Frequency valve not buzzing, no voltage

[BladeBronson]

I’m stumped on what to do next. I’ve jumpered the RPM relay on the pins with brown and red/yellow wires. I’ve clicked the key twice to turn on the accessory power (and even started the engine - I’m not sure when the FV gets power) and there’s no buzz. I’ve disconnected the plug at the bulkhead and get no voltage reading. When the accessory is on and I inset the jumper, I hear a click from a relay in the electrical compartment, so I think somethings trying to work. I’ve removed the rear wall and can see the two wires as they go into the bulkhead connector, but am not sure where I can trace them to.

Any thoughts on this?

[Bitsyncmaster]

Jumping the brown to double yellow/red wires on the RPM relay socket powers the lambda relay which then powers the FV and the lambda ECU. No key on is required. The click you hear must be the lambda relay. The power comes from fuse #7 but it your fuel pump was working than that fuse must be good. It may be your lambda relay (make sure you have the correct relay installed). That relay socket is the yellow one on most cars. The FV power wire does go through a bulkhead connector. So your not getting 12 volts at that connector with the jumper?

[BladeBronson]

That’s correct, no voltage at the bulkhead connector at all. I tried another lambda relay today with no difference. I’m trying to find my wiring diagram, but suspect that it could be the ECU.

[Bitsyncmaster]

That’s correct, no voltage at the bulkhead connector at all. I tried another lambda relay today with no difference. I’m trying to find my wiring diagram, but suspect that it could be the ECU.

The red/purple wire on the FV should get 12 volts no mater what the ECU is doing. You can use an ohm meter on the "lambda" relay center pin to the bulkhead. It could be your lambda relay is not getting the battery power so you can test that with a voltmeter.

[David T]

Make sure all the little ground wires on the intake manifold are there, one is for the frequency valve. Plug and unplug the frequency valve a couple of times. The connections corrode and can make a bad connection and by plugging and unplugging you might get it to buzz.

[BladeBronson]

I dove deeper into this today. I haven’t had to troubleshoot this system before so I made a mistake in reporting no voltage. I was measuring voltage across the 2 pins of the bulkhead connector. When the RPM relay is jumped, I get voltage on each of the two pins when measuring from a pin to ground. There’s just no buzzing.

[Bitsyncmaster]

I dove deeper into this today. I haven’t had to troubleshoot this system before so I made a mistake in reporting no voltage. I was measuring voltage across the 2 pins of the bulkhead connector. When the RPM relay is jumped, I get voltage on each of the two pins when measuring from a pin to ground. There’s just no buzzing.

The lambda ECU when powered should be driving one side of the FV to ground at a 50% duty cycle. So you would read about half the DC voltage on that pin of the FV. If the lambda ECU is not working, you will read voltage on both FV pins since one is powered and the other one is open but will get voltage through the powered pin.

[BladeBronson]

Ah, very interesting. This is really pointing toward the ECU not working. A local owner has a spare that I might be able to test with. Thanks a bunch for your expertise on this, Dave.

[BladeBronson]

Another owner came over today and I did a little experiment on his car. First, I verified that by jumpering his RPM relay, his FV started buzzing loudly. I disconnected the FV at the bulkhead connector and measured each pin's voltage while the negative probe was grounded to the engine block. The bottom pin had 12 volts. The top pin had 0.2 volts (which is way less than the 50% that I was expecting). Anyway, his car is running just fine. I unplugged his ECU and the ground wire and connected my ECU to his car and repeated the tests. Everything was exactly the same, pointing to my ECU being good.

So now I have to figure out why I would be getting ~11 volts on both pins (when measured with a multimeter with the positive probe on a pin at the bulkhead and the negative grounded to the engine block).

[Bitsyncmaster]

Another owner came over today and I did a little experiment on his car. First, I verified that by jumpering his RPM relay, his FV started buzzing loudly. I disconnected the FV at the bulkhead connector and measured each pin's voltage while the negative probe was grounded to the engine block. The bottom pin had 12 volts. The top pin had 0.2 volts (which is way less than the 50% that I was expecting). Anyway, his car is running just fine. I unplugged his ECU and the ground wire and connected my ECU to his car and repeated the tests. Everything was exactly the same, pointing to my ECU being good.

So now I have to figure out why I would be getting ~11 volts on both pins (when measured with a multimeter with the positive probe on a pin at the bulkhead and the negative grounded to the engine block).

When the FV is disconnected, there is no pull up to 12 volts on the pin the ECU is switching from ground to open. To see the 50% DC voltage you need to keep the FV connected.

[David T]

Now that you have proven the ECU, you should check the continuity of the wires going to the frequency valve. From the sound of it you aren't grounding the circuit.

[BladeBronson]

I’ve got buzzing! I took the ECU connector apart so I could backprobe the wires. Pin 5 has continuity to ground, but pin 16 does not. I jumpered the two and now I’m getting a nice loud FV buzz when the RPM relay is also jumpered.

So, I can fix this in a few different ways, but does anyone know offhand where the other end of that pin 16 ground wire terminates?

(Thin blue wire is the temporary patch)
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[Bitsyncmaster]

I’ve got buzzing! I took the ECU connector apart so I could backprobe the wires. Pin 5 has continuity to ground, but pin 16 does not. I jumpered the two and now I’m getting a nice loud FV buzz when the RPM relay is also jumpered.

So, I can fix this in a few different ways, but does anyone know offhand where the other end of that pin 16 ground wire terminates?

(Thin blue wire is the temporary patch)
66438

They go through the bulkhead and get joined to a terminal that bolts to your engine block.

B 19 X
L 20 R To diagnostic plug – read dwell here
U 21 X
E 22 B Engine block, from Lambda ECU pin 5
23 B Engine block, from Lambda ECU pin 16
24 X
25 WO Oil Pressure Switch
26 Y To WUR left pin
27 X

[BladeBronson]

Ah thanks, this was super helpful. Dave pasted the above from: https://www.dmc12.info/DeLorean%20Bulkhead%20Connectors.pdf I took a look at the blue plug and found that the number 23 pin receptacle had broken and the wire was probably making intermittent contact. I cut up an aluminum can and fashioned a sleeve to press the tabs so I could slide it out. I soldered it back together and my frequency valve is back in business!

Thank you for your time and expertise, Dave!

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