Speedometer & Fuel gauge troubleshooting

[KallyLC]

I've been trying to diagnose a bouncy speedo and a complete non-functional fuel gauge on my M1008, looking for some advice.



For the speedometer, the I think I might've figured it out, but I'd like to see what others think. The problem I have is that the speedo bounces and makes a whirring noise at highway speeds. I checked the gears in the transfer case, they both look fine. Turned the disconnected speedometer cable by hand and with a drill, it bounces. Pulled the gauge out of the dash and turned the input for the speedometer by hand, and you can see the magnet is 'slipping', so I think that's the issue. I'm in the UK, so I can get parts but there's nowhere that rebuilds gauges. Is it possible to get just the magnet and replace it? Or is it cheaper and easier to just replace the whole unit? As for the noise, the previous owner claims he replaced the cable to try and fix this problem but I don't know if he lubricated it.

The fuel gauge is the real head scratcher. It reads past full at all times, regardless of if the truck is running or not, as you can see above. First thing I did was check the fuel sender ground, and it turns out that the frame connector is there but half of the wire is missing and there's nothing connecting it to the tank/sending unit. So that's a problem, obviously, but when we earthed it manually by touching a wire from where the ground should attach on the sending unit to the frame, the needle stays where it is.



We poked around with a multimeter and the fuel pump/sending unit doesn't actually seem like it's isolated from the tank, and the tank isn't isolated from the frame, so is it already earthed? If so, isn't the separate earth for the sending unit a bit redundant? I'm no electrician so I'm probably misunderstanding how it's supposed to work, but it doesn't seem to make much sense.

The connector for the sending unit was a little stuck so I didn't want to pry too hard, but I guess the best way to check for sure if it's the gauge or sending unit is to earth it to see if the needle moves. What do you guys think?

 

[tom lovinggood]

I have the same problem with my M1009. The fuel gauge always reads beyond full until the actual fuel level is about 1/4 full. Have not been able to source the problem yet.

 

[cucvrus]

I struggle with electrical issues at times. I have been successful doing a process of elimination. I have the privilege of having extra parts on hand. On a fuel gauge first thing I do is check the ground on the sending unit and clean it. Next I use a GM sending unit and plug it in and see if I can get a reading by manually operating the float. Any old school same vintage GM sending unit will work. Most times the sending unit is the problem. I have had a few that were rusty and corroded at the gauge clutter connection. But yes diagnostics is time consuming. I would not just start replacing parts. I would get the problem located and go from there. In the mean time drop the tank and get everything cleaned up and replace the fuel sock in the tank. Good Luck. I hope that helps. This work is dirty and requires patience. I always get the M1009 fuel tank issues directly after the customers have filled the tank and have installed a class 3 receiver. Fun Times. 10 hours on one tank over the summer to get in, fix and reinstall.

 

[dependable]

Ground on sending unit usually cause of fuel gauge problems.

A slightly kinked or un lubed speedometer cable could cause the speedo bounce. Try disconnecting at dash and spraying some light lube down cable housing. It could be the speedometer itself, but would check cable first.

 

[INFChief]

All other gauges work properly? Get behind the dash and check the gauge wiring. Make sure the dash is also properly mounted.

If you can, disconnect the lead from the sending unit and from behind the dash. Perform a OHMS check (continuity check) to see if that lead is broken or has a bare spot touching ground (earth).

It’s been a while but the sending unit has a gasket that not only seals it but isolates the sender from the tank. But I somehow think there should be a separate ground from the sender body to the frame.

I need to research that a bit.

 

[INFChief]

I've been trying to diagnose a bouncy speedo and a complete non-functional fuel gauge on my M1008, looking for some advice.

