38 tank and had an early CJ tank and sender.

Try using a large jumper cable and clip it to the tank and the frame rail. Then connect your volt meter's ground lead to the frame rail as well and tell us what voltage reading you get.

dv3468 · Post by **dv3468** » Mon Feb 16, 2009 4:28 pm

Wes:

I did what you told me to do and no matter what, I get 3.0 volts off the sending unit screw!!! Just so you know, the gas tank is a large mouth M-38 tank, but I suspect my parts guy sent me a six volt 2A sending unit. Also, isn't the 2A gas tank shallower than the M-38 tank? Even if I get the fuel gauge working, it is going to read less gas than what is actually in the tank.

Awaiting your instructions.

Regards Dennis

Post by **wesk** » Mon Feb 16, 2009 10:47 pm

The ground quality must be identicle for both the sender/tank and the test volt meter. Using a lower voltage sender creates two issues.

1-quite an error in reading.
2-Heat as the resistor in the sender overheats from the higher voltage.

Also the sender must be matched to the gage for proper readings and operation. There are several ways to make a gage needle move and each type of movement has a diifferent sender requirement.

If you are staying with 12 volts then use a quality aftermarket universal 12 volt matched gage and sender. These come with a float arm that is adjustable for depth and throw.