New Brother and Question

One possibility...
I decided to replace every light bulb in my car. At that time all the light worked slowdown lights, check engine, belts, abs etc when key turned and engine started.
After replacing them 1/2 the lights stopped working. 2 things happend one may be your issue.
1) bulbs pushed too far down making them not work, fixing that fixed some but not all.
2) ribbon the bulbs make contact with got crimped, damaged, crumpled or otherwise failed to make contact properly. More you take bulbs out and replace them the worse it got. I bought a new faceplate with bulbs in it and swapped my gauges over. All the lights work now and I have new bulbs and I have spare parts.

good luck!

 

You can check the panel wiring by connecting, say, a small 9V battery to the instrument panel connector pins (or sockets?) applicable to the slow down lights. 9 volts should be enough to light up the lamps.

e.g. for the Left Hand Slow Down light, put power across the green and the green-white wire on the left hand instrument panel connector (I believe pins 4 and 6). For the right hand SDL, green-white and green-red (pins 4 and 9, I think) on the right hand instrument panel connector. Sorry, the diagrams are a little fuzzy.

If all good, then wiring checks from the TCUs to the instrument panel need to be carried out. I have wiring diagrams if you need them.

 

Sorry, I meant to say the contacts (pins?) on the instrument panel which correspond to those wire colours on the harness going to the instrument panel.

 

If the TCUs have been disconnected in the engine bay, maybe because they are faulty, the slow down lights won't illuminate on start up. This should throw a CEL though and you should have a code 4121 I believe.

 

The SDLs are briefly lit-up by the SDLs (if they are good) at ignition switch-on. On my non-cat UK spec 348, no CELs if the TCUs are bad or disconnected, just the codes (but no effect on the engine running).

 

When you turn the key before starting do the left and right check engine lights come on? Seems odd you have a code and no CEL do those bulbs work?

 

After doing some testing by disconnection of the TCUs I only had the 4121 TCU codes in both main ecus and the CELs were coming on but not constantly. No other codes present. When I reconnected and cleared those codes no CELs.

 

I had the same thing happen on with one of my slow down lights. I know you said the connectors are plugged in, but in my case one of the pins in the male side of the connector got pushed back and was not connected with the female side. It was just this one pin. Once I pushed the pin back in place the slow down light illuminated. Worth a look.

 

I presume your TCUs are like this one:
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You can bench test the TCUs by connecting 12V supply to the pins C and E as per the diagram below (you will find the markings on the connector). At the moment of connection, you should hear one click from the box and a second or so later, another click. The clicks are from an internal relay which goes on-off and this switches the SDL on-off at ignition "on". If you don't hear the clicks, the TCUs are bad. These type of TCUs always go bad, sooner or later. If your engine is non-cat, you can just let them be or disconnect them. Or you can spend more than $1,000 for a pair of new ones that will also fail (unless the currently sold ones are an improved version).

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