Wedge The TR7 Fuel Gauge Follow-up Problem !?!?

[sammyb]

So, I put the fuel gauge in (thanks ObiRichKanobi,) but I have no power to the gauge (I tried using a circuit tester, and I'm getting no power.)

Looked at the wiring diagram, and it looks like power comes via the sending unit. Is it common for the sending unit to fry and withold power? I checked under the car and it appears initially that the wires are connected to the sending unit. If I get a chance, I'll get under there and clean and reattach the wires (and check the the power at the unit.)

Any other ideas about something I might have missed? I'm hoping that it's not that the circuit board behind the dash that is broken.

 

[steve99]

Silly question Sam, but did you check all the fuses in the fuse panel? If Im not mistaken that is a fused circuit.

Steve

 

[sammyb]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by steve99:
Silly question Sam, but did you check all the fuses in the fuse panel? If Im not mistaken that is a fused circuit.

Steve<hr></blockquote>
Not a silly question, and the thought crossed my mind (typical problem-solving 101 -- start at the beginning and go to the end. Fuses at the start.)

I didn't visually check the fuses (didn't have a chance,) but I looked at the fuse descriptions, and the fuel gauge did not appear to be fused. I'll go check all the fuses.

 

[sammyb]

Spooky. The Battery/Aux fuse which is supposed to be a 50AMP fuse is a 25AMP fuse. I wonder if that is contributing to my courtesy lights and cigar lighter not working. Maybe it's heating up but not breaking or something (my battery/amp gauge works...I'll get a 50AMP tomorrow. Interestingly, the 15AMP fuse is also occupied by a 25AMP.

Dunno. I still haven't spent enough time with the wiring diagram.

 

[Mark Beiser]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by sammyb:
Spooky. The Battery/Aux fuse which is supposed to be a 50AMP fuse is a 25AMP fuse. I wonder if that is contributing to my courtesy lights and cigar lighter not working. Maybe it's heating up but not breaking or something (my battery/amp gauge works...I'll get a 50AMP tomorrow. Interestingly, the 15AMP fuse is also occupied by a 25AMP.<hr></blockquote>

WARNING, FIRE HAZARD!!!!!!!!!!!!!

Be very carefull with those fuse swaps! Considdering the cars you own, you are probably aware of this, but just in case...

British automotive fuses are rated acording to the amperage that will cause the fuse to blow. The automotive fuses you buy at the corner auto parts store are rated acording to the intended operating current of the load.

So, a 50 amp british fuse blows instantly if it is subjected to a 50 amp current.
A regular American 50 am automotive fuse will handle a continuous load of 50 amps and won't blow unless the amperage goes significantly higher.

I have seen a nice chart floating around that gives the equivelnt fuse sizes between the two. I'm sure someone will post a link to it shortly.

 

[ObiRichKanobi]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr> Looked at the wiring diagram, and it looks like power comes via the sending unit. Is it common for the sending unit to fry and withold power? <hr></blockquote>

Quite common...had the same problem when I bought my car. You can test the sending unit on the car by taking the colored wire off the sending unit and grounding it. With the ignition on, it should read full then.

PS. If you have to replace the sending unit, check the condition of the inside of the tank if you can. That was a major source of headache for me last year

[ 05-15-2004: Message edited by: ObiRichKanobi ]</p>

 

[sammyb]

I'll try O.R.K.'s sending unit check tomorrow.

The thing that gets me is that if the 25Amp fuse is designed to fail at 25Amps (it is a Lucas fuse) then why has it not failed in a 50Amp circuit?

I was quite surprised at the fuses in there. I have a habit of buying cars from people who are unaware they are driving around with bad fuses, botched electrical wiring etc...

 

[Izual Angel]

sammyb - The fuse hasn't blown because it hasn't yet reached 25A draw. Fuses are rated below or at the amperage they would damage the components they protect. The circuits don't always run close to that rating. For instance, in my daily driver the cooling fan is on a 30A fuse, but only draws 2 or 3 amps. But, 30 A is igh enough that if that much current were flowing it would have to have shorted in the fan motor, and the wires would melt. Same goes for that TR7 fuse. Also, there are a LOT of things wired through each of those fuses. Only 5 or so fuses and a couple relays to handle all the electrical functions of the whole car. Also, do you have a good wiring diagram? I know the reprint of hte 1977 owner handbook contains a copy of it (for that year).

 

[Izual Angel]

Well, I looked at my wiring diagram and it looks like there should be 3 wires to the fuel gage. 12V+, GND, and the sender wire. Is this what you have?

 

[sammyb]

I have the original owner's handbook, maintenance handbook and a Haynes manual. So, I'm set with reading material

Yes, there are three "wires"...in actuality, the fuel gauge plugs into three recepticals which appear to be attached to a printed circuit board. I threw a test light into the gauge holes, and couldn't get current.

According to the wiring diagram, power comes from the sending unit. I haven't had a chance to check per the suggestion of grounding the colored wire to see if the fuel gauge shows full.

My guess is that the former owner noticed the fuel gauge didn't work, figured it was a bad gauge, pulled it out in anticipation of finding another one and never found one, so he sold w/o it.

 

[Izual Angel]

I took a better look to-day (I'm having a problem with my own gauge). The three wires coming off the sender are GND, Fuel gauge, and another that goes to the fuel delay relay for the low fuel light.

What you may want to do is follow the traces on the rear circuit panel, and check the voltage at the guage (look to see that it's getting 12V from the shared source).

 

[sammyb]

I'm not getting any juice at the gauge, but I read the wiring diagram as power coming from the sending unit.

 

[Izual Angel]

No. If I'm not mistaken the fuel sender works like the temperature gauge sender. The gauge sends voltage through the sender, and gets it's reading from the resistance in the sender. That's why you get a certain reading when you ground the sender wires on the fuel, and temp gauges. Do all your other gauges work correctly? There are 3 wires to the gauge, are there not?