View attachment 848081

For the speedometer, the I think I might've figured it out, but I'd like to see what others think. The problem I have is that the speedo bounces and makes a whirring noise at highway speeds. I checked the gears in the transfer case, they both look fine. Turned the disconnected speedometer cable by hand and with a drill, it bounces. Pulled the gauge out of the dash and turned the input for the speedometer by hand, and you can see the magnet is 'slipping', so I think that's the issue. I'm in the UK, so I can get parts but there's nowhere that rebuilds gauges. Is it possible to get just the magnet and replace it? Or is it cheaper and easier to just replace the whole unit? As for the noise, the previous owner claims he replaced the cable to try and fix this problem but I don't know if he lubricated it.

The fuel gauge is the real head scratcher. It reads past full at all times, regardless of if the truck is running or not, as you can see above. First thing I did was check the fuel sender ground, and it turns out that the frame connector is there but half of the wire is missing and there's nothing connecting it to the tank/sending unit. So that's a problem, obviously, but when we earthed it manually by touching a wire from where the ground should attach on the sending unit to the frame, the needle stays where it is.

View attachment 848082

We poked around with a multimeter and the fuel pump/sending unit doesn't actually seem like it's isolated from the tank, and the tank isn't isolated from the frame, so is it already earthed? If so, isn't the separate earth for the sending unit a bit redundant? I'm no electrician so I'm probably misunderstanding how it's supposed to work, but it doesn't seem to make much sense.

The connector for the sending unit was a little stuck so I didn't want to pry too hard, but I guess the best way to check for sure if it's the gauge or sending unit is to earth it to see if the needle moves. What do you guys think?

M1009 Fuel sending unit ground wire

Hi guys, Just some simple questions... What gauge is the black ground wire from the fuel sending unit to the frame? Or what gauge should it be? And what size is the ring terminal at the end of that wire that connects it to the frame? Thank you...

www.steelsoldiers.com

I've been trying to diagnose a bouncy speedo and a complete non-functional fuel gauge on my M1008, looking for some advice.

View attachment 848081

For the speedometer, the I think I might've figured it out, but I'd like to see what others think. The problem I have is that the speedo bounces and makes a whirring noise at highway speeds. I checked the gears in the transfer case, they both look fine. Turned the disconnected speedometer cable by hand and with a drill, it bounces. Pulled the gauge out of the dash and turned the input for the speedometer by hand, and you can see the magnet is 'slipping', so I think that's the issue. I'm in the UK, so I can get parts but there's nowhere that rebuilds gauges. Is it possible to get just the magnet and replace it? Or is it cheaper and easier to just replace the whole unit? As for the noise, the previous owner claims he replaced the cable to try and fix this problem but I don't know if he lubricated it.

The fuel gauge is the real head scratcher. It reads past full at all times, regardless of if the truck is running or not, as you can see above. First thing I did was check the fuel sender ground, and it turns out that the frame connector is there but half of the wire is missing and there's nothing connecting it to the tank/sending unit. So that's a problem, obviously, but when we earthed it manually by touching a wire from where the ground should attach on the sending unit to the frame, the needle stays where it is.

View attachment 848082

We poked around with a multimeter and the fuel pump/sending unit doesn't actually seem like it's isolated from the tank, and the tank isn't isolated from the frame, so is it already earthed? If so, isn't the separate earth for the sending unit a bit redundant? I'm no electrician so I'm probably misunderstanding how it's supposed to work, but it doesn't seem to make much sense.

The connector for the sending unit was a little stuck so I didn't want to pry too hard, but I guess the best way to check for sure if it's the gauge or sending unit is to earth it to see if the needle moves. What do you guys think?

3-12. FUEL TANK AND SHIELD MAINTENANCE (M1009) (Cont) - TM-9-2320-289-20_159

trucks125ton.tpub.com

 

[ezgn]

All other gauges work properly? Get behind the dash and check the gauge wiring. Make sure the dash is also properly mounted.

If you can, disconnect the lead from the sending unit and from behind the dash. Perform a OHMS check (continuity check) to see if that lead is broken or has a bare spot touching ground (earth).

