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View Full Version : Goofy gas gauge even after new sending unit!!!


Chrisoswald
06-13-2012, 02:34 PM
So, got a new sending unit and dropped it in. Before i dropped it in i verified that it was operational with my meter. Also, verified that the gauge pegs at past full when touching the two wires together that attached to the sending unit. Here is the odd part...the gauge will not read full even though the tank is for sure full and the float is all the way up...what in the world???? Any ideas...thanks guys!!

Ski-me
06-13-2012, 02:55 PM
Depending on the boat....my 94' had some phillip head screws on top of the sending unit that I could adjust to dial in the gauge level. I had 2 screws up there.

Chrisoswald
06-13-2012, 03:04 PM
thanks but no such screws on my sending unit...any other suggestions?

pram
06-13-2012, 03:29 PM
These are the instructions that I received from MWM

Here are the instructions on calibrating your sender.

2. EMPTY CAL
It appears your Empty cal is set too high on the sender. Here's how to correct it:

--You'll need a jumper to short the White wire connection (Send) to the Black wire connection (Neg). An alligator clip is ideal. It needs to be something that you can remove after 2 seconds without doing much fumbling.

--You'll need an assistant to switch the power for you and watch the gauge for you.

--Have the sender out of the tank and connected, power OFF, and your jumper between Send and Neg (in addition to the normal connections, not instead of them).

--Have your power man turn ON the power and count "1000-1, 1000-2" (ie 2 seconds) at which time you should remove the white/black jumper.

--Your power man should have seen the reading be pegged above F for the 2 seconds you had the jumper applied__________. When you remove the jumper, the reading should first drop to below E____________, then do some bouncing____________, and finally finish at E.

--As a sanity check, you could insert the sender and see that the reading starts up scale as soon as the sender goes into the fuel________, instead of waiting until you get 3/4 of the way in.

Chrisoswald
06-13-2012, 06:04 PM
Thanks for the tips.

Here is where I am with this and I'm confused as is the manufacturer of the sending unit.

1. I confirmed that the sending unit is good by testing the ohm output at empty and full. It's correct per the manufacturer

2. I confirmed that when I touch the two wires that connect to the sending unit the gauge pegs past full.

Here is the part that has stumped the tech guy at moeller (maker of the sending unit). When the sending unit is hooked up to the gauge the gauge will not read more than 3/4 full.

I have a 88 prostar.

I will try anything so if you have a guess throw it at me...thanks guys.

HRC
06-13-2012, 06:13 PM
Is there some type of calibration screw on the gauge somewhere? Sounds to me like the problem is not in the sending unit if your resistance is smooth throughout the range of float movement.

Chrisoswald
06-13-2012, 06:23 PM
Nope. No adjustment. What's odd is that the individual parts work as they should but when hooked together no dice. Gotta be something simple I'm just missing.

pram
06-13-2012, 07:02 PM
you said the float? There is a float in the new gauge that you bought?


The only thing that I am thinking is that you did not get the correct length of sending unit

Chrisoswald
06-13-2012, 07:13 PM
Yes. I have a 88 prostar. The sending unit I bought is an exact replacement of what was in the boat.

Chrisoswald
06-13-2012, 07:15 PM
It's the correct length but even if it's not while the unit is out of the tank and I manually raise the float to the full position it will not register on the gauge more tha 3/4 of a tank.

I'm stumped

pram
06-13-2012, 07:48 PM
did your old one stop working?

you can fix the old one with some epoxy and try it out to see if that will work with your gauge

Chrisoswald
06-13-2012, 07:50 PM
The old one is dead.

mayo93prostar
06-13-2012, 07:57 PM
send that one back and buy one that has calibration screws. ISSPRO brand is what I put in my 93. can also get other type from SKIDIM. or contract moeller and have them tell you how to fix it. the problem is your resistance at full is not high enough resistance for gauge to read full. I guess you could add a resister in series to "calibrate" it to full but this is a little hokie.

Chrisoswald
06-13-2012, 08:19 PM
I called moeller and they were stumped since the sending unit was at roughly 30 ohms empty and 230 something full. Maybe it's just a crappy sending unit? I'm stumped because the sending unit is hitting the spec to drive that gauge.

AND

I just tried a new teleflex gauge from west marine and it is giving me the same reading which is really adding to the confusion so I'm wondering if it's someone other than the sending unit

Any ideas?

HRC
06-13-2012, 08:31 PM
Maybe the electronics in the gauge are loading the circuit down. Is it the original gauge?

Chrisoswald
06-13-2012, 09:12 PM
I thought of that and I do believe it's original. However when I take the wires to the sending unit and touch them together the gauge pegs past full. If the gauge was off, as I understand it, it would not peg past full. I'm gonna use a new gauge and see if the problem is still there.

Chrisoswald
06-13-2012, 09:25 PM
If I gotta spring for a new gauge any suggestions on a OEM replacement so I can keep the original look of the dash.....or am I just dreaming?

Chrisoswald
06-14-2012, 04:24 PM
OK...here we go. two new sending units both testing in spec...two new gas gauges...under no hook up scenario can i get either gauge hooked to either sending unit to read past 3/4 full

any ideas on where to turn??

mikeg205
06-14-2012, 04:47 PM
what's the spec of your guages...and what is the resistance reading on your senders... should be low like 30 ohms... will 30 ohms make your gauge read full? Only other thing is test ground. should be no resistance to ground.

mikeg205
06-14-2012, 04:50 PM
OK...here we go. two new sending units both testing in spec...two new gas gauges...under no hook up scenario can i get either gauge hooked to either sending unit to read past 3/4 full

any ideas on where to turn??

what are spec's of guages... what are spec's of senders... you should be able to bench test...in boat. Check connection wire resistance - could be bad wires that leads from tank to dash.


http://auto.howstuffworks.com/fuel-gauge1.htm

Chrisoswald
06-14-2012, 04:59 PM
Thanks for taking the time to reply. So both sending units are new and both go from 30 to 230 something. I bought two for testing purposes to insure I had working models. I verified the ohms with my meter. They are working properly. I also bought two gauges to insure one was not bad.

