Fuel gauge and sending unit match

[JBJeep]

Still trying to debug my recently restored stock, 24v M38A1.
New wiring, New sending unit, used gauge.

Fuel gauge will not register more than 1/2 full.
Needle will go down, however.

Restorer says it might me a ohms mismatch between gauge and sending unit. The one I provided didn't work, but he had one in his stash that did...but we never tested it with anything approaching a full tank.
He says there is something like three different versions out there for Miseries trucks.
True?

So...what to do?
I would like to stay away from reproduction units because the rest of the gauges are period SW units.
(One dealer says they have a NOS one, but they want $200+ for it, but even then, I'm guessing it may not work.)

Another suggestion I received was to ground the sending unit. It is grounded, but would grounding it again/better make a difference? (To me, either something is grounded or its not).

How (or where) can you buy a "matching" units?

[Kendall]

These are easy to test. Remove the sending unit from the tank and with power to it and it grounded, move the float up and down and watch the gauge move from full to empty if working properly. And you might find there is a movement issue with the arm of the sending unit.

[RonD2]

Agree with Kendall.
Could be mistaken (thinking M38 not M38A1), but can't the sending unit be installed "clocked wrong" where the baffle inside the tank interferes with full movement of the float arm?

Just a thought.

[JBJeep]

Kendall, RonD2

Thanks for the reply.
Considering there is some movement of the needle, that means that there is some connection between the gauge and sending unit.

Right???

So that seems to make sense to me,
but I have never been confused with Tesla, Westinghouse or Edison.

[wesk]

As always with the M38 & M38A1 instruments and senders it is essential that you properly ID them by part number and insure they are correctly matched to the appropriate sender. This valuable info is in your ORD 9 SNL G740 or 758.


Indicators


Indicators


Senders


Senders

I caution you that "Optional With" has two meanings on these parts pages and you should read two or three times to make sure that you understand that "Optional With" is used for both legitimate substitutions and the unit it is suppose to be matched to. As a basic rule those with note indication M170 only used on M170 and those with note M38A1 only used on A1.

In addition to substitutions listed in the ORD 9 the Army also published a TB-ORD-479 Dated Nov 52 which has additional substitutions approved for the MB/M38.

The following steps require a VOM and a knowledge of how to use it.

Then make sure all grounds are functional. Remember the gauge housing must be grounded thru the dash cluster then thru the dash structure. This grounding is often inadequate due to painters not leaving any bare metal contact at the fastening points. Make sure you fully evaluate gauge grounds. Don't be shy. Scratch the surfaces to insure your VOM leads are touching bare metal surfaces. The simple test on gauge housings is to touch one probe to the BARE METAL gauge housing and the other to a BARE METAL spot on the firewall. Your VOM should read "0" ohms. If not then leave the 1st lead on the gauge and move the second lead to a BARE METAL spot on the dash. Still not zero then Move the 2nd lead to a BARE METAL spot on the instrument cluster your VOM should read "0" ohms. Each connection that didn't read "0" needs a fix for the poor ground.

Then apply these same principals and needs to the sender, tank and tub ground path.

Next insure the sender float arm has unobstructed full travel in the tank.
This illustrations clearly shows the correct orientation of the sender.

Now with the basics covered we can move on to troubleshooting guides. In particular the Army TB 9-2300-228-20

[JBJeep]

Wes, thanks.

We will check out the orientation of the unit.
But being the pessimist, I'm doing research in buying new parts.

A couple of questions.
I really have the manual I used it while buying the parts for the restoration.
But to a novice, they do leave some room for error or misinterpretation.

So, allow some rookie questions....
-Would there be there a huge problem using the early fuel gauge in a late vehicle (mine is #82551)? (I think the main difference is the luminescent markings on the early unit).
The gauge in there now is a SW-505C/Ord # 7725352.
I have no idea what the sending unit is because I did not buy it.

Also, since I'm not in the Army and it's not 1963, I may not have access to real NOS stuff.
How do these recommendations work in today's repro world?
KW has SW-505J/Ord #8876374 Fuel gauge at a price about X4 the cost of a repro. with the same ORD number.
I hate to spend that kind of money, but I will if I have to.

The sending unit WO-810284/ORD# 8329687 is out of stock in many places, but Army Jeep parts says they have some.
Just to make sure I'm reading the manual correctly...will that work with a SW-505J???

