Aww, Come ON - It's a friggin' temp Gauge

[MahtyMaht]

WTF. There's only 3 terminals on the thang, for god's sakes. +Hot, Sender and ground? I never worked so hard for so long to do nothin' in my life. It'a a stupid ***** VDO temp gauge on a Merc 454. Originally had Medallion stuff. It wants a sender good for an Ohm range of 47.5 to about 450. I got that. I just dropped this engine in, and the harness had been raped some by the PO, so I hooked the VOM to what I think is the sender wire at the dash end, and what I think is the sender wire at the engine end. No Continuity, no problem, I got three more unused wires on the engine end of the harness to try, as I got no idea why they are there. One of'em coughs up a 0 ohms, I'm thinkin' 'Gotcha!'. I hook it to the sender, hook a switched 12v to the + side of the gauge, and it immediately pegs to the max. Riiiiight. I screwed around for 2 hours with this, and I got nothin'. I've got to think I have a really flawed idea about how this gauge works. Correct me if I'm wrong, but the +12v goes into one side of the gauge, comes out thru the S for sender terminal, goes to the sender, and thru the sender's heat driven variable resistance, to engine ground, so the gauge sees a varying voltage, dependant on engine temp. WTF am I missing? I HATE having to quit because it's dark, or I gotta go, but over all things on this earth, I really hate having to quit because I now realize that I'm cranked all off, and now, nothin'I think is useful, I'm not goping to successively approximate my way to a solution to the problem, I'm just wearing fstener threads, and risking breakin' thangs. Hey - this was 6 hours ago - can ya tell I'm still idlin' pretty fast over it? I don't stand stupid well, and haven't yet figured out an exemption for myself, and make no mistake, gentlemen - this was stupid. AAAAAAAAAAAAaaaaaaaaaaaaaaaaaaaaaaarrrrrrrrrrrrrrrggggggggggghhhhhhhhhhhhhhhh!!!!!!!!!!!

PS If this thang grouds thru the sender, why does the gauge itself have a dedicated ground?

 

[Bondo]

Re: Aww, Come ON - It's a friggin' temp Gauge

PS If this thang grouds thru the sender, why does the gauge itself have a dedicated ground?

Ayuh,... For the indicator light....

 

[dsiekman]

Re: Aww, Come ON - It's a friggin' temp Gauge

Okay, of the three, you should have sender, ground and hot. I think, and please correct me if I'm wrong, the hot is on the lighting circuit just to give you backlight at the gauge.

After six hours I think I would run a new wire back to the sender. Are you sure the sender is good?

 

[Bt Doctur]

Re: Aww, Come ON - It's a friggin' temp Gauge

Are you positive your hooking to the temp sender and not the overheat sender

 

[Bud B]

Re: Aww, Come ON - It's a friggin' temp Gauge

You dont say if this is an EFI motor.

Could be you are playing with the temp sensor for the computer maybe?

My 454 gauge SENDER is a SINGLE wire sender which is varying ground to the gauge. The hot is at the gauge in the panel. as is the bulb ground.

The temp SENSOR on the other hand is usually a 3 wire to the computer.One receives 5v from the comp, the other sends ground to the comp, and the third sends the variable + return back to the comp. All of which affect timing and the mixture.

 

[Silvertip]

Re: Aww, Come ON - It's a friggin' temp Gauge

You are right -- It is just a "friggin gauge" with three wires. But they work differently than you think. The gauge must have switched +12 volts on the "+" terminal which is what you said you did. You also need a good ground on the "-" terminal on the gauge. That terminal provides TWO functions. 1) it is ground for the gauge and 2) ground for the internal light. The sender modulates the gauge -- it is not a "series" element. Ok -- that's the gauge end. Now to the sender end. The shell of the sender is grounded via the threads and engine block so thats not likely screwed up. The sender wire goes to the "S" terminal on the gauge. If that wire has a ground (not just the resistance of the sender) it will peg the gauge. Check that wire again end-to-end with it disconnected at both ends. If you are sure it is nothing more than a length of wire and it is not grounded anywhere, connect it at both ends and try again. If you still have issues, either the gauge is a mismatch for the sender or the sender is wrong for the gauge. You may even be trying to use a "TEMPERATURE SWITCH" which is nothing more than an ON-OFF switch that trips at a specific engine temp. But you said you measured that so that is not likely either. Here is a diagram of how a fuel gauge works. A temp gauge is no different -- just a different sender.

