Car won't start when brake is on

My 1986 328 will not start when I have the brake pedal pressed. I can keep the key in position III and left my foot off the bake and the starter will engage. I reviewed the schematic and followed the white wire, (used to engage the solenoid), goes through a relay labeled "83 Brake warning light relay". I figure that is where the problem is but were is relay 83? The owners manual uses letters to reference the relays. Is this my problem? If so, where is the relay?

 

Wouldn't be my top suspect, but what 328 wiring diagram are you using? non-abs US, abs US, euro? and which Figure?

One thing is that the brake lights draw a surprising amount of current (like 8~10A) and this could take a marginal system from being able to start to not starting. One easy thing to do is to just measure the DC voltage between the battery terminals on the ends of the battery cables (not the battery posts):

1. With key "on", and brake pedal not pressed = should be something like 12V or more

and

2. with key "on" and brake pedal pressed = should be more than maybe 11.5V. If it drops a lot below this = that would explain a starter solenoid being unhappy when you try to start, and could be caused by a dirty battery terminal connection or a marginal battery.

 

Unless there has been some bizarre re-wiring done by a PO, I'd bet on Steve's response. As in another post about starting, I suggest you also check the batt cables for good/clean connections. I have to admit I'm surprised by the amperage load of the brake lights and, with a marginal system due to bad batt or poor cable connections,, it certainly could make the difference between starting and not.

I assume the battery is correct capacity-wise for the car. I had a friend who was a "save -weight" nutcase. He replaced an OEM-size battery with a much smaller one. It would only start the car on warm, summer days and even then it wasn't all that reliable!

 

Quick test that should eliminate the idea that it is the brake light draw would be to turn on the head lights and try to start the car with your foot off the brake. If it starts, it's likely not because when you have your foot on the brake the brake lights draw down the voltage.

I though you had this worked out Dean.

 

Nope, not completely solved. I've been dealing with a hot start promblem for years. I think I solved it by wrapping the starter in insulation. But this is treating the symptom and not fixing the problem. I've suspected low voltage and have cleaned all conections including ground straps. I've replace the battery, starter and ignition switch. I saw on another thread that there was a relay in the footwell related to the starter. The wire diagram I have is for the 308 so the are some minor differences. Would like to find that relay.

 

John, I think when you turn the key to start the headlights go off? I thought of removing the tail light bulbs. Obviously a real novice way to test this but I'm really out of my element when it comes to electrical.

 

Thank you, I will try this.

 

Thank you for your response. I have the correct battery and have changed all the components. I learned that there is a relay in play and I have never addressed that. Might be the only part left! Would like to inspect it if I could locate it.

 

I just checked. Headlights stay on while cranking on my 85 308. Probably the first time the headlights were in in 30 years.

I guess a 328 could be different.

 

That's really not a lot of amps. A quality battery will have over 600 cranking amps. My 308 starts after winter storage with the battery at 11.5 V, no load. Battery would have to be pretty for 8-10 amps to make a difference.

 

There is no "relay" involved in control of the starter on an unmodified 328 (although sometimes one gets added as a "fix", and that could give trouble, but it is usually added very close to the starter solenoid itself). You can get a copy of the non-abs US 328 wiring diagram here (although it is not without some errors):

http://ferrari.cdyn.com

 

Agree (and his problem is probably more diabolical as the starter does work sometimes). Like your suggestion to also try starting with the headlights "on" to see if that has the same effect or not.

 

I'll try the headlights test this weekend.