I Need Help With Re-Wiring Some Gauges

[Dave Siegel]

Here I am again, stumped with a minor electrical question (or two). I feel like a real dunce about this stuff, because I can put it together but I can't seem to diagnose it.

Four years ago I had the motor rebuilt in our coach (PD3751   Detroit 671) by a brilliant rebuilder in Sanford Florida. (He has since moved and I have no idea where he is.) He installed a VDO electronic tach that works off the Alternator and a VDO electronic speedometer that works off a sender on the transmission. They worked flawlessly for 4 years, until I decided to get in and rebuild my dash panel. Now I can't get them to work....period. I have downloaded the instruction sheet from VDO (these gauges are still available. But it really makes no sense to me. I suspect an interuption in the lines coming from the back to the dash, but I am not sure how to check them?? Does anyone know anyone in South West Florida that can help? Let's say Sarasota down, even over to Miami or Ft. Lauderdale. I disconnected the wires behind the gauges and now I can't seem to find the correct order to re-hook them up. I don't even know how to see if I have current running to the gauges. I know, I'm a mess, but I'm trying to get better, I really am. I figure by the time I am old enough to understand everything I what to know about these buses I'll be about 180 years old.

My second dilemma is: I have two fuel tanks with two gauges. I have a wire hooked to the tanks and then to the gauges. All of a sudden both gauges stopped working. I suspect either a break in the line running to the front or a connection has come off. Again I am stuck in trying to figure out the continuity of those lines. I suspect the answer to problem number one above will lead me to the answer of number 2???

Thanks in advance for any simply worded answers that a dummy might understand.

Dave Siegel

[Dreamscape]

A standard practice that I use is when taking apart wires/cutting them, number them, even if the numbers don't match your schematic. You can always remove the "wrong" numbers after all is checked and complete.
With ignition on you will have power coming out going to gauge, then back to gauge from sender. Are your wires color coded in any way? Do you have a wiring schematic? HTH

Happy Trails,

Paul

Dreamscape

[RTS/Daytona]

Hi Dave

Doesn't sound like you problem should be a great mystery.

First thing I would do is:

#1
Find the "HOT" wire for each of the gauges - just put your meter on a scale that will read 12 volts - hook the GROUND side meter lead to a GOOD GROUND (Something that bright shinny metal and diffinitly hooked to the bus chassic) - then TURN ON THE IGNITION - use the red meter lead and touch it to EVERY loose meter wire - I suspect 2 of them (tach/speedo) will shown 12-13 VOLTS - these wire will go to what is usually an "I" terminal (stands for ignition) on the meter

#2
Now that you found the "HOT" - go find the GROUNDS - - keep the ignition on that good ground - put your meter on the lowest OHM scale  and touch  the other meter to each of the other unkown wires - I susspect 2 of them will show (ZERO OHMS) - those will be your grounds - they go to the "G" ground terminal of the meter

#3
should be 2 wires left - with the engine running the tack (R terminal on the alternator) will read 13-14 volts with the engine running

as far as the fuel gages go (I) terminal is ignition (+) - (G) terminal is GROUND (-)  - (S) terminal goes to the SENDER

any questions call me **386***672***0571**

Pete RTS/Daytona  

[Dave Siegel]

Thanks Pete, I can understand that. My wires are color coded and I know which one is which. I guess I did that part right.

I will try tracing it like you suggested. I understand markings on the rear of the the fuel gauges, I guess my question was how do I test them?

How are you coming with that great looking RTS? How about sharing some pictures? For that matter, if the file is too big for here, what about sending me some pics and text and I'll put your RTS on my web site?

Thanks,
Dave

[RTS/Daytona]

Dave

Thanks for your kind comments about my RTS - here's a link to my Photo Albums
see --->   http://community.webshots.com/user/hawk_ii

On to the Fuel Gauge Testing

#1 - make sure you have 12v between the  (I) terminal and the (G) ground terminal (test at the meter)

#2 - us a jumper to short the (S-Sender) terminal to ground - (meter should read EMPTY)

#3 - remove the wire from the (S) terminal (METER SHOULD GO PAST FULL)

#4 - verify sender operation (Most likely you have a 0-90 OHM GM style sender)  some eary GM were 0-30 OHMS
       remove the (S) wire and measure between Ground with your meter OHM scale - read the resistance from the tank sender
       0=enpty 90=full

Pete RTS/Daytona

[Dave Siegel]

Wow Pete, you actually make that understandable. I'll give it a try.

