3.2 wiring dilemma, where does yellow #4 dme wire go? Options

[John]

This is baffling. DME#4 is getting +10.0 V while cranking, but it won't start. If I disconnect the starter and turn key to start then dme#4 gets 12+ V. I thought maybe 10V wasn't enough so I tried connecting another battery in parallel. DME#4 now gets 11V but still won't start. There is no fuel pressure. I put in another dme relay. Won't start. The fuel pump is new. If I bypass the dme relay to operate the fuel pump the engine won't start. But, putting key to "on" and jumping 12V to dme#4 with no other assistance, the engine will start and run every time. Further confounding matters, I got a generic automotive relay to supply 12V to dme#4. The fuel pump will turn on, but the starter won't operate! Preparing to go Goober with push button switch. Ugh.

I wouldn't suggest bypassing the wiring components. I suspect you may have some issues with how you have your DME relay wired. The DME relay supplies power to the computer AND controls the fuel pump.

I believe that you may be dropping out one of the voltage carrying wires required to run the car when you turn your ignition switch to the start position. (Your description of the problem seems to confirm this when you state that when the key is in the run position and you jumper the starter, that the car runs.)

It almost sounds like a circuit that should be powered with the key on and also with power while cranking. That sounds like circuit 15 to me.



Circuit 15 at a standard 914-4 ignition switch is hot in both start and run positions. This terminal usually has a Black/Blue Stripe wire. This terminal feeds Fuse #9. On 1974 cars, there is also a Red wire connected to this same terminal that feeds power to the seatbelt warning lamp.

The DME relay PIN 86 needs to connect to circuit 15. In a 911 wiring harness, this wire is BLACK. Circuit 15 feeds both the DME relay and powers the coil (also terminal 15). The circuit is +12V in both start and run key positions.

I hope you get your 3.2 wired correctly (running and starting with the key).

I have done a number of these and can potentially help.

[brp986s]

John, thanks for offering. I made some measurements today at the dme relay.

measurements versus ground (engine doesn't start, no jumping to dme box #4):

key off - #30 red = 12v and 85 brn = ground

key on - #87 red = 12v, and 85B blk/wht = 12v, and #86 blk = 12v (also 12v at coil)

key start - my dvom gives weird readings. Probably need a scope for #87 for injectors. But reading was ~0 fwiw. #87b grn/red ~0.6v

[Series9]

When everything is hooked up normally, 10v to the #4 while the starter is turning is totally normal. The starter is using a lot of energy. 10v is plenty of voltage to tell the computer what's happening, so let's forget about that.

It's unlikely that you have two bad DME relays, but not impossible. It sounds like either a DME relay, or the wiring to the relay.

The bottom line: you can make the engine run, so all that wiring is fine.

[jcd914]

There is a lot of great info in this thread! (IMG:style_emoticons/default/smile.gif)
I just wanted to clearify (maybe) the function of the DME relay.

DME Relay is actually 2 relays inside a single housing. 1 section powers the DME system and 1 section powers the fuel pump.
Here are the wire descriptions and there functions, as best I know.
Note: You may have to unplug the DME relay to read the terminal designation to determine the wire positions.
T30 - 4mm Red wire from the battery, provides power to the relay for DME system. Everything powered from the DME relay goes thru these wire.
T85 - 1mm Brown wire to ground
T85b - 1mm Black/White wire to pin 20 on DME Control unit (DME grounds this to power fuel pup)
T86 - 1mm Black wire from 15 circuit, 12volts with Key ON AND while cranking. (From the ignition switch)
T87 - 6mm Red wire to DME Control unit, injectors, idle control valve (power out of the relay)
T87b - 2.5mm Red/Green wire to fuel pump (power out of the relay)
also .75mm Yellow wire to O2 sensor heater (later cars)

Power from the ignition switch in the ON position AND in the start position goes to T86 on the DME relay. If T85 is properly grounded the relay then outputs power to T87 to the DME and injectors etc. It also provides power internally to fuel pump portion of the DME relay and when the DME control unit grounds the wire to T85b the DME relay outputs power to T87b to the fuel pump.

To me your symptoms seem like the wire to T86 at the DME relay is not getting power while cranking, only while in the Key ON position..
But never under estimate the havoc bad grounds can cause and double check them all.

Jim

[John]

There is a lot of great info in this thread! (IMG:style_emoticons/default/smile.gif)
I just wanted to clearify (maybe) the function of the DME relay.

DME Relay is actually 2 relays inside a single housing. 1 section powers the DME system and 1 section powers the fuel pump.
Here are the wire descriptions and there functions, as best I know.
Note: You may have to unplug the DME relay to read the terminal designation to determine the wire positions.
T30 - 4mm Red wire from the battery, provides power to the relay for DME system. Everything powered from the DME relay goes thru these wire.
T85 - 1mm Brown wire to ground
T85b - 1mm Black/White wire to pin 20 on DME Control unit (DME grounds this to power fuel pup)
T86 - 1mm Black wire from 15 circuit, 12volts with Key ON AND while cranking. (From the ignition switch)
T87 - 6mm Red wire to DME Control unit, injectors, idle control valve (power out of the relay)
T87b - 2.5mm Red/Green wire to fuel pump (power out of the relay)
also .75mm Yellow wire to O2 sensor heater (later cars)

Power from the ignition switch in the ON position AND in the start position goes to T86 on the DME relay. If T85 is properly grounded the relay then outputs power to T87 to the DME and injectors etc. It also provides power internally to fuel pump portion of the DME relay and when the DME control unit grounds the wire to T85b the DME relay outputs power to T87b to the fuel pump.