It’s been a while but the sending unit has a gasket that not only seals it but isolates the sender from the tank. But I somehow think there should be a separate ground from the sender body to the frame.

I need to research that a bit.

As CUCVRUS has said the sending unit is grounded from the unit to the frame of the truck.

 

[KallyLC]

Thanks for the advice, as I said in the original post, the sending unit ground wire just doesn't exist at all, so that's obviously a part of the problem. Since posting this we've gone through the troubleshooting process in the TM: grounding the wire from the gauge causes the needle to go back to E, but when we ground the sending unit to the frame, the gauge just reads past full. So, it looks like the sending unit is bad.

As for the speedometer, the gauge seemed a little suspect when we pulled it out and inspected it, but neither myself or my brother have seen the inside of a mechanical speedometer before, so we're not entirely sure what's normal. Next time I get a chance to tinker on it, I'll lube the cable and see if it helps.

 

[cucvrus]

On an M1008 I have encountered the sending unit ground wire broken off more than once. The best way to change the fuel tank and sending unit on an M1008 is to remove the cargo bed or at least block it up. I have removed beds many times to get to sending units. Yes, you can do it without removing the bed. But the job is tight and you will struggle longer getting things properly seated and correct with the bed in place. At this point a torch to heat the bed nuts is needed. And if not heat them cut them off. They are just fancy carriage bolts. I have had to cut a few but most times heating the nuts does the trick. Not as hard as it sounds. The M1008 has the 8 tie downs in the bed and they can lift the bed from the 4 corners with no problem. Good Luck. Do it right the first time and it gives you the opportunity to clean and paint the rear frame and under side of the bed. Do as you wish. Make it a project that makes other improvements. Happy Thanksgiving.

 

[KallyLC]

I'm working with next to no space, so removing the bed isn't an option. I planned to just drop the tank but I'll look into lifting the bed, the troop seats will have to come out for even just that, though.

Still just business as usual over here, but you guys have a good holiday. We don't get another one till Christmas.

 

[nyoffroad]

I'm working with next to no space, so removing the bed isn't an option. I planned to just drop the tank but I'll look into lifting the bed, the troop seats will have to come out for even just that, though.

Still just business as usual over here, but you guys have a good holiday. We don't get another one till Christmas.

Dropping the tank is a PIA especially when there is fuel in it, that tank is pretty long and even 5 gallons or so starts sloshing from one end to the other can make it fall off a jack or tip and spill. Removing the bed is a pain too but I lean that way. I don't have a torch set so how I do it is I cut the bolt heads in a # pattern with a air powered cut off tool, do that so the square thats left is 1/2" and wack it with a hammer and it falls out the bottom. ( I hate laying underneath getting rust in my eyes and ears).

 

[cucvrus]

I have had success with a standard propane torch and MAP gas. I have to make it work sometimes. And a good cordless Milwaukee impact changes the rules 10 fold. Heat is the secret. I can not always access the areas I work with Oxy/Ace tanks but the MAP gas works well. Less then $20. Do it right or forget it. You need a 3/4" impact wobble socket and an extension. A helper to stand on the bolt heads is vey helpful. Or have the helper push down on them with a board while you impact. Doing it by hand will surely strip the carriage bolt out of the bed floor. Happy Thanksgiving. If you were closer I would stop by and take the bolts out for you. But UK. Send the 2 way ticket and I'll be there Saturday. Take Care and Get it Done right the first time.

 

[cucvrus]

I hate laying underneath getting rust in my eyes and ears).

Safety googles and ear plugs every time I use a torch or am under a vehicle using an impact. Safety up front is a cheap alternative to injuries. Grinding the heads from a carriage bolt is 20 times harder then heating the nuts and dawning the safety gear. But not my truck or job. Do as you wish and enjoy what you are doing. Happy Thanksgiving.