I think you are right and I got a goofy ground somewhere. Here is the million dollar question. Where do I find the ground. Behind the dash is all original. All neatly run and bundled so Im pleased that its not all hacked up.

Any ideas where to look for ground on a 88 prostar

mikeg205
06-14-2012, 05:06 PM
Main ground on engine. Each switch and gauge will have a ground. Not sure if there's a shared ground cable under dash.

Chrisoswald
06-15-2012, 09:16 PM
So after looking behind the dash and at the rear of the engine I did not see any noticeable issue with the ground. Any suggestions on how to track this down.

j.mccreight@hotmail.com
06-15-2012, 10:06 PM
Are you positive that the sender matches the gauge in being a 240-33 ohm type? 0-90 ohm is another option that is used quite frequently. You have listed the sender value a few times but have not listed the gauge value. You can go to radio shack and purchase the stated ohm resistors and doubly verify the gauge is operating properly by using the resistors instead of the sender, thats what I would try next. The wires to the gauge themselves could also be the culprit.

Chrisoswald
06-15-2012, 10:28 PM
Good question and I thought I may have had a bad gauge so I bought a new one that is 30 to 240 ohms and I get the same 3/4 of a tank. I'm at a loss.

Other than the wiring at the back of the motor what else should I search for?

j.mccreight@hotmail.com
06-16-2012, 12:51 AM
Check the resistance of the wires from the sender to gauge individually and while both are disconnected, you basically want to see 0 ohm or very close to it, if the resistance is higher than this, your wires need to be traced and prolly replaced.

Chrisoswald
06-16-2012, 07:41 PM
Thanks. Will do. I did notice something odd today while out on the boat. When the blower is on all gauges kinda drop a bit.

mikeg205
06-16-2012, 07:54 PM
Thanks. Will do. I did notice something odd today while out on the boat. When the blower is on all gauges kinda drop a bit.

That would indicate a ground and/or old wire issue - probably corrosion... do what JMC. recommended. Corrosion and bad ground are the enemy....you can get contact cleaner at Radio Shack and then cover stuff with dielectric grease to protect. Full should be low resistance...i.e. 30... but maybe the wire is making it more like 50-75 (mad scientist guess). Did you measure the resistance from sender connection to ground at gauge connection? it should equal the reading you get at the fuel gauge at the tank.

Having an 88 I am sure you'll have some wires that have some corrosion.

Chrisoswald
06-17-2012, 08:23 PM
So I am clear I should test every neg and pos lead coming to each gauge and make certain that the ohms are close to 0. While I'm at it I should clean the contacts and dab with some dielectric grease?

j.mccreight@hotmail.com
06-17-2012, 08:48 PM
Since your issue is with the fuel gauge I would concentrate on that. Disconnect leads from sender and disconnect leads from gauge and then measure the resistance of each wire seperatly one to gauge the other to a good ground on engine or battery anything less than .5 ohm is ok, make sure you disconnect battery first. You could also run a new signal wire from the sender to the gauge as a trouble shooting method. Ensure the gauge is getting full battery voltage and ensure the ground on gauge is also good. On a tristar there is a large wire junction about 12 wires iirc, on the rear of engine near the bilge, check that and clean as necessary. fairly simply circuit, sender has a ground and a signal, the gauge has a power, ground and the signal and then the light circuit.

Chrisoswald
06-18-2012, 08:40 PM
Since your issue is with the fuel gauge I would concentrate on that. Disconnect leads from sender and disconnect leads from gauge and then measure the resistance of each wire seperatly one to gauge the other to a good ground on engine or battery anything less than .5 ohm is ok, make sure you disconnect battery first. You could also run a new signal wire from the sender to the gauge as a trouble shooting method. Ensure the gauge is getting full battery voltage and ensure the ground on gauge is also good. On a tristar there is a large wire junction about 12 wires iirc, on the rear of engine near the bilge, check that and clean as necessary. fairly simply circuit, sender has a ground and a signal, the gauge has a power, ground and the signal and then the light circuit.

I am determined to fix this so thanks a ton for taking the time to help. I understand what to do however I'm stumped a bit. And I may show some ignorance here but when I touch the two leads at the sending unit together the gauge pegs way past full. I assumed that would also means that the wiring was good but clearly this is not the case. Can you help me understand why?

mikeg205
06-18-2012, 09:13 PM
When you touch the 2 leads at the send unit your taking the resistance to 0. The sending unit is a variable resistor. Low resistance tells the gauge the tank is full....no resistance pegs the needle.

follow this and you will have your masters degree in fuel senders and gauges. Like suggested before measure the resistance at the sender. So multimeter red probe to sending wire black to ground wire. Then measure signal at gauge...signal wire to ground

j.mccreight@hotmail.com
06-18-2012, 10:00 PM
I am determined to fix this so thanks a ton for taking the time to help. I understand what to do however I'm stumped a bit. And I may show some ignorance here but when I touch the two leads at the sending unit together the gauge pegs way past full. I assumed that would also means that the wiring was good but clearly this is not the case. Can you help me understand why?

check your p.m.'s

Chrisoswald
06-30-2012, 10:19 PM
Got it all sorted out. Turned out to be a bad gauge....got lucky and found a vdo exact OEM matching gauge. HUGE thanks to j.mccreight for taking time out of his day to call me and walk me down the path of getting this fixed.