To me your symptoms seem like the wire to T86 at the DME relay is not getting power while cranking, only while in the Key ON position..
But never under estimate the havoc bad grounds can cause and double check them all.

Jim

(IMG:style_emoticons/default/agree.gif)

It also COULD be a bad ignition switch....

[ChrisFoley]

It also COULD be a bad ignition switch....

(IMG:style_emoticons/default/agree.gif)
There's a tiny spring inside the switch that is supposed to carry a trace current, but a short in other parts of the vehicle circuitry can cause the spring to burn up.
When the spring is damaged the switch may no longer engage all the circuits in start mode that it normally does.
If you replace the switch, the new one will fail as soon as the key is turned on, unless the short is discovered and corrected in advance.

[brp986s]

It also COULD be a bad ignition switch....

(IMG:style_emoticons/default/agree.gif)
There's a tiny spring inside the switch that is supposed to carry a trace current, but a short in other parts of the vehicle circuitry can cause the spring to burn up.
When the spring is damaged the switch may no longer engage all the circuits in start mode that it normally does.
If you replace the switch, the new one will fail as soon as the key is turned on, unless the short is discovered and corrected in advance.

I'm fully onboard with this theory and it will be easy to verify once I get home tonite. I had the same idea after Jim's post. Thanks to all. (IMG:style_emoticons/default/pray.gif)

[brp986s]

It also COULD be a bad ignition switch....

(IMG:style_emoticons/default/agree.gif)
There's a tiny spring inside the switch that is supposed to carry a trace current, but a short in other parts of the vehicle circuitry can cause the spring to burn up.
When the spring is damaged the switch may no longer engage all the circuits in start mode that it normally does.
If you replace the switch, the new one will fail as soon as the key is turned on, unless the short is discovered and corrected in advance.

(IMG:style_emoticons/default/smilie_pokal.gif) No 12v at relay 86 or 85b at key start. Ign switch is dead. Now why would the switch choose the moment of the engine conversion to crap out? This thing may have further tricks up it's sleeve, but I'm on the right path now. Thanks again!

[John]

(IMG:style_emoticons/default/smilie_pokal.gif) No 12v at relay 86 or 85b at key start. Ign switch is dead. Now why would the switch choose the moment of the engine conversion to crap out? This thing may have further tricks up it's sleeve, but I'm on the right path now. Thanks again!

weird stuff happens. Glad to see you just might be on the right path to success!

[John]

So is your car working correctly now?

[brp986s]

Yeah, don't you hate how threads evolve to climax, then...nothing?!

Yes, the car is running. The ign switch was replaced. Also, I measured 0.4 ohm from starter solenoid to ground. Is that an internal short or normal? That would pull alotta juice thru the switch - maybe not good for it. I had a spare low mile rblt starter that was open circuit at these terminals. So I swapped that one in, resulting in (IMG:style_emoticons/default/driving.gif)

Now the problem is the motronic. I bought a rblt motronic box from 911chips.com. (the rest of the set up I bought separately). Starting from cold or coming to stop the engine will often do 15 or so 500 to 1300 rpm rev cycles before settling to normal idle. I have another motronic box that doesn't result in this feature. I'll see if I can return/swap the 911chips box w/o alotta (IMG:style_emoticons/default/hissyfit.gif) . Sigh. I thought that going to motronic from cis I would be free from quirky old fuel systems. Instead it seems more of the same, but different.

[John]

Yeah, don't you hate how threads evolve to climax, then...nothing?!

Yes, the car is running. The ign switch was replaced. Also, I measured 0.4 ohm from starter solenoid to ground. Is that an internal short or normal? That would pull alotta juice thru the switch - maybe not good for it. I had a spare low mile rblt starter that was open circuit at these terminals. So I swapped that one in, resulting in (IMG:style_emoticons/default/driving.gif)

Now the problem is the motronic. I bought a rblt motronic box from 911chips.com. (the rest of the set up I bought separately). Starting from cold or coming to stop the engine will often do 15 or so 500 to 1300 rpm rev cycles before settling to normal idle. I have another motronic box that doesn't result in this feature. I'll see if I can return/swap the 911chips box w/o alotta (IMG:style_emoticons/default/hissyfit.gif) . Sigh. I thought that going to motronic from cis I would be free from quirky old fuel systems. Instead it seems more of the same, but different.

You may want to check the idle stabilizer as well. These have a tendency to stick causing idle hunting as you describe. Sometimes they can be freed up by removing them and spraying through them with some sort of solvent/cleaner. I have replaced several of these over the years. They are a little pricy, but it does fix the hunting idling. Tapping on them tends to get them temporarily freed up and helps with diagnosing them as the problem.