 

[ezgn]

I'm working with next to no space, so removing the bed isn't an option. I planned to just drop the tank but I'll look into lifting the bed, the troop seats will have to come out for even just that, though.

Still just business as usual over here, but you guys have a good holiday. We don't get another one till Christmas.

You don't need to remove the bed. Just get another guy and slide the bed back about 16'' inches. You will need some 4''x4'' to place under the bed on the frame in the rear as you slide the bed back. You will know what to do as you begin to slide the bed back by watching underneath as you move the bed backwards. A couple of 4' 4''x4'' will work.

 

[cucvrus]

You don't need to remove the bed. Just get another guy and slide the bed back about 16'' inches.
Excuse me but isn't that the same procedure to remove the cargo bed. I mean getting all 8 bolts and nuts out is the hardest part. If you go thru that trouble you may as well go the extra mile and clean and paint the exposed frame. But what do I know? I am just saying the obstacle he was having was getting rusty fasteners loose and off. Of course sliding the bed back is an option, but taking it off is the best way to do an excellent job. So yes do whatever you are comfortable with. I guess with a barn and 4 ratchet straps I am a cheater. Happy Thanksgiving.

 

[ezgn]

You don't need to remove the bed. Just get another guy and slide the bed back about 16'' inches.
Excuse me but isn't that the same procedure to remove the cargo bed. I mean getting all 8 bolts and nuts out is the hardest part. If you go thru that trouble you may as well go the extra mile and clean and paint the exposed frame. But what do I know? I am just saying the obstacle he was having was getting rusty fasteners loose and off. Of course sliding the bed back is an option, but taking it off is the best way to do an excellent job. So yes do whatever you are comfortable with. I guess with a barn and 4 ratchet straps I am a cheater. Happy Thanksgiving.

He said he did not have room to remove the bed. My bolts came out without any problem. I must have misread the part where he said he was having trouble removing the bed bolts. My truck is rust free, so I agree if he has a rust problem going through the effort to slide the bed back it would be wise to remove and to do an excellent job.

 

[KallyLC]

I never actually said a word about the bed bolts or rust, that's all coming from you guys. All I said was I didn't have the space to remove the bed.

Don't really have that much of a rust problem (surprisingly, considering the truck's spent most of its life in the UK), so i'm not really that concerned about painting the frame when the problem is the fact the fuel gauge doesn't work. If I'm going to have to fight rusty bolts and manhandle a heavy object then the fuel tank is definitely the lesser of two evils. Not planning on dropping the tank while it's full, so it shouldn't be that bad.

fully intend to do an excellent job of the task at hand, replacing the sending unit.

I appreciate all the advice, and I've no doubt those methods work for some people, but it's just not the way to go for me given the conditions I have to work with.

Thanks.

 

[Skinny]

I think even if you do a full sweep from zero ohms to infinite the gauge should stay within the E and F markings.

Welcome to old crappy GM electronics. If that's not bad enough the aftermarket tank senders aren't great quality either.

Both of my trucks do what you have going on. The Burb has the best speedo I have out of the pile of them but the gas gauge is junk.

The M1031 has a very nice set of digital aftermarket gauges that have been worth the money so far. I was tired of the gauges and lack of gauges. To replace a speedo brand new state side you are over $300.

Double that and you are entry level aftermarket digital. Plus you get oil pressure, temp, and tach. That's why I bit the bullet. I think the Burb will get a set sooner then later.

I think your fuel gauge is junk because it's reading outside of the scale. I'm sure you can get someone to rebuild them but you are stuck with the lack of instrumentation still. Personally wouldn't go used. They are all old at this point and not worth the money and time IMO.

 

[GM72K10]

Just did this in September. Helps to have a forklift. Got the sending unit from NAPA.

 

[Mullaney]

Just did this in September. Helps to have a forklift. Got the sending unit from NAPA.

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Definitely!

A forklift is that Soldier B that most of us don't have...