[sloboy89]

I've had two wiring harnesses for my electronic VVT go bad on me, so rather than look for another used replacement, I embarked on a journey to find another way to activate the VVT solenoid. Essentially, the printed circuit board that is located within the white plastic connector acts to put 12 volts across the two prongs of the solenoid when a trigger signal is sent from the throttle position sensor (TPS). Therefore, I figured that if I could find a suitable relay that is triggered by the TPS signal, I could activate the solenoid by grounding one prong of the solenoid and connecting the other solenoid to the relay output (with the relay input connected to 12VDC).

I tried the standard Bosch-style coil-driven relays, but the signal from the TPS was insufficient to trigger the relays, so I went looking for another solution. After some research, I decided to try a solid-state DC relay. I ordered the 10A relay below on eBay ($9.00), waited a few weeks for it to arrive from China, and then tested it out.

A little bit to my surprise, it actually worked, and the VVT is back in business. I mounted the relay on the side wall of the engine compartment near the intake plenum. It's only been a couple of days, so I will keep checking on it. For those interested, also below is a wiring diagram of my set-up.

 

[Stevew]

McGiver would be proud!

Rich,

Nice work at an attractive price! How did you draw the schematic?

10A for the relay is no doubt overkill, but who cares? And the input trigger range is impressive!

As you know, solid state electronics do not thrive in a high temp environment, but your mounting location on the sidewall should provide a satisfactory heat sink. And of course, it will only get hot when energized by the TPS at WOT. How long do you keep your pedal to the metal? Try to keep it floored for less than 20min at a time and you should be OK .

Only suggestion would be a fuse in the 12V power line (yellow?).

You didn't mention how the other two harnesses failed. I trust they did not combust.

All the best.

 

[sloboy89]

Rich,

Nice work at an attractive price! How did you draw the schematic?

10A for the relay is no doubt overkill, but who cares? And the input trigger range is impressive!

As you know, solid state electronics do not thrive in a high temp environment, but your mounting location on the sidewall should provide a satisfactory heat sink. And of course, it will only get hot when energized by the TPS at WOT. How long do you keep your pedal to the metal? Try to keep it floored for less than 20min at a time and you should be OK .

Only suggestion would be a fuse in the 12V power line (yellow?).

You didn't mention how the other two harnesses failed. I trust they did not combust.

All the best.

I just did the drawing in PowerPoint.

The green/black wire is the 12V power; the yellow is the signal from the TPS. There is an in-line fuse (8A) on the green/black wire before the block connector. I modified the diagram to show that fuse.

There is a big heat sink on the back of the relay that is flush against the steel compartment wall.

Nothing too exciting on the failed harnesses. First one was a broken solder connection on the inaccessible back side of the circuit board. Second one just stopped working. I think it was one of the transistors that failed.

 

[SteveB2175]

Rich,

Thanks for posting this !!

I had a VVT harness fail last year, and as far as I can tell there is no new source for these. I purchased a use one, and so far it is working, but it's nice to know there is an alternative solution.

I'm saving your post in my list of reference material.

-Steve

 

[Stevew]

Add to sticky list?

 

[sloboy89]

I updated the wiring diagram to annotate the existing wires.

 

[Alfonso75]

only 2 volts on yellow when throttle fully open (no vvt operational) any idea where 8 volts comes from

 

[sloboy89]

only 2 volts on yellow when throttle fully open (no vvt operational) any idea where 8 volts comes from

The yellow wire comes directly from the throttle position switch (TPS). Maybe there's something wrong with your TPS? You can check whether it is functioning using the procedure on pages 33-35 of the L-Jet diagnostics:

https://www.hpsimotorsports.com/l-jetronic--spider-

 

[Alfonso75]

switch checks ok voltage on yellow won't go above 2 volts

 

[sloboy89]

switch checks ok voltage on yellow won't go above 2 volts

Does the car start and run?

 

[Alfonso75]

yes

 

[sloboy89]

Other than the yellow wire to the VVT, the wires to the TPS all go to the Fuel ECU. It is beyond my knowledge as to what voltages/signals should be on the ECU pins.

 

[Alfonso75]

thanks for info I was leaning towards ECU Fuel pump runs all the time and below 1500 RPM car will miss from time to time

 

[factotum]

My VVT relay board was dead, as they all will be eventually. I was thinking along the same lines a Rich, putting a new relay in the system to activate the VVT solenoid. I had not thought about the very small current draw and the need for a solid state relay. I went to eBay and ordered the same SSR from Hong Kong or Taiwan for $11.97, which included shipping. About 3 weeks later, it was here. Took about 30 minutes to install after I spent over an hour looking everywhere that I could not install the relay.

I reused the housing on the end of the VVT solenoid because it was the best alternative. I took the back off of the housing, dug out the black stuff over the PCB, removed the single screw in the middle of the board, and pushed the two female connectors on the opposite side to get the PCB removed entirely. I attached two new female connectors to the black wire and the green/black wire and pushed them through the housing just flush with the opposite side like the originals were. On the wire side, I used hot glue to embed the new connectors and wire into the housing. After drilling a small hole through the center of back cover of the housing, I put in a new, longer screw to hold the cover in place. Easy. I cut off the yellow wire and pulled it out of vinyl sleeve after cutting the bundle. I reused the yellow wire to attach to the No. 1 screw on the SSB, like Rich shows, with the other end being connected to the green/black wire coming from the VVT housing.

The best part was when I tested the relay be turning on the ignition and opening up the throttle. Looking through the inspection hole, the solenoid worked perfectly. The road test showed the engine rpm accelerating noticeably quicker from 4k to the red line. Put a smile on my old face.

 

[sloboy89]

Glad to hear that it worked for you! You did exactly what I did in reusing the white connector on the solenoid after removing the internal circuit board.

 

[factotum]

Thank you for your initial inspiration, Rich. I am just happy that there was a good workaround for the old relay. Great minds think alike. Well, even DIYer minds.

 

[sepehr]

The circuit board components are all available at your local electronic hobby shop, Amazon, eBay, etc. It's just two old school BJT transistors and some resistors and diodes to run them properly. Someone drew a diagram of the circuit from their car and posted here previously. The one in my car was for whatever reason slightly different, but essentially functioned the same way. I just made a new circuit board and reused the housing, it was a bit fiddly but not terrible.

The SSR is also a good solution. I suggest you add a diode across the solenoid leads, in parallel with the SSR. The solenoid is an inductive load. It stores energy. That energy has to go somewhere when the solenoid is deenergized, and it does so in the form of a high voltage surge of current back through the relay. This happens every time it shuts off. This will severely shorten the life of the SSR. The diode will let the flyback current go around in a circle until its energy dissipates. Diode polarity is important, it should be installed "backwards" so it doesn't conduct when the solenoid is energized.

Some info

 

[sloboy89]

@sepehr, that is interesting about using the diode. If I were to show it in my wiring diagram, would it be like below in the red circle (or do I have it backwards)? (BTW, I've been using the SSR for over six years without issue)

 

[sepehr]

Yes, that's how you'd do it.
That being said, if it ain't broke don't fix it.

 

[sepehr]

Here's the wiring diagram for the factory VVT solenoid driver circuit as-found (factory) in the VVT wiring harness for my 88 Spider.

Edit: solenoid polarity does not actually matter, the figure only shows how it was wired at the factory.