The link is fine, got any background text that you would like to email me so I can add it into the web page?

Dave

[Don4107]

Dave,

Pete has given you some great directions.  The only thing I would add is to be sure the ground you use is connected to the main body.  That is, the panel might not have a good ground to the body.  Check with the meter for 0 ohms from the panel, assuming it is metal, to the body. 

Do this with all power off.  If you check it with power on, use the DC scale.  There should be 0 volts between the panel and body. (If there is voltage present it may blow the meter or meter fuse if you use ohms scale.)

Since you have multiple instruments not working is should be something they all have in common, either +12v or ground. 

Good luck and let us know what you find.

Don 4107

[Dave Siegel]

The panel has a separate ground  to the body. Of course the body (and the frame) are aluminum. (It's a PD3751) So I created a grounding strip just for grounds from the panel.

I don't understand "check from the panel to the body for 0 ohms". Do you mean to touch one leg of the meter to the panel and the other leg to the body ground?

First the tach and speedo went out and several months later the fuel guages took a nose dive. Sounding more and more like a bad ground,huh?

Dave

[gus]

Don is right, with so many gages not working it has to be something they all have in common-this appears to be the new dash.

You have lost a ground connection somewhere. If the new dash is a non-conductor you will need to have a ground to each gage.

Connect some gages to a good bus ground with alligator clips on jumper wires and you will probably have working gages.

[Don4107]

Dave

"I don't understand "check from the panel to the body for 0 ohms". Do you mean to touch one leg of the meter to the panel and the other leg to the body ground?"

Yes, just to confirm that you have a good ground.  What type material did you use for the new dash panel?  If metal is it painted or coated?

Another way to check the grounds is to turn on the power and using the DC voltage scale of your meter to check the cases of the gages and the dash panel for voltage with the meter negative probe on the main body.  If the gages and panel are well grounded there should be little or no DC.  If there is 12VDC then you have a ground problem. 

A little hint, whenever checking something like this always test a known 12 volt point to confirm that you do have the meter/test light grounded.  If you don't do this you never know for sure whether the point under test is really 0 volts or your tester is not grounded.

Another quick check.  Do the instrument lights work?  Has to be a ground for them to work unless they are a two wire system.

Ground problems can be a pain.  Just remember that the ground circuit has to carry the same current as the positive side.  Does not matter if its the starter pulling 400amps or a gauge pulling a few milliamps.  No ground, no circuit.  They sure can fool you because when you find good voltage you tend to go on and end up skipping half the circuit, the ground side.

You will get there.  Just keep at it.

Don 4107

[Dave Siegel]

Thanks Don, and everyone else that responded. The dash panel is made out of 1/2" plywood laminated with a motor turned (brushed) aluminum face. (The material is similar to what old vintage race cars used for a dash. (But of course their's were cut from real steel.) I knew that would not provide a ground and the case of the tach and speedo are plastic, so armed with that knowledge I provided a separate ground and then I grounded everything in that panel to a ground bus bar. The lights do work on the gauges. And other gauges on the dash work.

A very important thing to always remember with this particular buses circuitry. It is as simple as it can be. I wired it with a friend. It is color coded (most of it). The wires are tied in a harness (Most of them).  As I said in an earlier part of this thread, I seem to be able to assemble theses things pretty well, but after a while if there is a problem, I have a real problem diagnosing the issue. Brain fart time, I guess.

Ground wires for everything have always been an issue because this thing is entirely aluminum. I have been working my way through that. Over the years I have found connections that I made that were squeezed too tightly with my wire connector crimper, and that end would break or fail. I was most interested in how to provide a test for each line, first to determine that the line is in one piece, and then to reassure myself that the in place grounds are still working.

I really thank everyone for their patience and understanding. I will try what you have suggested, and keep everyone informed of the outcome. Maybe even post some pictures.

Dave Siegel

[gus]

Dave,

To second what Don said and be sure you didn't miss it.

It is very important that you make sure your meter is operating properly.

Always touch the two leads together when checking resistance to see if there is a meter reaction. An ever better ground check is with a very small scale digital meter. With power on you should have a very, very small or no voltage reading. Anything above micro volts means a poor ground. 12v means no ground.

When checking voltage just because you touch metal with the ground lead doesn't mean it is a good ground. Check the ground point before checking voltage. Alligator clips on meter leads are best.