 

[Alpheus]

Re: Aww, Come ON - It's a friggin' temp Gauge

I betcha that lone wire you found is an unused ground. Run a continuity check from the wire to ground to confirm...

 

[MahtyMaht]

Re: Aww, Come ON - It's a friggin' temp Gauge

Holy crapanoli! I got some heavy hitters responding here - I'm flattered, and optimistic. I've had time to 'take a tuhn about the hahbah' as they say up here, with the captain, (Morgan). Now -Bondude & Bt Doc, pleasure to hear from ya'll, Alpeus and SilverT too - Bondo and Dsiekman - This VDO gauge has 5 pins - 2 for the backlight, 3 for the gauge. The original Medallion didn't do that, but who cares, it should only mean I make a jumper between the light ground, and the gauge ground. Don't care about the backlight right now, need the gauge workin'. Bt Doc - I have the circuit rigged for the overheat, but the switch crunched, and so far, me and the Napa guys can't figure out which Echlin switch closes at 180, so I have the lead for it wire-tied out of the way. Sorry, BudB, it's carbureted, not injected & computed. Alpheus, the wire I 'found', was found with the ohm meter hooked to the sender wire at the dash, while I shopped my loose wires at the engine end with the VOM till I got continuity. Silvertip - you're killin' me here. What do you mean the ground is not a series dealio. Why does the ameter in the gauge need two grounds? I'm thinkin' the sender is the ground, albeit modulated, but what is the other one for? That'd mean this jackass gauge has 3 grounds; One for the backlight, one thru the sender, and one I'm lookin' for somebody to explain. Oh, by the way, Dsiekman, of course you would have done your own solution after several hours - any 'normal' would. I, however, am retarded, so I go into these things prepared to spend twice as much time as a 'normal' to get the same results. When it becomes apparent that I'm losing too much time even for a re-re, (as thatdurnwomantryinaruinmylife like to call us retarded people), I get cranked and have to quit. Then I'm really cranked because quitting, of course, is the primary ingredient in the recipe for failure. Failures are losers more often than not because they are quitters, and I'm fine with being retarded and even failing; losing, not so much. I feel like if I can't say I've won, I have to say I ..... er,uh, hack,choke,cough. Yeah. So, I'm gonna have to make this gauge my biatch. 'Preciate the feedback, gentlemen. I'm gonna have the stedkid bring that boat over here tomorrow, as the circulating pump's weep hole is also barfing into the bilge, and I was scared of takin' it off to replace it in the water, for fear of parkin' the boat 30 feet below it's usual spot. The Captain seems to be feeling talkative this evening, sorry guys, but not very.

 

[Alpheus]

Re: Aww, Come ON - It's a friggin' temp Gauge

Does the gauge and the temp sensor match? If you are trying to use a different temp gauge than the one that was originally paired with the sensor or vise versa you may have problems. Different manufactures use different resistance...

 

[MahtyMaht]

Re: Aww, Come ON - It's a friggin' temp Gauge

The VDO gauge says it wants a sender with an ohm range from 47 to 447. The Merc senders I have are showing about 450 ohms cold, and it drops when I torch the end with a lighter, so I figure they work, and are at probably close enough in ohms ratings to be fine.

 

[rodbolt]

Re: Aww, Come ON - It's a friggin' temp Gauge

VDO uses backwards resistance from every other manufacturer for their oil pressure and temp guages.
MUST use the correct single or dual station VDO sender or it reads backwards.
typically with most temp senders as temp goes up resistance comes down more current flows and the needle is pulled farther away from the 100* mark.
VDO is backwards and uses a positive coefficient.
same as their oil pressure senders.
a quick tester can be made with a radio shack variable potentiometer, cut it in in place of the sender and see what the guage wants.
does it want increasing resistance or decreasing resistance to move the needle correctly.