Thanks

[wesk]

It takes a bit of getting use to these manuals. I have the advantage of 22 years in the military using them.

WO (Willys Overland) part no. 810284 was never assigned an Ordinance part no. WO 805914 was assigned the Ord. part no. 8329687 and in the later parts manual also the FSN (Federal Stock Number) 6680-515-3456. Since both have the comments "optional with" indicating they are interchangeable. You have several numbers that may help you locate a useable one on Google. All those mentioned above as well as the AC # 1517756, the SW # (Stewart Warner) 507F and the DP # GG9096. Also the old eleven digit FSN's switched to new 13 digit NSN (National Stock Numbers) in the 1990's. You can convert the FSN to an NSN by just adding 2 stand alone digits after the original FSN's 1st 3 digits. For the FSN 6680-515-3456 we get an NSN 6680-00-515-3456. These two digits are a NATO thing and represent which country's supply arm procured the item. -00- is for the US.

Also the manual often assumes you'll know that no M170 or M38A1 note in part description means it is for the M38A1 and not the M170. This is because the M38A1 was introduced in Jun 1952 and the M170 wasn't produced until about 1955. So most of the time the manual writers only made special annotations in parts descriptions for the new guy on the block.

If accurate fuel level reading is your only concern I would simply switch to a direct reading mechanical gauge since the tank is right under your seat. Or buy an aftermarket electric gage and matching sender that will fit the instrument panel hole and the tank depth.

[JBJeep]

WesK

Ideally I would like stock looks AND an accurate reading.
I can't imagine that's impossible, but we are dealing with 70 year old vehicles, so who knows.

So, right now there are two sending units readily available:
810289 and 810284.
I have no idea what makes them different from each other.

I have a Fuel gauge: SW-505C/Ord # 7725352.
I can buy a SW-505J/Ord #8876374.

By your reading of the manual...which of these will work together?

[RonD2]

Hi JB,
I seem to recall your recent experience where you bought an expensive NOS speedometer you didn't need because you decided not to troubleshoot first and find the busted cable?

Just an observation, but it seems like a repeat here to buy expensive parts you may not need --- until you first complete the troubleshooting checks suggested above?

You know your gauge and sender both work --- they just aren't accurate.
My bet is the sender is clocked wrong in the tank (you said you didn't install it and your "restorer" did. Did he clock it correctly?).
Second bet is bad electrical connection somewhere.
It's a 20-minute job to pull the drivers seat and the sender.
Heck, with a flashlight you might even be able to look down the filler neck of the tank and see it.
I can't remember which side of the baffle its on.

Good luck!

[JBJeep]

RonD2

Thanks for the reminder!

The restorer, a guy who I trust, swears it is in correctly and properly wired..
Because I don't have the history of the components,
I have to look at them as well as installation issues.

As far as parts cost, if it takes another $75 for a different sending unit, at this point its a small price to finish the Jeep correctly. (Or you might call it "good money after bad". ).

[RonD2]

I think I'd ask him directly if he's aware of the tank baffle and clocking the sender for full range movement around it.

Or, if you think he'd be insulted by the question, like I said it's a 20-minute job to see for yourself.

I read where your restorer provided the sender so you don't know anything about it (including its part number).
Yes, it could easily be mis-matched (or junk after-market) parts is the problem.
I'm going to say this again: your gauge and sender are WORKING.

[wesk]

If you feel overly challenged to accomplish the troubleshooting listed perhaps this is something you should discuss with your TRUSTED RESTORER under the terms of WARRANTY WORK? Did this troop give you an invoice for the resto that included a parts listing with cost of each part? You also mentioned that you order most of the parts. So has your restorer told you where the sender came from?

By your reading of the manual...which of these will work together?

My reading of the manual is as I explained in my long post. The key to understanding is "DO NOT INSTALL ANY M-170 PARTS! Both of those part numbers you said are available are both for the M38A1. As I said in the previous post they are interchangeable.

[JBJeep]

Both of those part numbers you said are available are both for the M38A1. As I said in the previous post they are interchangeable.

Which parts are you referring to, the gauges or sending unit?

[RonD2]

So, right now there are two sending units readily available:
810289 and 810284.

Communicating over the interweb can be hard and confusing at times.
Does this help?

[JBJeep]

I was just questioning whether you were referring to the gauge or sender...since I mentioned two of each.
Thanks.