Instrument cluster troubleshooting

[rogersparknuisance]

Hi everybody, I'd like to get some advice on troubleshooting my instrument cluster. I have the original cluster that came with the car, which I will refer to as the VDO cluster, and I bought a temporary cluster to use while I sent this one to get refurbished by Bavarian Restorations, which I will refer to as the Motronic cluster.

The VDO cluster since I bought the car has never had the tachometer or fuel economy gauge work. The fuel gauge is fine, the temp gauge works but jumps around and sometimes you have to tap it. The speedo stopped working after a diff replacement, I had a separate thread here and that problem is still unresolved but possibly/probably not related to the cluster itself (https://www.r3vlimited.com/board/for...f-carrier-swap).

The Motronic cluster I purchased on eBay had essentially nothing working when I plugged it in. The fuel gauge read full even though I had a half tank, and the temp gauge stayed on cold even with the engine warmed up. Figuring the SI board batteries were probably dead on this cluster, I opened up both clusters. I found on the Motronic SI board the batteries were indeed dead, both registering 0.7V. I found to my surprise that the SI board in my VDO original cluster were restored by Bavarian Restorations in 2016 prior to my ownership, and both the batteries came in at exactly 3V as they should. So I set the Motronic SI board aside and plugged in the VDO SI board into the Motronic cluster and put it in the car. The results of this confused me, the fuel gauge still does not work and shows a full tank, now the temp gauge shows full hot, and the tach and econometer gauge started working. Speedo still doesn't work.


Since I can't use the cluster in this state at all to send one off for refurb, (I absolutely need a working temp gauge, and I also need either a working odometer or a working fuel gauge to determine when to fill up), I'm thinking I should maybe just send this one back and start over. I am about to start driving a lot more again and really don't like driving around without a working odometer.

Can anybody tell me if the SI boards are compatible across manufacturers? They look identical to me. The VDO unit is in MPH from a US '89 E30, the Motronic is a Canadian spec that has a manufacturer date in '91. Any advice for troubleshooting any of these would be helpful.

[wodcutr]

The motronic unit is probably from a 318 which only has one fuel sender so the fuel gauge is wrong. Also the coding chip will be for a 4 cylinder not 6 so the rpms will be off too. There are different SI boards for early model vs late model clusters and the speedo not working probably has more to do with the speed sensor in the diff or the wiring itself, not the cluster. The coding plug might also be in kilometers not mph since it is from Canada.

[rogersparknuisance]

I tried swapping the coding plug with the VDO SI board in each cluster. So the VDO SI board with the Motronic cluster and Motronic coding plug had a working tachometer, taking the coding plug from the VDO cluster in that setup made the tach stop working altogether. The Motronic plug in the VDO cluster/SI board combo also has a non working tach.

The Motronic cluster doesn't have working fuel or temp is the bigger problem so I can't really use it in that state.

Could the coding plug from my original VDO cluster be the problem here and the Motronic has a 318 plug that lets the tach work but not the fuel sender? The temp not working on this setup seems odd since I thought that was tied to the SI board.

[rogersparknuisance]

Maybe if I make a table it might be more clear since I have tried all combos (excluding the Motronic SI board because that has clearly dead batteries):

	VDO Cluster	Motronic Cluster
VDO Coding Plug	Temp works, fuel works, no tach no speedo	Temp shows max, fuel shows full, tach works, no speedo
Motronic Coding Plug	Temp works, fuel works, no tach no speedo	Temp shows max, fuel shows full, no tach, no speedo

[wodcutr]

Just because you think the batteries are good doesn't mean the SI board isn't damaged which usually happens from putting longer screws in where the short ones go to secure the cluster trim. If the 318 cluster temp gauge is pegged it is because the gauge is the wrong one for the sender you have. Same goes for the fuel gauge...different setup for each car. One has smaller fuel tank and one sender, the other has larger tank with 2 senders. Swap both gauges into the cluster as well. Inspect the SI board very carefully on the edge where the screws would contact it. Any damage there will render the board useless and unrepairable.

[rogersparknuisance]

All the other threads I looked up before I purchased the second cluster said that all e30 clusters are cross compatible as long as it has the same tach. I guess I have to send this one back as it is useless to me if I have to swap all these parts over from my other cluster if I were to send it off for refurb. Any tips for finding a different one that will work with my car specifically? 1989 325i US spec.

[rogersparknuisance]

Also FWIW neither SI board shows any visual damage from screws or otherwise. I only have the 2 outer screws for the bezel and none in the middle and they're both not very long.

[rogersparknuisance]

I'd like to revive this as I sent back the Motronic gauge cluster. Nobody who is selling a cluster on eBay seems to know what kind of car their cluster came out of, does anybody have a table of part number compatibility or something I can use to not purchase another cluster that won't work with my car? My car is an '89 325i US spec.

[Sunnyledge]

I believe the SI board only affects the tach and econometer. It doesn’t impact the speedometer, fuel or temp gauge.

[rogersparknuisance]

Right, what I am asking is is there any way to tell if a cluster came out of a 6 cylinder car vs a 4 cylinder car? 4 cylinder ones don't work for temp or fuel in 6 cylinder cars.

[E30_Driver]

I believe the SI board only affects the tach and econometer. It doesn’t impact the speedometer, fuel or temp gauge.

Hi! Apologies if I enter the discussion... Are you sure about that the SI board dosnt effect the speedo? I have a cluster that on the bench works perfectly and that on the car dosn't! I have already checked all the wires using the bentley Manual, replaced the sensor (which was workin) but with no success. So, my only doubt is on the SI board even if all the rest of the cluster works perfectly......

[Vincenze]

Hi! Apologies if I enter the discussion... Are you sure about that the SI board dosnt effect the speedo? I have a cluster that on the bench works perfectly and that on the car dosn't! I have already checked all the wires using the bentley Manual, replaced the sensor (which was workin) but with no success. So, my only doubt is on the SI board even if all the rest of the cluster works perfectly......

According to the wiring diagram, there are connections between the SI board and the speedometer, also between the SI board and the speed input (sensor).

[E30_Driver]

According to the wiring diagram, there are connections between the SI board and the speedometer, also between the SI board and the speed input (sensor).

I just checked the wiring diagram and it looks to me that the SI board affects just the econometer but not the speedometer and odometer. The speedo and odo are directly connected from the fuse 12 to the pin 13, and from the diff sensor to the pin 13 and 8.

[Vincenze]

I just checked the wiring diagram and it looks to me that the SI board affects just the econometer but not the speedometer and odometer. The speedo and odo are directly connected from the fuse 12 to the pin 13, and from the diff sensor to the pin 13 and 8.

I know that when the fuse #21 is disconnected, the speedometer in my car doesn't work. That fuse powers the SI board among other things.
It probably depends on the year of a car.

[Gregs///M]

The speedometer will work indepenant of the SI board. I know for a fact 100% that the speedometer will work even if you leave the SI board completely removed from the cluster. SI board powers and controls the tach, temp, MPG, OBC, and SI lights only. On the E23, E24, E28, the SI board powers and controls all clusters functions.

There were two fuel gauges uses in E30 clusters, the 55L and 63L. If your gauge reports wrong and you suspect the cluster to not be original, try swapping to the other fuel gauge size.