brocno fuel/ temp gauge

[Brian Gantert]

My fuel gauge and temp gauge work intermittent, mostly not, on my 1969 bronco 1/2 cab with a 6 cylinder engine. Oil pressure and amp gauge work fine, any ideas from the basic checks to the more advanced. All gauges are stock in the speedometer. thanks, BG

 

[70_Steve]

The oil pressure, water temp, and fuel level gauges are all powered by an Instrument Voltage Regulator (IVR). The ammeter is seperate in operation from the other three. The IVR is mounted to the rear of the gauge cluster, and the output of the IVR is daisy-chained to all three gauges.

Because your oil pressure gauge always works, I'd rule out any problem with the IVR. All three gauges work similarly, in that the sending unit varies a resistance, and the gauge reflects that resistance. The range of the sending units is ~73 ohms for the bottom of the scale and 10 ohms for the top of the scale. So, if you disconnect the wire from the sending unit on a working gauge, the gauge should, because of the increase in resistance, drop to the bottom of the scale. Conversly, if you connect the wire at the sending unit directly to ground, the gauge should peg full scale.

You didn't say whether, when the gauges were not working, what the gauge showed. But, because they're intermittant, it's most likely a connection problem, and possibly right at the sending unit. Of course, that being said, I had a similar probme with my water temp gauge, and the intermittant connection turned out to be a broken connection in the firewall connector. I repaired it by running a seperate wire around the connector.

 

[Brian Gantert]

69 bronco

thanks for all the info. What I am taking away from the reply is that first check ALL connections thru the firewall and check all grounds. I"ll try that this week. When fuel gauge does work it goes up to whatever fuel level is in the tank and the temp gauge goes up slightly when the engine is warmed up. Again thanks a lot...BG

 

[DirtDonk]

Especially the firewall connectors. Senders go bad of course, and wires and connectors deteriorate over time, but the two main rectangular through-the-firewall connectors are a fairly common source of headaches on the '70 and prior Broncos.
Do disconnect the battery first though, as the main alternator/battery loop wires go through there and have full voltage all the time.
In a perfect world, you can disconnect them without trouble, even with the battery connected. But since Broncos don't often live in a perfect world, it's a good idea to disconnect the battery first.
That way too, you can undo the two screws that hold the fuse panel to the floor/firewall, and check the condition of all the wires and soldered joints behind it as well. Easy to put it all back together again too, so it's not a big deal to check all this even though you're only looking for trouble on two of the circuits.
At that point, you're not just looking at the gauge circuits obviously, but now's as good a time as any to check on the whole system to make sure you are avoiding any other potential problems.

...he says, watching the counter tick over to 8000 posts! %)

Paul

 

[kc15842]

this post hits home. I have yet to tackle my electrical problem, headlights and dash. At times it is perfect headlight, parking lights,internal and cluster blinkers are perfect. Oil pressure and fuel level perfect, then it goes wild. But only with the headlight switch on, it begins with both cluster blinker lights go on, oil and fuel shut down and parking lights go off. I believe it's all in the two connectors between at the firewall. I tried junk yards, no luck. What is the best way to replace those connectors? Like I said it's not all of the time, any thoughts

 

[Viperwolf1]

Try fixing the grounds at the front turn signals/park lights.

 

[DirtDonk]

Not sure what your best course of action would be kc. It'll take some un-wrapping and re-wiring to replace those pesky little things. If there was enough wire on the cabin side, you could literally cut the plugs and simply butt-splice each wire together. You'd lose the ability to separate the two halves, but so what? How often do you need to unplug those things? Once every 40 years or so? If that?
You could make a little aluminum panel to cover the rectangular holes, and put round holes with grommets in their place.
If you needed to keep some wires with a way to disconnect them (like the engine senders and ignition maybe?), you can either use Weather-Pak connectors, or simple buss-bars and ring-terminals.

A LOT of work, but if you're having recurring problems due to bad connections there, it'd be worth the effort.
If, on the other hand, you think most of your wires need some help anyway, it'd be time for a completely new harness at that point.

Good luck, whichever way you go.
Oh, and add grounds no matter what. That cures a lot of problems. Including, in some cases, even the intermittent ones.

Paul