Ra23 electrical problem.

[rob1]

Having some issues which I thought I had rectified but haven't.

Firstly the brake lights stopped, found to be a blown fuse, but fitting of a new fuse blows instantly. I put that down to a short, the reverse lights also don't work, so I suspect it's somwhere between dash and the rear of the car.

Other problem is: Jumped in the car one night and out of the blue, the charge light came on. Also, the temp, fuel, and tacho were running low. The heater fan also didn't work. Well, this has been the case for around a month now. The car is running fine, no issues starting, battery draining, anything to do with the engine. I have checked the dash gauge regulator and it seems to be giving all guages 5.6 volts. I am pretty sure this should be around 7v. As this is what is printed on the back of the gauge circuit board.

I am confident it is not a dash regulator problem though, as fitting another guage panel resulted in the same output to gauges. (5.6v) The dash is also recieving 14.2 volts, so it's not a charge regulator problem either. (I have a corolla charge reg fitted which gives 14.2v)

So I am thinking it's a bad earth, (seeing as the tacho is also down) and all guages share the same loom branched together. This may also explain why the charge light is on?

Not sure why the heater fan doesn't work though. Maybe also share a common earth?

So anyone out there got any ideas? I know on these older cars this sort of stuff can be common so hoping someone has dealt with a similar problem, what was it?

Thanks, Rob

[Mos]

While not being able to provide direct help, keep in mind it is a perfectly valid fault finding technique to measure the voltage on the earth of anything with respect to either engine ground, chassis ground or battery negative (your post suggests you haven't tried that - apologies if you have). The voltage should be very low, certainly under 0.5V - otherwise you either have a very high current draw on that wire or the conductor to ground has high resistance.
Mos.

[rob1]

Thanks Mos, I'll get the m/meter out again and check.

So for example, if I check the earth side of the charge light I should be finding a reading of no greater than .5v? otherwise I have an earth resistance problem? (which is what I had sort of assumed was the problem)

I am not an auto elec obviously, but am quite capable of using m/meters etc as I fit elec gear to cars for a living. still learning. I should buy a logic probe.......

[oldcorollas]

charge light usually earths through the alternator, so try on the gauge earthing point...

was the charge light on bright? or dull?

yep, i think you are on track also you could make up a temp lead, going from an earth point on the gauges, directly to ground, and see if that helps things...

also for the lights, check that none of the bulbs ar eblown (incase they have blown and shorted.. unlikely but... never know)

[rob1]

Oldcorollas, the charge light is on dull, then gets stronger when you rev it. None of the lights are dull, just the needles of the gauges run low. My thought that the earth in the loom that runs to the tacho and all other gauges, (as they all come together) is weak, sounds like what you are saying. If I run a temp earth to the charge light I might find and establish the fault.

My problem is that I have no idea what the connections are in the round black plug that connects to the rear of the gauge panel. (exept the positves). I will go and see if I can trace them to each quage and find which is the common earth, then if the temp earth fixes the problem, I'll just run a new earth (s) up to the black plug. Rather than try to find where it's failing.

Also, I have just been informed that when the heater fan is activated (although not working) the charge light also comes on stronger. (my brothers car)

Thanks, Rob

[Mos]

So for example, if I check the earth side of the charge light I should be finding a reading of no greater than .5v? otherwise I have an earth resistance problem? (which is what I had sort of assumed was the problem)

Alternator/charge light usually has 12V going to both sides. The ignition switch supplies 12V to one side, alt supplies 12V to the other. When the alt dies (or is stationary) it supplies 0V so the bulb lights up. It should also have a diode so that when the alt 12V is higher than the ignition 12V it doesn't light the bulb (even a small voltage difference will cause a glow).

On some celicas (and some 1G alts) this operation is reversed - never played enough with them to know which ones did what, so it may not be 12/12 on both sides of the bulb.

I am not an auto elec obviously, but am quite capable of using m/meters etc as I fit elec gear to cars for a living. still learning. I should buy a logic probe.......

Logic probe wont really help in cars (tells you when a pulse occurs that is too fast for a DMM). Anything that involves digital electronics in cars is way too complex for a logic probe. A scope would be better, but still of limited use in the automotive digital arena but helps immensly with things like speed sensors

Mos.

[rob1]

Cheers Mos, I now understand, as I wired a temp earth to the alternator side of the charge light and the light came on strong. So it looks like earthing is not my problem. I checked the alternator voltage of the two sides of the charge light, I traced the circuit back to the plug and find that it is giving 6v. From what you have just said above, there must be a problem with the output from the alternator to the dash plug? strange...........as the car has been like this for over a month and the charge regulator is sitting steady on 14.2v.

[Mos]

I've seen alts where the charge light output wasn't working at all but the alt was still working in every other sense.
So your charge light signal coming from the alt has 6 v on it? Does it drop to 0 V when you shut off the engine?
Mos.

[oldcorollas]

I've seen alts where the charge light output wasn't working at all but the alt was still working in every other sense.

yup had that.. sometimes light comes on at startup.. sometimes not.. alt always fine...

i still reckon check with the other earths as well...

[rob1]

Ok. I fucked up a bit. One side of the charge light is constant. No problems there. On the other side, while connected, shows 6v. When I take the plug off the back of the circuit board, It drops to .6v. (I would assume it does this because it doesn't have the result of the full volts runing through the light and out to the other side.).

So, really I don't know whether it's the alternator side that is failing, it may be the ignition side.

aaaaaahhhhhh, I see why you ask if it drops when I switch the ignition off Mos, I pressume the alt side is constant, whereas the ignition side is activated. I will check now.

[rob1]

Yep, it's the ignition side.

[rob1]

I will connect a temp 12v to the ignition side and see if theat fixes the rest on the gauges aswell. (fused of course). I'll see how I go.

[rob1]

I got ahead of myself. They are both switched via ign.

[Mos]

Sorry, what I should have said is: Does it drop to 0 V when the engine is not running but the ignition is on?

[Mos]

I have checked the dash gauge regulator and it seems to be giving all guages 5.6 volts. I am pretty sure this should be around 7v. As this is what is printed on the back of the gauge circuit board.

If you're sure that it's supposed to be 7V, depending on where you measured 5.6 it could be a couple of things. You really need to find out what it should be for sure (eg check a car that works) otherwise you may be chasing the wrong problems.

If you measured 5.6V between the dash reg and chassis, then something is causing the 7V to droop - that could be a sensor drawing more current than it should (try unplugging things like water temp, fuel sender, etc) or something shorting out a wire that draws current from that supply.

If you measured 5.6V between the dash reg positive and the dash reg negative, then it's more likely to be a dash negative grounding issue but it could still be the same reasons as above. (Grounds in toyotas are normally white with black stripe, or brown in colour).

Mos.