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Discussion Starter #1
hi,
I have a 1997 Mustang Coupe (not GT) that has a tail light problem and a speedo problem.

The tail light problem:
The headlights work. The interior lights work. The dash lights work. The turn signals work. The brake lights work (with the brake pedal pressed) but the tail lights do not work.
I thought it was the headlight switch so I replaced that and still no tail lights. I doubt that ALL 4 of the rear brake light bulbs are bad. I checked every fuse and they are all ok as well.

The Speedo problem:
All of the gauges on the dash cluster work EXCEPT the speedometer. From Googling it, I read that the worm gear inside the speedo is usually a good culprit. so I checked that and it was ok. Oddly enough, when I put the speedo back in my Mustang, the speedo worked for about 45 minutes. Then it went back to the was it was before (the needle is either at the bottom or all over the place. Like I said earlier, the RMP gauge works fine as well as all others.

Anyone have a clue as to what I'm missing? Any help would be greatly appreciated.
 

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Confirm that the Odometer works. Why is this important? Because if the Odometer works and the Speedometer does not, this proves the VSS transmission sensor is good. Therefore the problem has to be in the cluster itself.

Me personally I wouldn't replace the cluster until after all of the cluster grounds have been confirmed good.

Regarding the other problem, it's not clear to me what exactly works and what does not work. Are you saying that the rear parking lights don't come on with the head lights? Where as all of the other rear lights (brake & turn) work?
 

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Discussion Starter #4
Confirm that the Odometer works. Why is this important? Because if the Odometer works and the Speedometer does not, this proves the VSS transmission sensor is good. Therefore the problem has to be in the cluster itself.

Me personally I wouldn't replace the cluster until after all of the cluster grounds have been confirmed good.

Regarding the other problem, it's not clear to me what exactly works and what does not work. Are you saying that the rear parking lights don't come on with the head lights? Where as all of the other rear lights (brake & turn) work?
Yes. that is exactly correct. Everything works EXCEPT the tail lights.
As for the speedo, the RPM and odometer gauges works properly as do all of the other gauges. I think the speedo is good but I can't figure why it doesn't work. It worked for a bit then died again. I am going to remove it again and take some of the grease off of the gears. Maybe that is causing the problem??
 

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Discussion Starter #5
ok here is another update.

I think the stepper motor on the cluster is gone as the needle does not move freely.

Regarding the tail lights, I checked the voltage at the head light switch and the BROWN wire is only .34V. I have no idea how to trace this back now.
 

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Confirm there is power to the head light T/W wire. If no power, then you fail as the odds favor a blown I/P #5 fuse.

If power to T/W and no power to the BN with the head lights on, then the head light switch is bad.

One would have "assumed" from the opening post that the front parking lights did work where the rear parking lights did not. This is not possible if the Headlight BN wire does not have 12 volts with the head light switch on (Bn wire feeds both front and back).

To anyone else with a similar problem. IF the head light BN wire has 12 volts AND the front parking lights work AND the rear parking lights do not, suspect a bad G300 ground.
 

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Discussion Starter #7
Well I got fed up with it and just took everything out. I found out that the previous owner had an alarm installed and it was wired to the headlight switch. So I tore all that out and then I removed the tail lights, head lights, head light switch, fuses, the works. Then I put it back together again.
Although it still doesn't work, I think it might make more sense to someone on here who is more knowledgeable than I.

The BRAKE lights work.
The turn signals work.
The head lights come on with ONE pull of the switch (I think one pull should only turn on the dash lights (and they do come on). The second pull of the switch doesn't seem to do anything.
The daytime running lights AND dome lights do not work. I checked all of the fuses and they are all good. I used 2 new switches so I am assuming that they are good. I'm not a mechanic but I tried to check the power to the switch with my multi meter. This is what I got...
Connector B1 (black with Orange), B2 (T/W) and D1 (Green with yellow) all have 12.4V
the "R" connector (Brown) is only 0.03V until you activate the switch at which point it goes to 12.4V.
Connector D2 (Gray) has 0.02V
Connector H (Red with yellow) has 0.03V

Any other ideas? Is it possible for a fuse to be bad but still look good (i.e. not blown)?

Thanks for all the help thus far.

pony.jpg
 

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Connector H (Red with yellow) has 0.03V
^^^^THIS is your problem. The R/Y wire powers the head lights. It should have full power when the head light switch is pulled to the 2nd position. I suspect that the prior owner has re-wired the head light connector (that's why the head lights come on in the 1st position). Or something has burned out inside the connector (heat damage).

NOTE: there is an automatic reset circuit breaker inside the head light switch. So if the wiring is bad and thus the head light switch gets HOT, this will cause the CB to trip.

Regardless, IMO if you figure out why the R/Y wire isn't hot in the 2nd position, this will give you the clue needed to fix it.

IMO, trouble shooting the DRL should be done after the reason for no head lights is found as it's possible it may be related.
 

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Discussion Starter #9
Thanks WMBURNS. The headlight switch connector was melted a bit so I replaced it with a new one. I soldered all the wires together so I know those connections are good. I also replaced the headlight switch but I guess it could also be bad. Someone else mentioned that the ecu could be bad??
 

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Someone else mentioned that the ecu could be bad??
Who ever told you this ^^^^ shouldn't be considered a friend. Replace a PCM because the Head light don't work???? The head light control circuits are about as straight forward as it gets. Just wire, Fuses, and heavy duty switches.

Does the head lights still come on with the first pull? If so, isn't a simpler explanation that either the repair has an error or something was over looked?

See Occam's Razor: Occam's razor - Wikipedia, the free encyclopedia

No one want's to believe they may have made a mistake but in this case, I strongly suggest that you go back and double check your work.

Note, the DRL module is an add on option. A bad DRL module will NOT stop the head lights from working. So I stand by my 1st recommendation. Get the head lights working before digging deeper into the DRL problem.

A set of wiring diagrams my help you figure out where the wiring problem is. If interested in getting a copy for yourself I may be able to help. PM if interested.
 
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