Gauges Reading Backwards

The Dog House · Post by **The Dog House** » Sun Jun 03, 2012 9:47 pm

I just installed a set of new Faria gauges. According to the wiring diagram in my Trojan F26 manual, the yellow wire from the accessory harness is the ignition wire and the white wire from the accessory harness is the ground. I connected the gauges accordingly. My gauges are working, but in reverse. My oil pressure gauge starts at 80 psi with the engine off, "decreases" to 50 psi with the engine running at idle, and "decreases" to 40 psi with the engine running at a fast idle. I added fuel tonight, and my fuel gauge went from 3/8 full to 1/8 full. I believe the tank should be almost ( ~7/8 ) full.

It seems that all of my gauges are working in reverse. I read that the ABYC standard for 12V ground is yellow. Is it possible that sometime in the last 40 years the yellow and white were reversed? Would switching the yellow to ground and white to ignition potentially fix the problem? If the wires are not reversed, would I damage the gauges by switching them? Since it is DC, I'm guessing that switching the two wires would reverse the direction of the gauges. Am I on the right track?

prowlersfish · Post by **prowlersfish** » Sun Jun 03, 2012 9:52 pm

Your senders don't match the gages . Get the senders that match .

alexander38 · Post by **alexander38** » Mon Jun 04, 2012 12:10 am

from what I've read yellow is the old DC black, white is still the same. they went to yellow so the 110v black wouldn't get in to the 12v system, But some things we buy have a Yellow standby hot wire that keeps thing hot but not on, Lowrance has some things like this...I just never use it. I'd call them and check your work out before something gets broke..

prowlersfish · Post by **prowlersfish** » Mon Jun 04, 2012 6:39 am

A volt meter will quickly answer the positive vs negative question .

The Dog House · Post by **The Dog House** » Mon Jun 04, 2012 6:55 am

This weekend I plan on removing both the yellow and white wires and using a voltmeter to confirm which is 12V and which is ground. It will be a little difficult since there is not another ground point near the gauge, but I think I can reach the ground bar with the leads I have.

davidsmith · Post by **davidsmith** » Mon Jun 04, 2012 9:48 am

If you had the original AC Delco guages then you will have to change the senders. The AC Delco guages did not follow the same standard as most other manufactures.

The Dog House · Post by **The Dog House** » Mon Jun 04, 2012 10:04 am

davidsmith wrote:If you had the original AC Delco guages then you will have to change the senders. The AC Delco guages did not follow the same standard as most other manufactures.

The engine is a Chrysler 318. Did the Chrysler engines use AC Delco senders?

davidsmith · Post by **davidsmith** » Mon Jun 04, 2012 10:49 am

My F36 has 440 Chryslers and still has the factory AC Delco guages.

prowlersfish · Post by **prowlersfish** » Mon Jun 04, 2012 11:59 am

The Dog House wrote:This weekend I plan on removing both the yellow and white wires and using a voltmeter to confirm which is 12V and which is ground. It will be a little difficult since there is not another ground point near the gauge, but I think I can reach the ground bar with the leads I have.

Use a DVOM it will read - if backwards

Big D · Post by **Big D** » Mon Jun 04, 2012 9:13 pm

Confirm polarity, but I suspect as Paul and David that you`re dealing with the wrong resistance standard for matching gauge to sender; they must match. Check Faria`s website, they have a lot of info there including resistances of the senders to match thier gauges. You can then check your senders.

The Dog House · Post by **The Dog House** » Tue Jun 05, 2012 12:06 pm

My gauges are American standard gauges. If the polarity is correct, I'll look at the senders. I never even considered that a 1971 Chrylser V8 would have European senders. I'm still hoping it's a polarity issue, but we'll find out on Saturday when I check things out with my digital voltmeter.

larryeddington · Post by **larryeddington** » Tue Jun 05, 2012 5:16 pm

See below URL:

http://www.wemausa.com/specifications/T ... OTING2.pdf

And yess chrysler used on these boats delco "standard" gauges.

I am dealing with the newer fuel sending units are 33 to 240 ohm. The old ones in Trojan tank was 30 to 120 ohm. Talked to a well known mfg of gauges and said the only way they will recommend dual fuel gauges is having two sending units in each tank. That said if you have a lawn mower with two fuel gauges one sender will work them fine.

Then further shock, I have not fuel gauges on fly bridge, a guess is the same on all Trojans.

The Dog House · Post by **The Dog House** » Thu Jun 07, 2012 7:15 am

I checked the voltage at my gauges last night, and the polarity was correct (yellow = 12V and white = ground). Since the polarity is correct, the senders must not be matching the gauges. I'll eventually need to switch the senders, but for now I'll just be labeling the gauges to remind me they are working backwards.

The temperature sender will probably be the first one I change. The current sender takes a 9/16" wrench and I measured the threads at 1/2". Does anyone know if 1/2" is the correct temperature sender size for a Chrysler LM318?

prowlersfish · Post by **prowlersfish** » Thu Jun 07, 2012 12:07 pm

I would belive its 3/8 NPT or 1/4 npt . 3/8 npt would be aprox 1/2" on the outside .