The RH side low beam in my 85 went out (high beam ok) and I replaced it with a new factory bulb. It worked with the new bulb for a few days then lost just the low beam again. The new bulb did not get low beam when connected to the LH side socket either. I have another factory bulb in now that is working. The fuses are ok, but I've noticed both sides light as long as a fuse is in either the HEAD (LH) or HEAD (RH) socket. So they will both light up with just one fuse inserted in either socket. I could be wrong but I thought they couldn't bypass the fuse box without a jumper, so could this indicate a short or bad ground somewhere?

I checked voltage between terminals of the headlight with LB then HB switched. I measured at the contact blades of the bulbs with the connectors pushed back slightly and the lights running.

LB Switched - LH 11.3v RH 11.2v
HB Switched - LH 10.41v RH 10.21v

With the HB switched as above, I measure .247v between pins 1 & 3 (LB and Common) on the RH side. So getting some draw from the LB circuit there when it should be near zero. How could I further diagnose?

I have been checking things out with the lights on intermittently and the engine off, so that may account for some of the lower voltage measures.

 

Need to start by absolutely making sure your wiring, connectors,etc are good.

The headlight circuit is poorly-designed. Toyota's electrical engineers back then were not as brilliant as its mechanical engineers. (Google bad horn circuit, starter wiring flaw).
Yes, there is expected current leakage through the HB and LB filaments of the side without the fuse. Both HB and LB of that side MAY light but very dimly.
Try below:

 

Thank you for your reply. Results below.




I measured at the following points:

A - At battery terminal
B - At back of black connector inside fuse box
C - At LH fuse (partially pulled out from socket)
D - At RH fuse (partially pulled out from socket)
E - At LH bulb fin (connector partially pulled)
F - At RH bulb fin (connector partially pulled)
G - At LH & RH bulb fins (connectors partially pulled)
H - At LH & RH bulb fins (connectors partially pulled)

So if I measured correctly I have:

- B should be 12v with headlight switched on reads .66v when switched to LB and 12v when switched to HB
- G should be 0v with HB switched on reads .8v
- H should be 0v with HB switched on reads 1.4v

 

you have negative probe to body or batt ground for all readings, correct?
"B" 0.6V in LB does not look right. B should be 12V regardless of LB or HB.
This suggests inconsistent connections between relay contacts and wiring.
Work on points from A through F. Once you consistently get 12V there, we work on G and H readings. Voltage readings suggest bad stalk-dimmer switch contacts. This is partly because of the bad design of the HL circuit which subjects that switch to high current, when it's meant to only be to "control". H4 conversion harness will address that problem, ONCE you fix current issue.LB ON; TRY GROUNDING G DIRECTLY
HB ON, GROUND H DIRECTLY

 

Yes this is all with the negative probe to battery ground. I test B from the back of the pin in the connector and I always get .6v for LB and ~11.5v for HB.



I just tested the relay as below and found I do not have continuity between terminals 1 and 3. I'll start with replacing that. I'm not completely sure how the relay works. Would this cause the voltage drop?

 

Hold on a minute. That relay looks different from our 1986 to 1988. Let's double-check our schematics. Do you have correct one? Mine posting it, similar to one i posted above?

 

I've been using that same schematic but it looks identical to the schematic in my 85 shop manual. The relay is Toyota part no. 90987-04002 and it's been in this truck all the 3-4 years I've owned it. Not sure if the previous owner replaced the original or if this is it.

 

I received the new relay today. Oddly, when I check for continuity as above, I have continuity between terminals 2 and 4, but not between 1 and 3 just like the relay I just replaced... When I check voltage at point B, I still get about .6v with the new relay in. Am I measuring in the correct place? That is, (+) probe on the back of the connector with the red and green wire in the fuse box (circled above), and (-) on the battery (-) terminal?