Electrical spike causing Tacho to surge

[PeeBee]

I have my 1997 GQ Patrol over at Brunswick Diesel in WA for 6.5 conversion - it has finally left them and is on the way to Perth then Melb. The past week has ben a series of 'release delays' being one problem after another popped up, so rather than simply work to the script, the guys at Brunswick stuck with it and have solved them all, or at least I think I may still have the one problem and its going to bounce back at me.

The tacho on my vehicle started bouncing around crazy and irratically before I sent it over there, then it settled down so I thought nothing of it. The conversion was more or less complete and then the original tacho dies completely. A s/h tacho was sourced and put in, only for this one to run ok, then go crazy as well, finally dying. A third tacho has now gone in and with 'some fancy re-wiring, all is good'. I have had a discussion with Greg at Brunswick and he can't recall seeing this before, nor knows whats causing it. There are no auxiliary items hooked up in the conversion right now to be giving a feedback from what I know that might be causing a 'voltage spike' as it was described. The entire aux circuits or the car are dead as the dual battery system is removed for the conversion.

Does anyone have any idea what might be happening here? I don't know the detail around the bypass, but will find out, however its root cause that remains a mystery. For all intensive purposes its a 2.8 wiring loom with a piggy back adapter loom for the engine. The fault started pre the conversion, but remains after it - I am sure of that. I fear its going to pop up again after i get it back, and although it only stops the tacho, will remain annoying.

Hope those brighter than me electrically have some clues. I have been over every visible earth point and removed, cleaned, and reseated that i can find.

[mudnut]

Very interesting fault. Since it has appeared in a couple of tachos, then it points to a wiring fault or maybe a faulty sender. Does it occur on rough roads or when the engine is hot?

[PeeBee]

Very interesting fault. Since it has appeared in a couple of tachos, then it points to a wiring fault or maybe a sender faulty. Does it occur on rough roads or when the engine is hot?

Unfortunately all this is happening in WA and I am in Vic waiting for the converted vehicle to return. Funny enough the original problem used to settle down after the car had been running a while, and I thought I might have had a dry joint or similar, however it simply became unpredictable and then stabilized until rearing its head again last week at the converters. I hate these intermittant problems, very difficult to get to root cause.

[dom14]

Hi mate,
The popular tacho problem with GQs I've heard and experienced myself is the loose connection, corrosion issues on the back of the tacho / display unit. In my case, I removed it and gave it a clean with some carby cleaner and tooth brush. If I recall correctly, in mine both tacho & speedo was affected from the electrical connection issue of the back of the unit.
It's been few years & it hasn't given any trouble since.
I doubt yours is a different scenario.
PM @Yendor.
He might be able offer some good tips.
Cheers

P.S. Happy to hear the engine conversion is finished. Cheers.

[PeeBee]

Car finally in Perth, latest date to Melb Oct 23 - yes!!!!

[dom14]

Car finally in Perth, latest date to Melb Oct 23 - yes!!!!

Cool!!!